WO2006002844A1 - Apparatus comprising a sigma-delta modulator and method of generating a quantized signal in a sigma-delta modulator - Google Patents

Apparatus comprising a sigma-delta modulator and method of generating a quantized signal in a sigma-delta modulator Download PDF

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Publication number
WO2006002844A1
WO2006002844A1 PCT/EP2005/006875 EP2005006875W WO2006002844A1 WO 2006002844 A1 WO2006002844 A1 WO 2006002844A1 EP 2005006875 W EP2005006875 W EP 2005006875W WO 2006002844 A1 WO2006002844 A1 WO 2006002844A1
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Prior art keywords
sigma
signal
delta modulator
dither
modulator
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PCT/EP2005/006875
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French (fr)
Inventor
Johannes Wilhelmus Eikenbroek
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Ericsson Technology Licensing Ab
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Publication date
Priority claimed from EP04388043A external-priority patent/EP1612946B1/en
Application filed by Ericsson Technology Licensing Ab filed Critical Ericsson Technology Licensing Ab
Priority to KR1020067027904A priority Critical patent/KR101156877B1/en
Priority to US11/570,580 priority patent/US7394418B2/en
Publication of WO2006002844A1 publication Critical patent/WO2006002844A1/en

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    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03L—AUTOMATIC CONTROL, STARTING, SYNCHRONISATION OR STABILISATION OF GENERATORS OF ELECTRONIC OSCILLATIONS OR PULSES
    • H03L7/00—Automatic control of frequency or phase; Synchronisation
    • H03L7/06—Automatic control of frequency or phase; Synchronisation using a reference signal applied to a frequency- or phase-locked loop
    • H03L7/16—Indirect frequency synthesis, i.e. generating a desired one of a number of predetermined frequencies using a frequency- or phase-locked loop
    • H03L7/18—Indirect frequency synthesis, i.e. generating a desired one of a number of predetermined frequencies using a frequency- or phase-locked loop using a frequency divider or counter in the loop
    • H03L7/197—Indirect frequency synthesis, i.e. generating a desired one of a number of predetermined frequencies using a frequency- or phase-locked loop using a frequency divider or counter in the loop a time difference being used for locking the loop, the counter counting between numbers which are variable in time or the frequency divider dividing by a factor variable in time, e.g. for obtaining fractional frequency division
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03L—AUTOMATIC CONTROL, STARTING, SYNCHRONISATION OR STABILISATION OF GENERATORS OF ELECTRONIC OSCILLATIONS OR PULSES
    • H03L7/00—Automatic control of frequency or phase; Synchronisation
    • H03L7/06—Automatic control of frequency or phase; Synchronisation using a reference signal applied to a frequency- or phase-locked loop
    • H03L7/16—Indirect frequency synthesis, i.e. generating a desired one of a number of predetermined frequencies using a frequency- or phase-locked loop
    • H03L7/18—Indirect frequency synthesis, i.e. generating a desired one of a number of predetermined frequencies using a frequency- or phase-locked loop using a frequency divider or counter in the loop
    • H03L7/197—Indirect frequency synthesis, i.e. generating a desired one of a number of predetermined frequencies using a frequency- or phase-locked loop using a frequency divider or counter in the loop a time difference being used for locking the loop, the counter counting between numbers which are variable in time or the frequency divider dividing by a factor variable in time, e.g. for obtaining fractional frequency division
    • H03L7/1974—Indirect frequency synthesis, i.e. generating a desired one of a number of predetermined frequencies using a frequency- or phase-locked loop using a frequency divider or counter in the loop a time difference being used for locking the loop, the counter counting between numbers which are variable in time or the frequency divider dividing by a factor variable in time, e.g. for obtaining fractional frequency division for fractional frequency division
    • H03L7/1976—Indirect frequency synthesis, i.e. generating a desired one of a number of predetermined frequencies using a frequency- or phase-locked loop using a frequency divider or counter in the loop a time difference being used for locking the loop, the counter counting between numbers which are variable in time or the frequency divider dividing by a factor variable in time, e.g. for obtaining fractional frequency division for fractional frequency division using a phase accumulator for controlling the counter or frequency divider
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
    • H03M3/00—Conversion of analogue values to or from differential modulation
    • H03M3/30—Delta-sigma modulation
    • H03M3/322—Continuously compensating for, or preventing, undesired influence of physical parameters
    • H03M3/324—Continuously compensating for, or preventing, undesired influence of physical parameters characterised by means or methods for compensating or preventing more than one type of error at a time, e.g. by synchronisation or using a ratiometric arrangement
    • H03M3/326—Continuously compensating for, or preventing, undesired influence of physical parameters characterised by means or methods for compensating or preventing more than one type of error at a time, e.g. by synchronisation or using a ratiometric arrangement by averaging out the errors
    • H03M3/328—Continuously compensating for, or preventing, undesired influence of physical parameters characterised by means or methods for compensating or preventing more than one type of error at a time, e.g. by synchronisation or using a ratiometric arrangement by averaging out the errors using dither
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
    • H03M7/00—Conversion of a code where information is represented by a given sequence or number of digits to a code where the same, similar or subset of information is represented by a different sequence or number of digits
    • H03M7/30—Compression; Expansion; Suppression of unnecessary data, e.g. redundancy reduction
    • H03M7/3002—Conversion to or from differential modulation
    • H03M7/3004—Digital delta-sigma modulation
    • H03M7/3006—Compensating for, or preventing of, undesired influence of physical parameters
    • H03M7/3008—Compensating for, or preventing of, undesired influence of physical parameters by averaging out the errors, e.g. using dither
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
    • H03M3/00—Conversion of analogue values to or from differential modulation
    • H03M3/30—Delta-sigma modulation
    • H03M3/39—Structural details of delta-sigma modulators, e.g. incremental delta-sigma modulators
    • H03M3/412—Structural details of delta-sigma modulators, e.g. incremental delta-sigma modulators characterised by the number of quantisers and their type and resolution
    • H03M3/422—Structural details of delta-sigma modulators, e.g. incremental delta-sigma modulators characterised by the number of quantisers and their type and resolution having one quantiser only
    • H03M3/424—Structural details of delta-sigma modulators, e.g. incremental delta-sigma modulators characterised by the number of quantisers and their type and resolution having one quantiser only the quantiser being a multiple bit one
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
    • H03M3/00—Conversion of analogue values to or from differential modulation
    • H03M3/30—Delta-sigma modulation
    • H03M3/39—Structural details of delta-sigma modulators, e.g. incremental delta-sigma modulators
    • H03M3/436—Structural details of delta-sigma modulators, e.g. incremental delta-sigma modulators characterised by the order of the loop filter, e.g. error feedback type
    • H03M3/438—Structural details of delta-sigma modulators, e.g. incremental delta-sigma modulators characterised by the order of the loop filter, e.g. error feedback type the modulator having a higher order loop filter in the feedforward path
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
    • H03M7/00—Conversion of a code where information is represented by a given sequence or number of digits to a code where the same, similar or subset of information is represented by a different sequence or number of digits
    • H03M7/30—Compression; Expansion; Suppression of unnecessary data, e.g. redundancy reduction
    • H03M7/3002—Conversion to or from differential modulation
    • H03M7/3004—Digital delta-sigma modulation
    • H03M7/3015—Structural details of digital delta-sigma modulators
    • H03M7/302—Structural details of digital delta-sigma modulators characterised by the number of quantisers and their type and resolution
    • H03M7/3024—Structural details of digital delta-sigma modulators characterised by the number of quantisers and their type and resolution having one quantiser only
    • H03M7/3026—Structural details of digital delta-sigma modulators characterised by the number of quantisers and their type and resolution having one quantiser only the quantiser being a multiple bit one
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
    • H03M7/00—Conversion of a code where information is represented by a given sequence or number of digits to a code where the same, similar or subset of information is represented by a different sequence or number of digits
    • H03M7/30—Compression; Expansion; Suppression of unnecessary data, e.g. redundancy reduction
    • H03M7/3002—Conversion to or from differential modulation
    • H03M7/3004—Digital delta-sigma modulation
    • H03M7/3015—Structural details of digital delta-sigma modulators
    • H03M7/3031—Structural details of digital delta-sigma modulators characterised by the order of the loop filter, e.g. having a first order loop filter in the feedforward path
    • H03M7/3033—Structural details of digital delta-sigma modulators characterised by the order of the loop filter, e.g. having a first order loop filter in the feedforward path the modulator having a higher order loop filter in the feedforward path, e.g. with distributed feedforward inputs

Definitions

  • the invention relates to a method of generating a quantized signal in a Sigma-delta modulator arranged to operate together with a fractional divider in a frequency synthesizer for use in a receiver for modulated signals, the method comprising the steps of feeding a modulator input signal to a quan ⁇ tizer via at least one integrator; generating in said quantizer a quantized sig ⁇ nal; feeding back said quantized signal to be subtracted from said modulator input signal; and generating a dither signal to be applied to a point in the Sigma-delta modulator.
  • the invention further relates to an apparatus com ⁇ prising a Sigma-delta modulator.
  • Sigma-Delta modulators or converters are used in several different applica ⁇ tions, such as phase locked loops and frequency synthesizers.
  • an average division-factor constitutes an inte ⁇ ger part and a fractional part.
  • the fractional part is generated by a Sigma- Delta modulator which has a static input signal (the fractional part of the divi ⁇ sion-factor), while the output (the quantizer of the Sigma-Delta converter) is a multi-level signal which on its turn determines the division-factor of the syn ⁇ thesizer.
  • the division-factor is "randomly" switched between several integer division-values (e.g. N-1 , N and N+1) in order to arrive - on average - at the desired division-factor.
  • a dither signal (i.e. a random signal) is applied just prior to quantization in the Sigma-Delta modulator, which reduces the likelihood that the Sigma-Delta modulator might enter a limit cycle. Thereby, the output oscillating signal generated by the phase locked loop is unlikely to contain the unwanted tones.
  • dither signal i.e. a random signal
  • these un ⁇ wanted idle tones can be eliminated or reduced, but it is at the expense of some increase in noise-power at the output of the Sigma-Delta modulator.
  • the dither-signal can also be applied to other points of the Sigma-Delta modulator, such as the actual input of the modulator, which means that it is added to the static input-signal which sets the fractional-part of the overall division-factor.
  • a Sigma-Delta modulator has two integrators coupled in series, and two different dither signals are added; a first dither signal is added between the two integrators while a second dither signal is added to the input of the quantizer.
  • the dither signal may be added at any point in the Sigma-Delta modulator, included its input, provided suitable filtering of the dither signal.
  • the dither signal is applied to the input of the quantizer, because the closed-loop signal-transfer of the dither signal to the output of the modulator, i.e. the output of the quantizer, has a high-pass transfer.
  • This is beneficial because the white noise of the dither signal is shaped to a high-pass spectral shape, but the wanted signal spectrum usually is low-pass, resulting in a fre ⁇ quency separation between the wanted signal and the shaped noise.
  • the high frequency part of the shaped noise will be suppressed by the low-pass closed loop transfer of the synthesizer once the converter is used in a syn ⁇ thesizer.
  • the Sigma-Delta induced phase noise is still the dominant phase noise contributor for frequencies outside the closed- loop bandwidth of the synthesizer.
  • this noise can be reduced, but due to stability reasons it is not easy to increase the order of the loop, while decreasing the loop bandwidth will increase the settling-time for a frequency-step which is not desired in many applications.
  • the trend is to integrate the passive loop-filter components as well and this will be easier to realise when the order of the loop is as low as possible.
  • a Sigma-Delta modulator is frequency synthesiz ⁇ ers for use in Bluetooth-receivers.
  • the wanted signal can be obtained from the transmitted signal by a frequency demodulator which converts the wanted- signal to a signal whose amplitude is proportional to the derivative of the momentary phase of this wanted-signal. Due to this differentiation, noise- components with a higher frequency than others will be enhanced in level compared to noise with a lower frequency. This will influence the "ripple" of the demodulated wanted-signal in practice. This happens to the Bluetooth- signal due to the Sigma-Delta modulator induced phase-noise of the synthe- sizer.
  • the dither-signal may be applied to the input of the Sigma-Delta converter instead of the quantizer-input. Note that in this case the level of the in-band phase noise of the synthesizer output-signal will increase. However due to the dif ⁇ ferentiation-action in a frequency-demodulator, the low-frequency part of the noise will be less harmful. Thus for normal Bluetooth applications it is pre ⁇ ferred to apply the dither signal to the input of the Sigma-Delta modulator.
  • phase-modulation instead of frequency-modulation as the modulation-format.
  • a phase-demodulator is more sensi ⁇ tive to the level of the in-band phase noise of the low-pass shaped synthe ⁇ sizer phase noise.
  • coherent phase demodulators will exhibit this phenomenon.
  • this modulation-format i.e. phase-modulation
  • a Bluetooth-connection will typically start in the basic Bluetooth-mode with frequency-modulation and then switch to phase-modulation once the higher data-rate is initiated. If a Sigma-Delta modulator in which the dither signal is added to its input is used, the receiver in frequency modulation mode does not work optimally, and if, on the other hand, a Sigma-Delta modulator in which the dither signal is added to the quantizer input is used, the phase modulation mode will not be optimal, because the Sigma-Delta modulator has to be designed to one of the modes with the result that the performance in the other mode is not satisfactory. Similar problems also exist for other applications having different modes and using Sigma-Delta modulators.
  • US 2003/174799 shows a frequency synthesizer including a voltage con ⁇ trolled oscillator and a Sigma-Delta controlled phase-locked loop that deter ⁇ mines the output frequency synthesizer.
  • a dither signal is introduced to the Sigma-Delta modulator.
  • the Sigma-Delta modulator includes a first order M- N accumulator and a second order M-N accumulator, and two AND gates allow enabling signals to connect the dither signal to the first order and/or the second order M-N accumulator.
  • the object is achieved in that the method further comprises the step of applying said dither signal to a selected one of a num ⁇ ber of different points in the Sigma-delta modulator in dependence of a con ⁇ trol signal indicating a modulation format for said modulated signals.
  • the per ⁇ formance of the Sigma-Delta modulator can easily be adapted to the different modes of the application.
  • one of the number of different points in the Sigma-delta modula ⁇ tor may be an input to the quantizer.
  • another one of the number of different points in the Sigma-delta modulator may be a terminal for the modu ⁇ lator input signal. This allows the application of the dither signal to be switched between the two positions mentioned above.
  • the dither signal is applied to a first point in the Sigma-Delta modulator when the modulated signals are modulated according to a first modulation format and applied to a second point in the Sigma-Delta modulator when the modulated signals are modulated according to a second modulation format.
  • the frequency synthesizer may be used in a receiver for Bluetooth signals.
  • the dither signal may be applied to a first point in the Sigma-Delta modulator when the Bluetooth signals are frequency modulated and applied to a second point in the Sigma-Delta modulator when the Bluetooth signals are phase modulated.
  • the method may further comprise the step of applying an additional dither signal to at least one other point in the Sigma-Delta modulator. This allows dither to be applied to some points permanently and to other points in de ⁇ pendence of the control signal.
  • the invention also relates to an apparatus comprising a Sigma-delta modulator arranged to operate with a fractional divider in a fre ⁇ quency synthesizer adapted for use in a receiver for modulated signals, said Sigma-delta modulator comprising at least one integrator having an input connected to a modulator input signal; a quantizer arranged to generate a quantized signal and having an input connected to an output of said integra ⁇ tor; means for feeding back said quantized signal to be subtracted from said modulator input signal; and a dither signal generator arranged to generate a dither signal to be applied to a point in the Sigma-delta modulator.
  • the Sigma-delta modulator further comprises means for applying said dither sig- nal to a selected one of a number of different points in the Sigma-delta modu ⁇ lator in dependence of a control signal indicative of a modulation format for said modulated signals, a quantized signal can be generated in a Sigma- delta modulator which provides optimal results for different modes of the ap ⁇ plication, such as phase modulation mode and frequency modulation mode in a Bluetooth receiver.
  • one of the number of different points in the Sigma-delta modula ⁇ tor may be an input to the quantizer.
  • another one of the number of different points in the Sigma-delta modulator may be a terminal for said modulator input signal. This allows the application of the dither signal to be switched between the two positions mentioned above.
  • the means for applying the dither signal is ar ⁇ ranged to apply the dither signal to a first point in the Sigma-Delta modulator when said modulated signals are modulated according to a first modulation format and to apply the dither signal to a second point in the Sigma-Delta modulator when said modulated signals are modulated according to a sec ⁇ ond modulation format.
  • the frequency synthesizer may be adapted for use in a receiver for Bluetooth signals.
  • the means for applying the dither signal may be ar ⁇ ranged to apply the dither signal to a first point in the Sigma-Delta modulator when said Bluetooth signals are frequency modulated and to apply the dither signal to a second point in the Sigma-Delta modulator when said Bluetooth signals are phase modulated.
  • the apparatus may further be arranged to apply an additional dither signal to at least one other point in the Sigma-Delta modulator. This allows dither to be applied to some points permanently and to other points in dependence of the control signal.
  • figure 1 shows a block diagram of a Sigma-Delta controlled synthesizer in which the invention can be used
  • figure 2 shows a Sigma-Delta modulator with a dither signal coupled to the quantizer input
  • figure 3 shows a Sigma-Delta modulator with a dither signal coupled to the input of the modulator
  • figure 4 shows an improved Sigma-Delta modulator in which the dither signal can be switched between the quantizer input and the input of the modulator
  • figure 5 shows a block diagram of a Sigma-Delta controlled synthesizer with the improved Sigma-Delta modulator of figure 4,
  • figure 6 shows a Sigma-Delta modulator in which the dither signal is coupled to the two points through different filters
  • figure 7 shows a Sigma-Delta modulator in which two different dither signals can be coupled to the two points
  • figure 8 shows the Sigma-Delta modulator of figure 6 with an additional dither signal coupled to a further point in the modulator
  • figure 9 shows the Sigma-Delta modulator of figure 7 with an additional dither signal coupled to a further point in the modulator.
  • a diagram of a Sigma-Delta controlled frequency synthesizer 1 in which the invention can be used, is shown in figure 1.
  • the synthesizer 1 includes a phase locked loop circuit.
  • Such synthesizers may be used in several applica ⁇ tions, e.g. transceiver systems or stand-alone receivers, which may contain or use Bluetooth demodulators.
  • a reference signal f re f to which the phase locked loop circuit is to be regu ⁇ lated, is fed to a phase and/or frequency detector (PFD) 2.
  • PFD phase and/or frequency detector
  • This signal is usually a signal having high frequency stability.
  • the output of the phase/fre ⁇ quency detector 2 is fed through a filter 3, which is typically a low pass filter (LPF), to a voltage controlled oscillator (VCO) 4 generating the output signal f o u t -
  • VCO voltage controlled oscillator
  • the signal f out is further fed back to a frequency divider (DIV) 5, and a frequency divided signal generated by the divider 5 is then applied to another input of the phase and/or frequency detector 2, so that a feedback loop is provided.
  • DIV frequency divider
  • phase/frequency relationship between the frequency di ⁇ vided signal generated by the divider 5 and the reference signal f r ⁇ f is com ⁇ pared, and the output of the phase/frequency detector 2 is representative of the phase/frequency difference between these two signals.
  • the voltage con ⁇ trolled oscillator 4 is thus provided with a filtered voltage signal representative of this phase/frequency difference, and f out is regulated according thereto.
  • the f out frequency is divided by a division factor be ⁇ fore it is compared with the reference signal in the phase and/or frequency detector 2.
  • fractional synthesis may be utilized.
  • the division factor may also be regulated, and the average division factor thus constitutes an integer part and a fractional part.
  • the fractional part may be controlled by a division control signal generated by a Sigma-Delta modulator 6.
  • the Sigma-Delta modulator 6 receives a static fraction setting input signal and provides a multi-level signal, which on its turn determines the division factor of the synthesizer.
  • the division factor is ran ⁇ domly switched between several integer division values (e.g.
  • FIG. 2 shows an example of a Sigma-Delta modulator 6.
  • the input signal of the Sigma-Delta modulator 6 is fed to a quantizer 15 via two integrators 12, 13, and the quantized output signal is fed back through a filter 16 to subtract from the input signal in the summation point 11.
  • This feedback forces the av ⁇ erage value of the quantized signal to track the average input signal.
  • a differ ⁇ ence between these signals accumulates in the integrators 12, 13 and even ⁇ tually corrects itself.
  • a single integrator may also be used.
  • the response of the Sigma-Delta modulator to e.g. a ramp input is so that the quantized output signal oscillates between two levels that are adjacent to the input in such a manner that its local average equals the average input value.
  • the dither signal is added to the quantizer input.
  • the closed-loop signal-transfer of the dither signal to the output of the Sigma-Delta modulator, i.e. the output of the quantizer 15, has a high-pass transfer.
  • This is beneficial, especially for synthesizers used in receivers of phase modulated signals, because the white noise of the dither signal is shaped to a high-pass spectral shape, but the wanted signal spectrum usually is low-pass, resulting in a fre ⁇ quency separation between the wanted signal and the shaped noise.
  • the high frequency part of the shaped noise will be suppressed by the low-pass closed loop transfer of the synthesizer once the converter is used in a syn ⁇ thesizer. It can be shown that a phase receiver is more sensitive to the level of the in-band phase noise of the low-pass shaped synthesizer phase noise. Especially coherent phase receivers will exhibit this phenomenon.
  • the Sigma-Delta induced phase noise is still the dominant phase noise contributor for frequencies out ⁇ side the closed-loop bandwidth of the synthesizer.
  • this noise can be reduced, but due to stability reasons it is not easy to increase the order of the loop, while decreasing the loop bandwidth will increase the settling-time for a frequency- step which is not desired in many applications.
  • the trend is to in ⁇ tegrate the passive loop-filter components as well and this will be easier to realise when the order of the loop is as low as possible.
  • the wanted signal can be obtained from the transmitted signal by a frequency demodula ⁇ tor which converts the wanted-signal to a signal whose amplitude is propor ⁇ tional to the derivative of the momentary phase of this wanted-signal. Due to this differentiation, noise-components with a higher frequency than others will be enhanced in level compared to noise with a lower frequency. This will in ⁇ fluence the "ripple" of the demodulated wanted-signal in practice for e.g. a Bluetooth-signal due to the Sigma-Delta modulator induced phase-noise of the synthesizer. (This phase noise was added to the wanted-signal in the transmitter during up conversion to the desired RF-frequency). In practice it turns out that this mechanism adversely influences the Bluetooth "Modulation Characteristics" performance, e.g. the test-result during the alternating bit- sequence test.
  • simulations of a phase modulated system shows that a high pass spectral shape of the phase noise, which is obtained by adding the dither signal to the quantizer input, gives a better Error Vector Magnitude (EVM) performance than low pass spectral shaped phase noise, especially when a coherent demodulator is used for the EVM measurement.
  • EVM Error Vector Magnitude
  • Standard Bluetooth uses frequency modulation, and in Sigma-Delta modula ⁇ tors used in synthesizers for demodulation of standard Bluetooth signals it is therefore preferred to apply the dither signal at the input of the Sigma-Delta modulator.
  • the extensions to Bluetooth, medium- rate and high-rate will use phase modulation instead of frequency modula ⁇ tion as the modulation format, and here it is, as mentioned above, preferred to apply the dither signal at the quantizer input.
  • a Bluetooth-connection will typically start in the basic Bluetooth-mode with frequency-modulation and then switch to phase-modulation once the higher data-rate is initiated. If a Sigma-Delta modulator in which the dither signal is added to its input is used, the receiver in frequency modulation mode does not work optimally, and if, on the other hand, a Sigma-Delta modulator in which the dither signal is added to the quantizer input is used, the phase modulation mode will not be optimal, because the Sigma-Delta modulator has to be designed to one of the modes with the result that the performance in the other mode is not satisfactory. Similar problems also exist for other applications having different modes and using Sigma-Delta modulators.
  • FIG 4 shows an improved Sigma-Delta modulator 25, which as an exam ⁇ ple can be used in a Bluetooth receiver designed to handle both modulation formats.
  • the improved Sigma-Delta modulator 25 has a switching element 22 to which the dither signal is connected. Under control of a modulation control signal the dither signal can be switched between the two possible input posi ⁇ tions described above, depending on which modulation format is used, and the phase noise introduced by the Sigma-Delta modulator can be optimally suppressed for both modulation formats.
  • This can simply be implemented in the digital domain, because typically the complete Sigma Delta modulator is implemented in the digital domain.
  • the baseband signal that initiates the conversion between modulation formats can be used as the control signal that applies the dither signal to either the input of the Sigma-Delta modulator or the quantizer input.
  • the phase noise contribution of the Sigma-Delta modulator to the demodulated signal will be minimised for both modulation formats. This will increase the likeli ⁇ hood that the test specification of the transmitter signal will be met more eas ⁇ ily in practice.
  • the standard Bluetooth test that is mostly affected is the "Modulation Characteristics" of the transmitted signal, while the test that is mostly affected for medium rate and high rate will be the EVM-test of the transmitted signal.
  • the received signal will also be less affected by phase noise induced by the Sigma-Delta modulator, so the BER-floor (Bit Error Rate) might improve.
  • Figure 5 shows the use of the improved Sigma-Delta modulator 25 in a syn ⁇ thesizer 21. Except for the use of the improved Sigma-Delta modulator 25 and the additional modulation control signal the synthesizer 21 is similar to the synthesizer 1 in figure 1.
  • the switching element 22 applies the same dither signal to either the summation point 11 or the summation point 14.
  • the dither signal may also be added to the respective summation points through different filters 27, 28, so that the resulting dither signal added to one of the summation points is specifically adapted to be added at this summation point.
  • two separate and different dither sig ⁇ nals generated in two separate dither generators 31 , 32 may be used.
  • the two dither signals are coupled to the summation points 11 and 14 by means of the double switching element 33, which is controlled by a control signal.
  • the type of the control signal may depend on the application. It could be the modulation control signal mentioned above, but in other applications other signals may be used.
  • the dither signal or dither signals may also be added at more than two differ ⁇ ent points in the Sigma-Delta modulator, and some may be added dependent of the control signal while others may be added permanently.
  • An example is shown in the Sigma-Delta modulator 36 in figure 8.
  • the dither signal may be applied selectively, as in figure 6, to one of the summation points 11 and 14 under control of the switching element 22, but in addition the dither signal is permanently added to the summation point 38 between the two inte ⁇ grators 12 and 13.
  • it could just as well be one of the other summa ⁇ tion points to which the dither signal is added permanently, or it could be added permanently to two of them, while it is added to a third one in depend ⁇ ence of the control signal.
  • dither signal generators could also be used, as shown in the Sigma-Delta modulator 40 in figure 9, where a dither signal from the dither signal generator 41 is added permanently to summation point 42, while one of the signals from the dither signal genera ⁇ tors 31 and 32 is added to one of the summation points 11 and 14 in de ⁇ pendence of the control signal.
  • the dither signals and the summa ⁇ tion points may be combined in other ways.
  • the dither signals are added to the respective summation points through filters.
  • each dither signal may be filtered or not, or the dither generator itself may include a filter.
  • the dither signals may be filtered in the dither generator, outside the dither gen ⁇ erator or not at all. One signal may be filtered, one may not be filtered, etc. In this respect, all combinations are possible.
  • the Sigma-Delta modulators described above may be implemented either in the analog domain or the digital domain.

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  • Engineering & Computer Science (AREA)
  • Theoretical Computer Science (AREA)
  • Compression, Expansion, Code Conversion, And Decoders (AREA)

Abstract

A method of generating a quantized signal in a Sigma-Delta modulator (25) comprises the steps of feeding a modulator input signal to a quantizer (15) via at least one integrator (12, 13); generating in the quantizer (15) a quantized signal; feeding back the quantized signal to be subtracted from the modulator input signal; and generating a dither signal to be applied to a point in the Sigma-Delta modulator. The dither signal is applied to a selected one of a number of different points (11, 14) in the Sigma-Delta modulator (25) in dependence of a control signal. In this way a method of generating a quantized signal in a Sigma-Delta modulator is provided which provides optimal results for different modes of the application, such as phase modulation mode and frequency modulation mode in a Bluetooth receiver.

Description

APPARATUS COMPRIS ING A SIGiXLA- DELTA MODULATOR AND METHOD OF GENERATING A QUANTIZED SIGNAL IN A SIGMA-DELTA MODULATOR
Technical Field of the Invention
The invention relates to a method of generating a quantized signal in a Sigma-delta modulator arranged to operate together with a fractional divider in a frequency synthesizer for use in a receiver for modulated signals, the method comprising the steps of feeding a modulator input signal to a quan¬ tizer via at least one integrator; generating in said quantizer a quantized sig¬ nal; feeding back said quantized signal to be subtracted from said modulator input signal; and generating a dither signal to be applied to a point in the Sigma-delta modulator. The invention further relates to an apparatus com¬ prising a Sigma-delta modulator.
Description of Related Art
Sigma-Delta modulators or converters are used in several different applica¬ tions, such as phase locked loops and frequency synthesizers. In Sigma- Delta controlled synthesizers, an average division-factor constitutes an inte¬ ger part and a fractional part. The fractional part is generated by a Sigma- Delta modulator which has a static input signal (the fractional part of the divi¬ sion-factor), while the output (the quantizer of the Sigma-Delta converter) is a multi-level signal which on its turn determines the division-factor of the syn¬ thesizer. The division-factor is "randomly" switched between several integer division-values (e.g. N-1 , N and N+1) in order to arrive - on average - at the desired division-factor.
Because the input-signal of the Sigma-Delta modulator is static, so called "idle tones" may appear at the quantized output-signal. These periodic sig¬ nals are unwanted and must be avoided in many applications like synthesiz¬ ers for radio-equipment. US 5 986 512 shows a phase locked loop circuit which is controlled by a Sigma-Delta modulator and in which the output oscil¬ lating signal could exhibit such unwanted tones, because the Sigma-Delta modulator is susceptible to enter into what is referred to as a "limit cycle". It should be noted that the input signal to the Sigma-Delta modulator is static, i.e. a DC level, when using the Sigma-Delta modulator in phase locked loops, because the Sigma-Delta modulator usually generates the fractional part of the division number. Every time a new channel or a new reference frequency is selected, the input signal is updated. Some properties of the Sigma-Delta modulators can be explained by linear theory, but at the output of the Sigma- Delta modulator the signal is quantized, which is a nonlinear operation. Limit cycles or "idle tones" may appear due to this nonlinear operation.
According to US 5 986 512 a dither signal (i.e. a random signal) is applied just prior to quantization in the Sigma-Delta modulator, which reduces the likelihood that the Sigma-Delta modulator might enter a limit cycle. Thereby, the output oscillating signal generated by the phase locked loop is unlikely to contain the unwanted tones. However, it should be noted that by adding the dither signal to the quantizer-input of the Sigma-Delta modulator, these un¬ wanted idle tones can be eliminated or reduced, but it is at the expense of some increase in noise-power at the output of the Sigma-Delta modulator. Regarding the effect of the dither signal, it is noted that when the input signal of the Sigma-Delta modulator is static, the output signal will in average equal the input signal, but there could be a repetitive behaviour in the "output oscil¬ lations" that generates idle tones. If on the other hand the input is white noise the quantization error is less correlated and the influence of idle tones is re¬ duced. Dithering of the Sigma-Delta modulator does the same thing - decor- relate the quantization error.
The dither-signal can also be applied to other points of the Sigma-Delta modulator, such as the actual input of the modulator, which means that it is added to the static input-signal which sets the fractional-part of the overall division-factor. In US 6 175 321 a Sigma-Delta modulator has two integrators coupled in series, and two different dither signals are added; a first dither signal is added between the two integrators while a second dither signal is added to the input of the quantizer. In EP 709 969 the dither signal may be added at any point in the Sigma-Delta modulator, included its input, provided suitable filtering of the dither signal.
Usually the dither signal is applied to the input of the quantizer, because the closed-loop signal-transfer of the dither signal to the output of the modulator, i.e. the output of the quantizer, has a high-pass transfer. This is beneficial because the white noise of the dither signal is shaped to a high-pass spectral shape, but the wanted signal spectrum usually is low-pass, resulting in a fre¬ quency separation between the wanted signal and the shaped noise. The high frequency part of the shaped noise will be suppressed by the low-pass closed loop transfer of the synthesizer once the converter is used in a syn¬ thesizer.
In many applications it turns out that the Sigma-Delta induced phase noise is still the dominant phase noise contributor for frequencies outside the closed- loop bandwidth of the synthesizer. By increasing the order of the loop and/or decreasing the loop bandwidth this noise can be reduced, but due to stability reasons it is not easy to increase the order of the loop, while decreasing the loop bandwidth will increase the settling-time for a frequency-step which is not desired in many applications. In addition, the trend is to integrate the passive loop-filter components as well and this will be easier to realise when the order of the loop is as low as possible.
One example of the use of a Sigma-Delta modulator is frequency synthesiz¬ ers for use in Bluetooth-receivers. For Bluetooth frequency-modulation is used as the modulation format. The wanted signal can be obtained from the transmitted signal by a frequency demodulator which converts the wanted- signal to a signal whose amplitude is proportional to the derivative of the momentary phase of this wanted-signal. Due to this differentiation, noise- components with a higher frequency than others will be enhanced in level compared to noise with a lower frequency. This will influence the "ripple" of the demodulated wanted-signal in practice. This happens to the Bluetooth- signal due to the Sigma-Delta modulator induced phase-noise of the synthe- sizer. (This phase noise was added to the wanted-signal in the transmitter during upconversion to the desired RF-frequency). In practice it turns out that this mechanism adversely influences the Bluetooth "Modulation Characteris¬ tics" performance, e.g. the test-result during the alternating bit-sequence test.
In order to minimise the degradation due to the induced phase-noise, the dither-signal may be applied to the input of the Sigma-Delta converter instead of the quantizer-input. Note that in this case the level of the in-band phase noise of the synthesizer output-signal will increase. However due to the dif¬ ferentiation-action in a frequency-demodulator, the low-frequency part of the noise will be less harmful. Thus for normal Bluetooth applications it is pre¬ ferred to apply the dither signal to the input of the Sigma-Delta modulator.
However, in the near future the extensions to Bluetooth, medium-rate and high-rate, will use phase-modulation instead of frequency-modulation as the modulation-format. It can be shown that a phase-demodulator is more sensi¬ tive to the level of the in-band phase noise of the low-pass shaped synthe¬ sizer phase noise. Especially coherent phase demodulators will exhibit this phenomenon. For this modulation-format (i.e. phase-modulation) it is benefi¬ cial to add the dither-signal to the guantizer-input of the converter in order to exhibit a high-pass transfer of the dither-noise.
A Bluetooth-connection will typically start in the basic Bluetooth-mode with frequency-modulation and then switch to phase-modulation once the higher data-rate is initiated. If a Sigma-Delta modulator in which the dither signal is added to its input is used, the receiver in frequency modulation mode does not work optimally, and if, on the other hand, a Sigma-Delta modulator in which the dither signal is added to the quantizer input is used, the phase modulation mode will not be optimal, because the Sigma-Delta modulator has to be designed to one of the modes with the result that the performance in the other mode is not satisfactory. Similar problems also exist for other applications having different modes and using Sigma-Delta modulators. US 2003/174799 shows a frequency synthesizer including a voltage con¬ trolled oscillator and a Sigma-Delta controlled phase-locked loop that deter¬ mines the output frequency synthesizer. To avoid repetitive cycles in the Sigma-Delta controlled phase-locked loop, a dither signal is introduced to the Sigma-Delta modulator. The Sigma-Delta modulator includes a first order M- N accumulator and a second order M-N accumulator, and two AND gates allow enabling signals to connect the dither signal to the first order and/or the second order M-N accumulator.
Sudhakar Pamarti et al., "A Wideband 2.4-GHz Delta-Sigma Fractional-N PLL With 1-Mb/s In-Loop Modulation", IEEE JOURNAL OF SOLID-STATE CIRCUITS, vol. 39, no. 1 , January 2004, pages 49-62 shows a fractional-N phase locked loop implemented with a Delta-Sigma modulator for use in a Bluetooth receiver. A dither signal may be applied to the input of the Delta- Sigma modulator to eliminate spurious tones.
Therefore, it is an object of the invention to provide a method of generating a quantized signal in a Sigma-delta modulator which provides optimal results for different modes of the application, such as phase modulation mode and frequency modulation mode in a Bluetooth receiver.
Summary
According to the invention the object is achieved in that the method further comprises the step of applying said dither signal to a selected one of a num¬ ber of different points in the Sigma-delta modulator in dependence of a con¬ trol signal indicating a modulation format for said modulated signals.
When the injection of the dither signal can be switched between selected points in the Sigma-Delta modulator under control of a control signal the per¬ formance of the Sigma-Delta modulator can easily be adapted to the different modes of the application. Expediently, one of the number of different points in the Sigma-delta modula¬ tor may be an input to the quantizer. Similarly, another one of the number of different points in the Sigma-delta modulator may be a terminal for the modu¬ lator input signal. This allows the application of the dither signal to be switched between the two positions mentioned above.
In an expedient embodiment, the dither signal is applied to a first point in the Sigma-Delta modulator when the modulated signals are modulated according to a first modulation format and applied to a second point in the Sigma-Delta modulator when the modulated signals are modulated according to a second modulation format.
The frequency synthesizer may be used in a receiver for Bluetooth signals. In that case the dither signal may be applied to a first point in the Sigma-Delta modulator when the Bluetooth signals are frequency modulated and applied to a second point in the Sigma-Delta modulator when the Bluetooth signals are phase modulated.
The method may further comprise the step of applying an additional dither signal to at least one other point in the Sigma-Delta modulator. This allows dither to be applied to some points permanently and to other points in de¬ pendence of the control signal.
As mentioned, the invention also relates to an apparatus comprising a Sigma-delta modulator arranged to operate with a fractional divider in a fre¬ quency synthesizer adapted for use in a receiver for modulated signals, said Sigma-delta modulator comprising at least one integrator having an input connected to a modulator input signal; a quantizer arranged to generate a quantized signal and having an input connected to an output of said integra¬ tor; means for feeding back said quantized signal to be subtracted from said modulator input signal; and a dither signal generator arranged to generate a dither signal to be applied to a point in the Sigma-delta modulator. When the Sigma-delta modulator further comprises means for applying said dither sig- nal to a selected one of a number of different points in the Sigma-delta modu¬ lator in dependence of a control signal indicative of a modulation format for said modulated signals, a quantized signal can be generated in a Sigma- delta modulator which provides optimal results for different modes of the ap¬ plication, such as phase modulation mode and frequency modulation mode in a Bluetooth receiver.
Expediently, one of the number of different points in the Sigma-delta modula¬ tor may be an input to the quantizer. Similarly, another one of the number of different points in the Sigma-delta modulator may be a terminal for said modulator input signal. This allows the application of the dither signal to be switched between the two positions mentioned above.
In an expedient embodiment, the means for applying the dither signal is ar¬ ranged to apply the dither signal to a first point in the Sigma-Delta modulator when said modulated signals are modulated according to a first modulation format and to apply the dither signal to a second point in the Sigma-Delta modulator when said modulated signals are modulated according to a sec¬ ond modulation format.
The frequency synthesizer may be adapted for use in a receiver for Bluetooth signals. In that case the means for applying the dither signal may be ar¬ ranged to apply the dither signal to a first point in the Sigma-Delta modulator when said Bluetooth signals are frequency modulated and to apply the dither signal to a second point in the Sigma-Delta modulator when said Bluetooth signals are phase modulated.
The apparatus may further be arranged to apply an additional dither signal to at least one other point in the Sigma-Delta modulator. This allows dither to be applied to some points permanently and to other points in dependence of the control signal. Brief Description of the Drawings
The invention will now be described more fully below with reference to the drawings, in which
figure 1 shows a block diagram of a Sigma-Delta controlled synthesizer in which the invention can be used,
figure 2 shows a Sigma-Delta modulator with a dither signal coupled to the quantizer input,
figure 3 shows a Sigma-Delta modulator with a dither signal coupled to the input of the modulator,
figure 4 shows an improved Sigma-Delta modulator in which the dither signal can be switched between the quantizer input and the input of the modulator,
figure 5 shows a block diagram of a Sigma-Delta controlled synthesizer with the improved Sigma-Delta modulator of figure 4,
figure 6 shows a Sigma-Delta modulator in which the dither signal is coupled to the two points through different filters,
figure 7 shows a Sigma-Delta modulator in which two different dither signals can be coupled to the two points,
figure 8 shows the Sigma-Delta modulator of figure 6 with an additional dither signal coupled to a further point in the modulator, and
figure 9 shows the Sigma-Delta modulator of figure 7 with an additional dither signal coupled to a further point in the modulator. Detailed Description of Embodiments
A diagram of a Sigma-Delta controlled frequency synthesizer 1 , in which the invention can be used, is shown in figure 1. The synthesizer 1 includes a phase locked loop circuit. Such synthesizers may be used in several applica¬ tions, e.g. transceiver systems or stand-alone receivers, which may contain or use Bluetooth demodulators.
A reference signal fref, to which the phase locked loop circuit is to be regu¬ lated, is fed to a phase and/or frequency detector (PFD) 2. This signal is usually a signal having high frequency stability. The output of the phase/fre¬ quency detector 2 is fed through a filter 3, which is typically a low pass filter (LPF), to a voltage controlled oscillator (VCO) 4 generating the output signal fout- The signal fout is further fed back to a frequency divider (DIV) 5, and a frequency divided signal generated by the divider 5 is then applied to another input of the phase and/or frequency detector 2, so that a feedback loop is provided. Thus the phase/frequency relationship between the frequency di¬ vided signal generated by the divider 5 and the reference signal frΘf is com¬ pared, and the output of the phase/frequency detector 2 is representative of the phase/frequency difference between these two signals. The voltage con¬ trolled oscillator 4 is thus provided with a filtered voltage signal representative of this phase/frequency difference, and fout is regulated according thereto.
In the frequency divider 5 the fout frequency is divided by a division factor be¬ fore it is compared with the reference signal in the phase and/or frequency detector 2. To improve resolution, fractional synthesis may be utilized. The division factor may also be regulated, and the average division factor thus constitutes an integer part and a fractional part. As shown in figure 1 , the fractional part may be controlled by a division control signal generated by a Sigma-Delta modulator 6. The Sigma-Delta modulator 6 receives a static fraction setting input signal and provides a multi-level signal, which on its turn determines the division factor of the synthesizer. The division factor is ran¬ domly switched between several integer division values (e.g. N-1 , N and N+1) in order to arrive - on average - at the desired division-factor. Because the input-signal of the Sigma-Delta modulator is static, so called "idle tones" may appear at the modulator output signal and the output oscil¬ lating signal fout- These periodic signals are unwanted and must be avoided in many applications like synthesizers for radio-equipment. This can be achieved by adding a dither signal to the Sigma-Delta modulator, as also shown in figure 1. Thereby, the output oscillating signal generated by the phase locked loop is unlikely to contain the unwanted tones. However, it should be noted that by adding the dither signal to the Sigma-Delta modula¬ tor, these unwanted idle tones can be eliminated or reduced, but it is at the expense of some increase in noise power at the output of the Sigma-Delta modulator.
Figure 2 shows an example of a Sigma-Delta modulator 6. The input signal of the Sigma-Delta modulator 6 is fed to a quantizer 15 via two integrators 12, 13, and the quantized output signal is fed back through a filter 16 to subtract from the input signal in the summation point 11. This feedback forces the av¬ erage value of the quantized signal to track the average input signal. A differ¬ ence between these signals accumulates in the integrators 12, 13 and even¬ tually corrects itself. Instead of the two integrators 12, 13, a single integrator may also be used. The response of the Sigma-Delta modulator to e.g. a ramp input is so that the quantized output signal oscillates between two levels that are adjacent to the input in such a manner that its local average equals the average input value. In the summation point 14 the dither signal is added to the quantizer input.
When the dither signal is applied to the input of the quantizer, the closed-loop signal-transfer of the dither signal to the output of the Sigma-Delta modulator, i.e. the output of the quantizer 15, has a high-pass transfer. This is beneficial, especially for synthesizers used in receivers of phase modulated signals, because the white noise of the dither signal is shaped to a high-pass spectral shape, but the wanted signal spectrum usually is low-pass, resulting in a fre¬ quency separation between the wanted signal and the shaped noise. The high frequency part of the shaped noise will be suppressed by the low-pass closed loop transfer of the synthesizer once the converter is used in a syn¬ thesizer. It can be shown that a phase receiver is more sensitive to the level of the in-band phase noise of the low-pass shaped synthesizer phase noise. Especially coherent phase receivers will exhibit this phenomenon.
However, in many applications it turns out that the Sigma-Delta induced phase noise is still the dominant phase noise contributor for frequencies out¬ side the closed-loop bandwidth of the synthesizer. By increasing the order of the loop and/or decreasing the loop bandwidth this noise can be reduced, but due to stability reasons it is not easy to increase the order of the loop, while decreasing the loop bandwidth will increase the settling-time for a frequency- step which is not desired in many applications. In addition, the trend is to in¬ tegrate the passive loop-filter components as well and this will be easier to realise when the order of the loop is as low as possible.
When frequency-modulation is used as the modulation format, the wanted signal can be obtained from the transmitted signal by a frequency demodula¬ tor which converts the wanted-signal to a signal whose amplitude is propor¬ tional to the derivative of the momentary phase of this wanted-signal. Due to this differentiation, noise-components with a higher frequency than others will be enhanced in level compared to noise with a lower frequency. This will in¬ fluence the "ripple" of the demodulated wanted-signal in practice for e.g. a Bluetooth-signal due to the Sigma-Delta modulator induced phase-noise of the synthesizer. (This phase noise was added to the wanted-signal in the transmitter during up conversion to the desired RF-frequency). In practice it turns out that this mechanism adversely influences the Bluetooth "Modulation Characteristics" performance, e.g. the test-result during the alternating bit- sequence test.
This degradation due to the induced phase-noise may be minimized when the dither-signal is applied to the input of the Sigma-Delta modulator instead of the quantizer-input. This is illustrated in figure 3, where the dither signal is added to the loop of the Sigma-Delta modulator 20 in the summation point 11. In this case the level of the in-band phase noise of the synthesizer out¬ put-signal will increase. However due to the differentiation-action in a fre¬ quency-demodulator, the low-frequency part of the noise will be less harmful. Thus for frequency modulation applications it is preferred to apply the dither signal to the input of the Sigma-Delta converter. Measurements show that the performance with respect to the Bluetooth "Modulation Characteristics" is improved when the dither signal is fed to the signal input of the Sigma-Delta modulator compared to the quantizer input.
On the other hand, simulations of a phase modulated system shows that a high pass spectral shape of the phase noise, which is obtained by adding the dither signal to the quantizer input, gives a better Error Vector Magnitude (EVM) performance than low pass spectral shaped phase noise, especially when a coherent demodulator is used for the EVM measurement.
Standard Bluetooth uses frequency modulation, and in Sigma-Delta modula¬ tors used in synthesizers for demodulation of standard Bluetooth signals it is therefore preferred to apply the dither signal at the input of the Sigma-Delta modulator. However, in the near future the extensions to Bluetooth, medium- rate and high-rate, will use phase modulation instead of frequency modula¬ tion as the modulation format, and here it is, as mentioned above, preferred to apply the dither signal at the quantizer input.
A Bluetooth-connection will typically start in the basic Bluetooth-mode with frequency-modulation and then switch to phase-modulation once the higher data-rate is initiated. If a Sigma-Delta modulator in which the dither signal is added to its input is used, the receiver in frequency modulation mode does not work optimally, and if, on the other hand, a Sigma-Delta modulator in which the dither signal is added to the quantizer input is used, the phase modulation mode will not be optimal, because the Sigma-Delta modulator has to be designed to one of the modes with the result that the performance in the other mode is not satisfactory. Similar problems also exist for other applications having different modes and using Sigma-Delta modulators.
Figure 4 shows an improved Sigma-Delta modulator 25, which as an exam¬ ple can be used in a Bluetooth receiver designed to handle both modulation formats. In addition to the components of the Sigma-Delta modulator 6 of figure 2 the improved Sigma-Delta modulator 25 has a switching element 22 to which the dither signal is connected. Under control of a modulation control signal the dither signal can be switched between the two possible input posi¬ tions described above, depending on which modulation format is used, and the phase noise introduced by the Sigma-Delta modulator can be optimally suppressed for both modulation formats. This can simply be implemented in the digital domain, because typically the complete Sigma Delta modulator is implemented in the digital domain. The baseband signal that initiates the conversion between modulation formats can be used as the control signal that applies the dither signal to either the input of the Sigma-Delta modulator or the quantizer input.
When the modulation-format dependent dither control is applied, the phase noise contribution of the Sigma-Delta modulator to the demodulated signal will be minimised for both modulation formats. This will increase the likeli¬ hood that the test specification of the transmitter signal will be met more eas¬ ily in practice. The standard Bluetooth test that is mostly affected is the "Modulation Characteristics" of the transmitted signal, while the test that is mostly affected for medium rate and high rate will be the EVM-test of the transmitted signal. In addition, the received signal will also be less affected by phase noise induced by the Sigma-Delta modulator, so the BER-floor (Bit Error Rate) might improve.
Figure 5 shows the use of the improved Sigma-Delta modulator 25 in a syn¬ thesizer 21. Except for the use of the improved Sigma-Delta modulator 25 and the additional modulation control signal the synthesizer 21 is similar to the synthesizer 1 in figure 1. In figure 4 the switching element 22 applies the same dither signal to either the summation point 11 or the summation point 14. However, as shown in the Sigma-Delta modulator 26 in figure 6, the dither signal may also be added to the respective summation points through different filters 27, 28, so that the resulting dither signal added to one of the summation points is specifically adapted to be added at this summation point. Alternatively, as shown in the Sigma-Delta modulator 30 in figure 7, two separate and different dither sig¬ nals generated in two separate dither generators 31 , 32 may be used. The two dither signals are coupled to the summation points 11 and 14 by means of the double switching element 33, which is controlled by a control signal. The type of the control signal may depend on the application. It could be the modulation control signal mentioned above, but in other applications other signals may be used.
The dither signal or dither signals may also be added at more than two differ¬ ent points in the Sigma-Delta modulator, and some may be added dependent of the control signal while others may be added permanently. An example is shown in the Sigma-Delta modulator 36 in figure 8. Here the dither signal may be applied selectively, as in figure 6, to one of the summation points 11 and 14 under control of the switching element 22, but in addition the dither signal is permanently added to the summation point 38 between the two inte¬ grators 12 and 13. Naturally, it could just as well be one of the other summa¬ tion points to which the dither signal is added permanently, or it could be added permanently to two of them, while it is added to a third one in depend¬ ence of the control signal. Again separate dither signal generators could also be used, as shown in the Sigma-Delta modulator 40 in figure 9, where a dither signal from the dither signal generator 41 is added permanently to summation point 42, while one of the signals from the dither signal genera¬ tors 31 and 32 is added to one of the summation points 11 and 14 in de¬ pendence of the control signal. Also here the dither signals and the summa¬ tion points may be combined in other ways. In the embodiments shown in figures 6 and 8 the dither signals are added to the respective summation points through filters. However, it should be noted that in all the embodiments shown in figures 4 and 6 to 9 each dither signal may be filtered or not, or the dither generator itself may include a filter. The dither signals may be filtered in the dither generator, outside the dither gen¬ erator or not at all. One signal may be filtered, one may not be filtered, etc. In this respect, all combinations are possible.
The Sigma-Delta modulators described above may be implemented either in the analog domain or the digital domain.
Although a preferred embodiment of the present invention has been de¬ scribed and shown, the invention is not restricted to it, but may also be em¬ bodied in other ways within the scope of the subject-matter defined in the following claims.

Claims

P a t e n t c l a i m s :
1. A method of generating a quantized signal in a Sigma-Delta modulator (25; 26; 30; 36; 40) arranged to operate together with a fractional divider (5) in a frequency synthesizer (21) for use in a receiver for modulated signals, the method comprising the steps of:
• feeding a modulator input signal to a quantizer (15) via at least one in¬ tegrator (12, 13);
• generating in said quantizer (15) a quantized signal;
• feeding back said quantized signal to be subtracted from said modula¬ tor input signal; and
• generating a dither signal to be applied to a point in the Sigma-Delta modulator, c h a r a c t e r i z e d in that the method further comprises the step of:
• applying said dither signal to a selected one of a number of different points (11, 14) in the Sigma-Delta modulator (25; 26; 30; 36; 40) in dependence of a control signal indicating a modulation format for said modulated signals.
2. A method according to claim 1, c h a r a c t e r i z e d in that one (14) of said number of different points in the Sigma-Delta modulator is an input to said quantizer (15).
3. A method according to claim 1, c h a r a c t e r i z e d in that one (11) of said number of different points in the Sigma-Delta modulator is a terminal for said modulator input signal.
4. A method according to any one of claims 1 to3, c h a r a c t e r ¬ i z e d in that the dither signal is applied to a first point (11) in the Sigma-Delta modulator when said modulated signals are modulated accord¬ ing to a first modulation format and applied to a second point (14) in the Sigma-Delta modulator when said modulated signals are modulated accord¬ ing to a second modulation format.
5. A method according to any one of claims 1 to4, c h a r a c t e r ¬ i z e d in that said frequency synthesizer (21) is used in a receiver for Bluetooth signals.
6. A method according to claim 5, c h a r a c t e r i z e d in that the dither signal is applied to a first point (11) in the Sigma-Delta modulator when said Bluetooth signals are frequency modulated and applied to a sec¬ ond point (14) in the Sigma-Delta modulator when said Bluetooth signals are phase modulated.
7. A method according to any one of claims 1 to6, c h a r a c t e r ¬ i z e d in that the method further comprises the step of applying an addi¬ tional dither signal to at least one other point (38; 42) in the Sigma-Delta modulator.
8. An apparatus comprising a Sigma-Delta modulator (25; 26; 30; 36; 40) arranged to operate together with a fractional divider (5) in a frequency syn¬ thesizer (21) adapted for use in a receiver for modulated signals, said Sigma- delta modulator comprising:
• at least one integrator (12, 13) having an input connected to a modula¬ tor input signal;
• a quantizer (15) arranged to generate a quantized signal and having an input connected to an output of said integrator;
• means for feeding back said quantized signal to be subtracted from said modulator input signal; and
• a dither signal generator arranged to generate a dither signal to be applied to a point in the Sigma-Delta modulator, c h a r a c t e r i z e d in that the Sigma-Delta modulator further comprises: • means for applying said dither signal to a selected one of a number of different points (11, 14) in the Sigma-Delta modulator (25; 26; 30; 36; 40) in dependence of a control signal indicative of a modulation format for said modulated signals.
9. An apparatus according to claim 8, c h a r a c t e r i z e d in that one (14) of said number of different points in the Sigma-Delta modulator is an input to said quantizer (15).
10. An apparatus according to claim 8, c h a r a c t e r i z e d in that one (11 ) of said number of different points in the Sigma-Delta modulator is a terminal for said modulator input signal.
11. An apparatus according to any one of claims 8 to 10, c h a r a c ¬ t e r i z e d in that said means for applying the dither signal is ar¬ ranged to apply the dither signal to a first point (11 ) in the Sigma-Delta modu¬ lator when said modulated signals are modulated according to a first modula¬ tion format and to apply the dither signal to a second point (14) in the Sigma- Delta modulator when said modulated signals are modulated according to a second modulation format.
12. An apparatus according to any one of claims 8 to 11, c h a r a c ¬ t e r i z e d in that said frequency synthesizer (21) is adapted for use in a receiver for Bluetooth signals.
13. An apparatus according to claim 12, c h a r a c t e r i z e d in that said means for applying the dither signal is arranged to apply the dither signal to a first point (11) in the Sigma-Delta modulator when said Bluetooth signals are frequency modulated and to apply the dither signal to a second point (14) in the Sigma-Delta modulator when said Bluetooth signals are phase modulated.
14. An apparatus according to any one of claims 8 to 13, c h a r a c ¬ t e r i z e d in that the apparatus is further arranged to apply an addi¬ tional dither signal to at least one other point (38; 42) in the Sigma-Delta modulator.
PCT/EP2005/006875 2004-07-01 2005-06-23 Apparatus comprising a sigma-delta modulator and method of generating a quantized signal in a sigma-delta modulator WO2006002844A1 (en)

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