US20010002804A1 - Control circuit for programmable frequency synthesizer - Google Patents
Control circuit for programmable frequency synthesizer Download PDFInfo
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- US20010002804A1 US20010002804A1 US09/373,035 US37303597A US2001002804A1 US 20010002804 A1 US20010002804 A1 US 20010002804A1 US 37303597 A US37303597 A US 37303597A US 2001002804 A1 US2001002804 A1 US 2001002804A1
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- 239000013642 negative control Substances 0.000 abstract description 7
- 239000013641 positive control Substances 0.000 abstract description 2
- 230000010355 oscillation Effects 0.000 description 13
- 238000010586 diagram Methods 0.000 description 7
- 238000004891 communication Methods 0.000 description 5
- 230000008859 change Effects 0.000 description 4
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- 230000007423 decrease Effects 0.000 description 3
- 230000003534 oscillatory effect Effects 0.000 description 3
- 230000001413 cellular effect Effects 0.000 description 2
- 230000001276 controlling effect Effects 0.000 description 2
- 239000013078 crystal Substances 0.000 description 2
- 230000006978 adaptation Effects 0.000 description 1
- 238000010276 construction Methods 0.000 description 1
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- 238000012938 design process Methods 0.000 description 1
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- 239000000126 substance Substances 0.000 description 1
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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
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- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03B—GENERATION OF OSCILLATIONS, DIRECTLY OR BY FREQUENCY-CHANGING, BY CIRCUITS EMPLOYING ACTIVE ELEMENTS WHICH OPERATE IN A NON-SWITCHING MANNER; GENERATION OF NOISE BY SUCH CIRCUITS
- H03B5/00—Generation of oscillations using amplifier with regenerative feedback from output to input
- H03B5/08—Generation of oscillations using amplifier with regenerative feedback from output to input with frequency-determining element comprising lumped inductance and capacitance
- H03B5/12—Generation of oscillations using amplifier with regenerative feedback from output to input with frequency-determining element comprising lumped inductance and capacitance active element in amplifier being semiconductor device
- H03B5/1203—Generation of oscillations using amplifier with regenerative feedback from output to input with frequency-determining element comprising lumped inductance and capacitance active element in amplifier being semiconductor device the amplifier being a single transistor
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03B—GENERATION OF OSCILLATIONS, DIRECTLY OR BY FREQUENCY-CHANGING, BY CIRCUITS EMPLOYING ACTIVE ELEMENTS WHICH OPERATE IN A NON-SWITCHING MANNER; GENERATION OF NOISE BY SUCH CIRCUITS
- H03B5/00—Generation of oscillations using amplifier with regenerative feedback from output to input
- H03B5/08—Generation of oscillations using amplifier with regenerative feedback from output to input with frequency-determining element comprising lumped inductance and capacitance
- H03B5/12—Generation of oscillations using amplifier with regenerative feedback from output to input with frequency-determining element comprising lumped inductance and capacitance active element in amplifier being semiconductor device
- H03B5/1231—Generation of oscillations using amplifier with regenerative feedback from output to input with frequency-determining element comprising lumped inductance and capacitance active element in amplifier being semiconductor device the amplifier comprising one or more bipolar transistors
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03B—GENERATION OF OSCILLATIONS, DIRECTLY OR BY FREQUENCY-CHANGING, BY CIRCUITS EMPLOYING ACTIVE ELEMENTS WHICH OPERATE IN A NON-SWITCHING MANNER; GENERATION OF NOISE BY SUCH CIRCUITS
- H03B5/00—Generation of oscillations using amplifier with regenerative feedback from output to input
- H03B5/08—Generation of oscillations using amplifier with regenerative feedback from output to input with frequency-determining element comprising lumped inductance and capacitance
- H03B5/12—Generation of oscillations using amplifier with regenerative feedback from output to input with frequency-determining element comprising lumped inductance and capacitance active element in amplifier being semiconductor device
- H03B5/1237—Generation of oscillations using amplifier with regenerative feedback from output to input with frequency-determining element comprising lumped inductance and capacitance active element in amplifier being semiconductor device comprising means for varying the frequency of the generator
- H03B5/124—Generation of oscillations using amplifier with regenerative feedback from output to input with frequency-determining element comprising lumped inductance and capacitance active element in amplifier being semiconductor device comprising means for varying the frequency of the generator the means comprising a voltage dependent capacitance
- H03B5/1243—Generation of oscillations using amplifier with regenerative feedback from output to input with frequency-determining element comprising lumped inductance and capacitance active element in amplifier being semiconductor device comprising means for varying the frequency of the generator the means comprising a voltage dependent capacitance the means comprising voltage variable capacitance diodes
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- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03B—GENERATION OF OSCILLATIONS, DIRECTLY OR BY FREQUENCY-CHANGING, BY CIRCUITS EMPLOYING ACTIVE ELEMENTS WHICH OPERATE IN A NON-SWITCHING MANNER; GENERATION OF NOISE BY SUCH CIRCUITS
- H03B5/00—Generation of oscillations using amplifier with regenerative feedback from output to input
- H03B5/08—Generation of oscillations using amplifier with regenerative feedback from output to input with frequency-determining element comprising lumped inductance and capacitance
- H03B5/12—Generation of oscillations using amplifier with regenerative feedback from output to input with frequency-determining element comprising lumped inductance and capacitance active element in amplifier being semiconductor device
- H03B5/1237—Generation of oscillations using amplifier with regenerative feedback from output to input with frequency-determining element comprising lumped inductance and capacitance active element in amplifier being semiconductor device comprising means for varying the frequency of the generator
- H03B5/1262—Generation of oscillations using amplifier with regenerative feedback from output to input with frequency-determining element comprising lumped inductance and capacitance active element in amplifier being semiconductor device comprising means for varying the frequency of the generator the means comprising switched elements
-
- 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
- H03L7/08—Details of the phase-locked loop
- H03L7/099—Details of the phase-locked loop concerning mainly the controlled oscillator of the loop
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03B—GENERATION OF OSCILLATIONS, DIRECTLY OR BY FREQUENCY-CHANGING, BY CIRCUITS EMPLOYING ACTIVE ELEMENTS WHICH OPERATE IN A NON-SWITCHING MANNER; GENERATION OF NOISE BY SUCH CIRCUITS
- H03B2201/00—Aspects of oscillators relating to varying the frequency of the oscillations
- H03B2201/02—Varying the frequency of the oscillations by electronic means
- H03B2201/0275—Varying the frequency of the oscillations by electronic means the means delivering several selected voltages or currents
- H03B2201/0283—Varying the frequency of the oscillations by electronic means the means delivering several selected voltages or currents the means functioning digitally
- H03B2201/0291—Varying the frequency of the oscillations by electronic means the means delivering several selected voltages or currents the means functioning digitally and being controlled by a processing device, e.g. a microprocessor
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- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03L—AUTOMATIC CONTROL, STARTING, SYNCHRONISATION OR STABILISATION OF GENERATORS OF ELECTRONIC OSCILLATIONS OR PULSES
- H03L2207/00—Indexing scheme relating to automatic control of frequency or phase and to synchronisation
- H03L2207/06—Phase locked loops with a controlled oscillator having at least two frequency control terminals
Definitions
- the present invention relates generally to programmable frequency synthesizers. More specifically, the present invention relates to a programmable frequency synthesizer utilizing a voltage controlled oscillator circuit and associated control circuitry for use in a circuit having a low supply voltage.
- Voltage controlled oscillators are widely used in communication devices such as cellular telephones to generate oscillatory signals.
- a voltage controlled oscillator is one of the components of a phase-locked loop or PLL, which is an electronic circuit used to generate a stable oscillatory signal having a desired frequency of oscillation.
- PLL phase-locked loop
- Such circuits are commonly referred to as frequency synthesizers.
- the signal output by a PLL frequency synthesizer may be used, for example, as a carrier signal in a transmitter or as a local oscillator signal in a receiver.
- the DC voltage control signal VC used as a control input to the VCO has a range of between about 0.7 and 4.3 volts, or approximately 3.6 volts.
- VC has a range of between about 0.6 and 3.0 volts, or 2.4 volts.
- the measure of the frequency responsiveness of a VCO with respect to input voltage VC is known as the gain of the VCO, and is measured in megahertz per volt (MHz/V).
- the gain of the VCO is measured in megahertz per volt (MHz/V).
- a voltage controlled oscillator To be useful in an RF communication device such as a dual band cellular telephone, a voltage controlled oscillator must be tunable over a range of approximately 90 MHz for some applications.
- a VCO for use in a device having a five-cell supply must have a gain of about 26 MHz/Volt.
- a VCO for use in a device having a four-cell supply must have a gain of about 40 MHz/Volt.
- phase noise is directly proportional to the gain of the oscillator. This is due to the fact that the varactor diode in a VCO exhibits a higher internal series resistance when a low control voltage is applied across its terminals. This resistance decreases the quality factor of the oscillator's resonant circuit, leading to increased phase noise.
- a three-cell battery provides a control voltage range of from about 0.5 to 2.2 volts, or 1.7 volts.
- a conventional VCO having a gain corresponding to such a control voltage range would exhibit unacceptable phase noise characteristics.
- a voltage controlled oscillator and related control circuitry which exhibits acceptable phase noise performance in a device having a low supply voltage level.
- a voltage controlled oscillator comprising a voltage controlled capacitor having a first terminal and a second terminal. A positive control voltage is applied to the first terminal of the voltage controlled capacitor and a negative control voltage is applied to the second terminal of the voltage controlled capacitor.
- a circuit for generating a negative control voltage is provided in a phase-locked loop circuit.
- the circuit includes a negative DC generator for generating a negative DC voltage from an-AC signal or a transitioning logic signal, and a programmable variable attenuator for selectably attenuating the negative control voltage.
- FIG. 1 is a block diagram of a conventional phase locked loop circuit.
- FIG. 2 is a schematic diagram of a conventional voltage controlled oscillator.
- FIG. 3 is a graph showing a representative C-V characteristic of a varactor diode.
- FIG. 4 is a graph showing the series resistance of a varactor diode versus the varactor bias voltage.
- FIG. 5 is a schematic diagram of a voltage controlled oscillator according to the present invention.
- FIG. 6 is a block diagram of a phase locked loop according to the present invention.
- FIG. 1 A block diagram of a typical phase-locked loop incorporating a voltage controlled oscillator is shown in FIG. 1.
- a reference oscillator 170 generates a reference signal S ref at a predetermined frequency of oscillation.
- the frequency of the reference signal S ref is fixed at f ref , and is dependent on the construction of the reference oscillator 170 .
- reference oscillator 170 is a temperature compensated crystal oscillator (TCXO) or oven-controlled crystal oscillator (OCXO) having a stable frequency of oscillation of 19.44 MHz; Such oscillators are highly stable, capable of maintaining a frequency of oscillation within approximately 3-5 parts per million.
- TCXO temperature compensated crystal oscillator
- OXO oven-controlled crystal oscillator
- the reference signal S ref is passed through a divide-by-M circuit 150 which divides the frequency of oscillation of S ref by a selected integer value (M).
- M the frequency of oscillation of S ref
- the divided signal is then provided to an input of a phase detector 140 .
- Phase detector 140 generates a DC voltage signal that is proportional to the phase difference between two input oscillatory signals, one of which is the reference signal having a frequency of f ref /M.
- phase detector 140 which is passed through loop filter 120 to remove residual AC elements, is used to control the frequency of voltage controlled oscillator 110 .
- the output signal of VCO 110 (which is also the output signal of PLL 100 ) is passed through a divide-by-N circuit 160 and fed into a second input port of phase detector 140 .
- phase detector 140 compares the phase of a signal of frequency f ref /M with the phase of a signal of frequency f vco /N.
- the PLL 100 output signal S vco will eventually settle on a stable frequency of oscillation that is equal to f ref (N/M).
- a microprocessor may be used to change the values of M and N. In this manner, the frequency of oscillation of the output signal fvco may be digitally controlled.
- a voltage controlled oscillator is a key element of a frequency synthesizer, since it permits a frequency of oscillation to be selected based on an applied input voltage.
- FIG. 2 illustrates a schematic diagram of a well known voltage controlled oscillator (VCO) circuit 200 .
- VCO 200 includes a control voltage input VC applied to an inductor L 1 , which functions as an RF choke.
- Inductor L 1 is operatively coupled to an LC network comprising varactor diode D 1 , inductor L 3 , and capacitors C 1 and C 2 .
- C 2 is also coupled to the base of transistor Q 1 .
- a capacitor C 3 is coupled between the base and emitter of transistor Q 1
- a capacitor C 4 is coupled between the emitter of transistor Q 1 and ground.
- Varactor diode D 1 , capacitor C 1 and inductor L 3 form a resonant circuit which determines the frequency of operation of VCO 200 .
- the capacitance of varactor diode D 1 is determined by the DC voltage applied across its terminals by VC.
- the frequency of oscillation of VCO 200 is determined by the input DC voltage level VC.
- the capacitance of varactor diode D 1 is a function of the DC voltage applied across its terminals. As the voltage across diode D 1 increases, its capacitance decreases. Since varactor diode D 1 is an element of the resonant circuit which determines the frequency of oscillation of VCO 200 , a change in the capacitance of varactor diode D 1 (due to change in the level of control voltage VC) will result in a change in the frequency of oscillation of the VCO 200 .
- VCO 500 includes a positive DC control input VC 1 and a negative DC control input VC 2 .
- VC 1 is coupled to the cathode 510 of varactor diode D 1 through RF choke inductor L 1 , causing diode D 1 to be reverse biased.
- VC 2 is coupled to the anode 520 of varactor diode D 1 through RF choke inductor L 2 .
- a capacitor C 5 is coupled between the anode 520 of varactor diode D 1 and ground. Capacitor C 5 provides an AC ground to the control voltage VC 2 .
- the remaining elements of VCO 500 are similar in location and function to the corresponding elements described with reference to FIG. 2.
- FIG. 6 An embodiment of a phase-locked loop circuit employing the VCO 500 described above is illustrated in FIG. 6.
- PLL 600 includes a phase detector 640 , loop filter 620 , VCO 500 , divide-by-M circuit 650 and divide-by-N circuit 660 arranged in a conventional fashion.
- VCO 500 is a two-input VCO as illustrated in FIG. 5, having a positive DC input VC 1 and a negative DC input VC 2 .
- the output of loop filter 320 is coupled to the positive DC input VC 1 .
- a negative DC generator 665 is coupled to the output of divide-by-M circuit 650 .
- Negative DC generator 665 generates a negative DC signal from an input sinusoidal signal.
- the implementation and operation of negative DC generators may be a charge switching device or a conventional recitifier with voltage multiplication, the structure and operation of both of which are well known. Such devices are described for example in J. Ryder, ELECTRONIC FUNDAMENTALS AND APPLICATIONS (5th Ed. 1976) Prentice-Hall, Inc., Ch. 16.
- Negative DC generator 665 outputs a negative DC signal to programmable variable attenuator (PVA) 670 .
- PVA 670 selectively attenuates the negative DC signal generated by negative DC generator 665 according to an instruction word supplied by microprocessor 680 via bus 385 .
- the attenuated negative DC signal is then output from PVA 670 to a low pass filter 675 , which filters out unwanted AC signal components.
- the filtered attenuated negative DC signal is applied as control input VC 2 to VCO 500 .
- a plurality of negative DC levels may be applied to negative control voltage input VC 2 .
- the range of control voltage applied to VCO 500 may be extended beyond that which would normally be possible in a low voltage device.
- PVA 700 includes a plurality of control input lines 710 , each of which is coupled to the base of a PNP transistor 720 .
- the emitter of each of the transistors 720 is coupled to a supply terminal 730 , and the collector of each transmitter is coupled to a first terminal of one of a plurality of resistors 740 .
- the second terminal of each of the plurality of resistors is coupled to an output terminal 750 .
- a negative DC voltage source 760 is also coupled to the output terminal 750 through resistor Ry.
- a binary control word is applied as a digital signal to the control inputs 710 .
- the value of the binary control word will determine the level of attenuation of the input negative DC voltage by PVA 700 .
- the programmable variable attenuator has four control inputs. However, more control lines may be easily added as desired for greater flexibility in the selection of attenuation levels.
- a control word consisting of the binary value 0111 may be applied.
- the voltage appearing at b 1 will be logical 0 and the voltage appearing at terminals b 2 -b 4 will be logical 1.
- the voltage appearing at output terminal 750 will now be an attenuated version of the negative DC voltage level 760 .
- Various control words will result in various combinations of resistances in series with Ry, resulting in various levels of attenuation in the negative DC voltage level.
- a negative DC voltage signal VC 2 If the generation of a negative DC voltage signal VC 2 is performed using a charge switching device, an unwanted AC ripple may be present in the control signal. However, since the AC signal input to the negative DC generator 665 has a highly stable frequency of oscillation, the unwanted AC components will have a highly predictable and stable frequency. In order to reduce these unwanted components, a low pass filter 675 is used to filter the control signal VC 2 prior to applying it to the VCO 610 .
- the VCO and associated circuitry of the present invention provide a number of additional advantages.
- the VCO of the present invention may be manufactured using inexpensive varactor technology and design processes.
- the design is highly integratable, and would require little if any additional circuit board area to implement.
- the control circuitry can be easily integrated into existing devices.
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Abstract
Description
- 1. Field of Invention
- The present invention relates generally to programmable frequency synthesizers. More specifically, the present invention relates to a programmable frequency synthesizer utilizing a voltage controlled oscillator circuit and associated control circuitry for use in a circuit having a low supply voltage.
- 2. Description of Related Art
- Voltage controlled oscillators are widely used in communication devices such as cellular telephones to generate oscillatory signals. In particular, a voltage controlled oscillator is one of the components of a phase-locked loop or PLL, which is an electronic circuit used to generate a stable oscillatory signal having a desired frequency of oscillation. Such circuits are commonly referred to as frequency synthesizers. The signal output by a PLL frequency synthesizer may be used, for example, as a carrier signal in a transmitter or as a local oscillator signal in a receiver.
- In order to tune a transmitter or receiver to a particular frequency, it is necessary to alter the frequency of oscillation of the carrier or local oscillator signal in a controlled manner. This can be achieved by synthesizing the carrier and local oscillator signals using a programmable frequency synthesizer including a VCO. A VCO is tuned by varying an input DC control voltage level.
- Most portable electronic devices use a five-cell or four-cell battery to generate DC power. Devices having five-cell and four-cell batteries can generate a regulated DC power supply voltage level of about 4.8 volts and 3.8 volts, respectively. In the case of a five-cell battery, the DC voltage control signal VC used as a control input to the VCO has a range of between about 0.7 and 4.3 volts, or approximately 3.6 volts. In the case of a four-cell battery, VC has a range of between about 0.6 and 3.0 volts, or 2.4 volts. The measure of the frequency responsiveness of a VCO with respect to input voltage VC is known as the gain of the VCO, and is measured in megahertz per volt (MHz/V). Thus, if the input control voltage has a lower range, the VCO must have a greater gain in order to be tunable over the same frequency range.
- To be useful in an RF communication device such as a dual band cellular telephone, a voltage controlled oscillator must be tunable over a range of approximately 90 MHz for some applications. Thus, a VCO for use in a device having a five-cell supply must have a gain of about 26 MHz/Volt. A VCO for use in a device having a four-cell supply must have a gain of about 40 MHz/Volt.
- One drawback to a VCO is the fact that it produces a certain amount of phase noise, which can degrade the performance of a communication system. A VCO must be designed such that phase noise is kept within certain predetermined limits. In a voltage controlled oscillator, phase noise is directly proportional to the gain of the oscillator. This is due to the fact that the varactor diode in a VCO exhibits a higher internal series resistance when a low control voltage is applied across its terminals. This resistance decreases the quality factor of the oscillator's resonant circuit, leading to increased phase noise.
- There is currently a trend in the communication industry to design and produce electronic devices capable of operating with a three-cell battery. A three-cell battery provides a control voltage range of from about 0.5 to 2.2 volts, or 1.7 volts. A conventional VCO having a gain corresponding to such a control voltage range would exhibit unacceptable phase noise characteristics. Thus, there is a need for a voltage controlled oscillator and related control circuitry which exhibits acceptable phase noise performance in a device having a low supply voltage level.
- It is therefore an object of the present invention to provide a voltage controlled oscillator having improved phase noise performance when operated using a low supply voltage.
- It is a further object of the invention to provide a circuit for controlling the voltage controlled oscillator according to the present invention.
- It is a further object of the invention to provide a phase locked loop incorporating a voltage controlled oscillator according to the present invention.
- The foregoing objects are achieved in a voltage controlled oscillator comprising a voltage controlled capacitor having a first terminal and a second terminal. A positive control voltage is applied to the first terminal of the voltage controlled capacitor and a negative control voltage is applied to the second terminal of the voltage controlled capacitor.
- A circuit for generating a negative control voltage is provided in a phase-locked loop circuit. The circuit includes a negative DC generator for generating a negative DC voltage from an-AC signal or a transitioning logic signal, and a programmable variable attenuator for selectably attenuating the negative control voltage.
- These and other objects of the invention, together with features and advantages thereof will become apparent from the following detailed specification when read with the accompanying drawings in which like reference numerals refer to like elements.
- FIG. 1 is a block diagram of a conventional phase locked loop circuit.
- FIG. 2 is a schematic diagram of a conventional voltage controlled oscillator.
- FIG. 3 is a graph showing a representative C-V characteristic of a varactor diode.
- FIG. 4 is a graph showing the series resistance of a varactor diode versus the varactor bias voltage.
- FIG. 5 is a schematic diagram of a voltage controlled oscillator according to the present invention.
- FIG. 6 is a block diagram of a phase locked loop according to the present invention.
- The present invention will now be described with reference to the accompanying drawings, in which a preferred embodiment of the invention is shown. However, this invention may be embodied in many different forms and should not be construed as limited to the specific embodiment shown.
- A block diagram of a typical phase-locked loop incorporating a voltage controlled oscillator is shown in FIG. 1. A
reference oscillator 170 generates a reference signal Sref at a predetermined frequency of oscillation. The frequency of the reference signal Sref is fixed at fref, and is dependent on the construction of thereference oscillator 170. - In a typical communications application,
reference oscillator 170 is a temperature compensated crystal oscillator (TCXO) or oven-controlled crystal oscillator (OCXO) having a stable frequency of oscillation of 19.44 MHz; Such oscillators are highly stable, capable of maintaining a frequency of oscillation within approximately 3-5 parts per million. - The reference signal Sref is passed through a divide-by-
M circuit 150 which divides the frequency of oscillation of Sref by a selected integer value (M). The divided signal is then provided to an input of aphase detector 140.Phase detector 140 generates a DC voltage signal that is proportional to the phase difference between two input oscillatory signals, one of which is the reference signal having a frequency of fref/M. - The output signal of
phase detector 140, which is passed throughloop filter 120 to remove residual AC elements, is used to control the frequency of voltage controlledoscillator 110. The output signal of VCO 110 (which is also the output signal of PLL 100) is passed through a divide-by-N circuit 160 and fed into a second input port ofphase detector 140. Thus,phase detector 140 compares the phase of a signal of frequency fref/M with the phase of a signal of frequency fvco/N. Depending on the values of the circuit elements included in theVCO 110, thePLL 100 output signal Svco will eventually settle on a stable frequency of oscillation that is equal to fref(N/M). - A microprocessor may be used to change the values of M and N. In this manner, the frequency of oscillation of the output signal fvco may be digitally controlled.
- Thus, it is apparent that a voltage controlled oscillator is a key element of a frequency synthesizer, since it permits a frequency of oscillation to be selected based on an applied input voltage.
- FIG. 2 illustrates a schematic diagram of a well known voltage controlled oscillator (VCO) circuit200. VCO 200 includes a control voltage input VC applied to an inductor L1, which functions as an RF choke. Inductor L1 is operatively coupled to an LC network comprising varactor diode D1, inductor L3, and capacitors C1 and C2. C2 is also coupled to the base of transistor Q1. A capacitor C3 is coupled between the base and emitter of transistor Q1, and a capacitor C4 is coupled between the emitter of transistor Q1 and ground. Varactor diode D1, capacitor C1 and inductor L3 form a resonant circuit which determines the frequency of operation of VCO 200. The capacitance of varactor diode D1 is determined by the DC voltage applied across its terminals by VC. Thus, the frequency of oscillation of VCO 200 is determined by the input DC voltage level VC.
- As illustrated in FIG. 3, the capacitance of varactor diode D1 is a function of the DC voltage applied across its terminals. As the voltage across diode D1 increases, its capacitance decreases. Since varactor diode D1 is an element of the resonant circuit which determines the frequency of oscillation of VCO 200, a change in the capacitance of varactor diode D1 (due to change in the level of control voltage VC) will result in a change in the frequency of oscillation of the VCO 200.
- However, as illustrated in FIG. 4, as the voltage across varactor diode D1 decreases, the internal resistance of diode D1 increases. The internal resistance of diode D1 directly affects the resistance of the resonant circuit of VCO 200, lowering the quality factor of the resonant circuit and affecting the noise performance of VCO 200. Thus, when a low-range control voltage is used to control VCO 200, VCO 200 will exhibit unsatisfactory noise characteristics. And, as noted above, in a three-cell device, there is a reduced DC voltage range available to control VCO 200.
- In order to provide an increased DC voltage range for use in controlling the operation of a VCO in a PLL circuit, the present invention provides a VCO having a positive DC control input and a negative DC control input. An embodiment of a VCO according to the present invention is illustrated in FIG. 5. As shown in FIG. 5,
VCO 500 includes a positive DC control input VC1 and a negative DC control input VC2. VC1 is coupled to thecathode 510 of varactor diode D1 through RF choke inductor L1, causing diode D1 to be reverse biased. VC2 is coupled to theanode 520 of varactor diode D1 through RF choke inductor L2. A capacitor C5 is coupled between theanode 520 of varactor diode D1 and ground. Capacitor C5 provides an AC ground to the control voltage VC2. The remaining elements ofVCO 500 are similar in location and function to the corresponding elements described with reference to FIG. 2. - Since a positive DC voltage is applied to the
cathode 510 of varactor diode D1 and a negative DC voltage is applied to theanode 520 of varactor diode D1, a greater range of control voltages may be applied to varactor diode D1 than would be available using only one control voltage, leading to a lower average internal series resistance and consequent improved phase noise performance. - An embodiment of a phase-locked loop circuit employing the
VCO 500 described above is illustrated in FIG. 6. As shown in FIG. 6, PLL 600 includes aphase detector 640,loop filter 620,VCO 500, divide-by-M circuit 650 and divide-by-N circuit 660 arranged in a conventional fashion.VCO 500 is a two-input VCO as illustrated in FIG. 5, having a positive DC input VC1 and a negative DC input VC2. The output ofloop filter 320 is coupled to the positive DC input VC1. - In the PLL600 illustrated in FIG. 6, a negative DC generator 665 is coupled to the output of divide-by-
M circuit 650. Negative DC generator 665 generates a negative DC signal from an input sinusoidal signal. The implementation and operation of negative DC generators may be a charge switching device or a conventional recitifier with voltage multiplication, the structure and operation of both of which are well known. Such devices are described for example in J. Ryder, ELECTRONIC FUNDAMENTALS AND APPLICATIONS (5th Ed. 1976) Prentice-Hall, Inc., Ch. 16. - Negative DC generator665 outputs a negative DC signal to programmable variable attenuator (PVA) 670.
PVA 670 selectively attenuates the negative DC signal generated by negative DC generator 665 according to an instruction word supplied by microprocessor 680 via bus 385. The attenuated negative DC signal is then output fromPVA 670 to alow pass filter 675, which filters out unwanted AC signal components. Finally, the filtered attenuated negative DC signal is applied as control input VC2 toVCO 500. - By altering the contents of the instruction word supplied to
PVA 670, a plurality of negative DC levels may be applied to negative control voltage input VC2. In this manner, the range of control voltage applied toVCO 500 may be extended beyond that which would normally be possible in a low voltage device. - A circuit diagram of a programmable variable attenuator is illustrated in FIG. 7.
PVA 700 includes a plurality ofcontrol input lines 710, each of which is coupled to the base of aPNP transistor 720. The emitter of each of thetransistors 720 is coupled to asupply terminal 730, and the collector of each transmitter is coupled to a first terminal of one of a plurality ofresistors 740. The second terminal of each of the plurality of resistors is coupled to anoutput terminal 750. A negativeDC voltage source 760 is also coupled to theoutput terminal 750 through resistor Ry. A binary control word is applied as a digital signal to thecontrol inputs 710. The value of the binary control word will determine the level of attenuation of the input negative DC voltage byPVA 700. In the embodiment illustrated in FIG. 7, the programmable variable attenuator has four control inputs. However, more control lines may be easily added as desired for greater flexibility in the selection of attenuation levels. - For example, in a PVA having four input lines, a control word consisting of the binary value 0111 may be applied. In such case, the voltage appearing at b1 will be logical 0 and the voltage appearing at terminals b2-b4 will be logical 1. Thus, only transistor Q1 will be in an “on” state and transistors Q2-Q3 will be in an “off” state. Depending on the values of R1 and Ry, the voltage appearing at
output terminal 750 will now be an attenuated version of the negativeDC voltage level 760. Various control words will result in various combinations of resistances in series with Ry, resulting in various levels of attenuation in the negative DC voltage level. - If the generation of a negative DC voltage signal VC2 is performed using a charge switching device, an unwanted AC ripple may be present in the control signal. However, since the AC signal input to the negative DC generator 665 has a highly stable frequency of oscillation, the unwanted AC components will have a highly predictable and stable frequency. In order to reduce these unwanted components, a
low pass filter 675 is used to filter the control signal VC2 prior to applying it to the VCO 610. - Besides providing a VCO with acceptable phase noise performance in a low voltage system, the VCO and associated circuitry of the present invention provide a number of additional advantages. The VCO of the present invention may be manufactured using inexpensive varactor technology and design processes. The design is highly integratable, and would require little if any additional circuit board area to implement. Moreover, the control circuitry can be easily integrated into existing devices.
- While the present invention has been described with respect to its preferred embodiment, those skilled in the art will recognize that the present invention is not limited to the specific embodiment described and illustrated herein. Different embodiments and adaptations besides those shown herein and described as well as many variations, modifications and equivalent arrangements will now be apparent or will be reasonably suggested by the foregoing specification and drawings, without departing from the substance or scope of the invention.
Claims (16)
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US09/373,035 US6271731B2 (en) | 1997-04-15 | 1997-04-15 | Control circuit for programmable frequency synthesizer |
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US09/373,035 US6271731B2 (en) | 1997-04-15 | 1997-04-15 | Control circuit for programmable frequency synthesizer |
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US20010002804A1 true US20010002804A1 (en) | 2001-06-07 |
US6271731B2 US6271731B2 (en) | 2001-08-07 |
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FR2501434B1 (en) * | 1981-03-03 | 1985-10-11 | Cepe | CONTROLLED FREQUENCY OSCILLATOR COMPRISING A PIEZOELECTRIC ELEMENT AND HAVING AN EXTENDED FREQUENCY VARIATION RANGE |
US4580107A (en) * | 1984-06-06 | 1986-04-01 | The United States Of America As Represented By The Secretary Of The Air Force | Phase lock acquisition system having FLL for coarse tuning and PLL for fine tuning |
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