EP1745544A1

EP1745544A1 - Pulling-free lo generation system and method

Info

Publication number: EP1745544A1
Application number: EP05718801A
Authority: EP
Inventors: Johannes Petrus Antonius Frambach; Paulus Thomas Maria Zeijl
Original assignee: Koninklijke Philips Electronics NV
Current assignee: NXP BV
Priority date: 2004-04-30
Filing date: 2005-04-25
Publication date: 2007-01-24
Also published as: JP2007535855A; KR20070004890A; CN1985436A; WO2005107059A1

Abstract

A system and method for generating an LO signal for an RF transmitter is provided that eliminates the risk of LO-pulling. A filtering technique is employed to select the desired harmonic of a VCO signal, usually the third. The required in-phase and quadrature LO signals can be derived by means of dividers. Any harmonic relation is avoided between the transmit signal and the VCO frequency, without generation of intermediate signals close to the VCO frequency, thus circumventing pulling of the VCO.

Description

PULLING-FREE LO GENERATION SYSTEM AND METHOD

The present invention relates to local oscillator or LO-pulling of a single die radio frequency transceiver. More particularly, the present invention relates to a system and method for at least one way to generate an LO signal for the transceiver. Most particularly, the present invention relates to a system and method for at least one way to generate the

LO signal without pulling of a VCO of a transmitter of the transceiver. Today's radio transceivers contain most of their functionality in a single chip. This allows for high scale integration, making radio frequency (RF) transceivers very cost effective for use in consumer products. However, when all RF functionality is placed into a single die, 'LO-pulling' or 'VCO-pulling' presents itself as problem for designers of such chips to solve. LO-pulling occurs when a fraction of the transmit signal couples back to the voltage controlled oscillator (VCO) of the frequency synthesizer. The transmit signal is modulated with the information to be transmitted, so the VCO is modulated as well. Since the VCO is used to generate the transmit signal, e.g., by means of a mixer, a feedback loop exists that severely degrades the quality of the transmit signal. Due to the regenerative nature of the VCO, even the smallest coupling can have a severe impact on the quality of the transmit signal. Several alternative schemes for generating an LO signal for a transmit mixer have been proposed in the literature. All proposed schemes have the objective of reducing the pulling of the VCO, see, e.g., Zolfaghari, Alireza and Razavi, Behzad: 'A Low-Power 2.4 GHz Transmitter/Receiver CMOS IC\ IEEE-JSSC, vol. 38, no. 2, February 2003, pps. 176-183 and Darabi, Hooman et al.: "A 2.4-GHz CMOS Transceiver for Bluetooth', IEEE- JSSC, vol. 36, no. 12, December 2001, pps. 2016-2023. These proposed schemes either employ a two-stage up-conversion of the transmit signal or a two-stage generation of the LO signal, in order to avoid a harmonic relation between the transmit signal (or its harmonics) and the VCO frequency. In either case, however, a large amount of harmonics, sub-harmonics and mixing products are present and some of these mixing products can still cause VCO pulling. In some such proposed schemes a significant amount of filtering is required to suppress unwanted spectral components. Zolfaghari et al propose the use of a double intermediate frequency (IF) in the up- conversion process. The frequency approach is selected such that no spectral components coincide with the VCO, hence circumventing the pulling problem at the cost of a complex signal path. Another approach is a combination of a classical direct up-conversion transmitter with an alternative LO generation. Here again, the frequency plan is selected so that no coinciding spectra are generated. The solution proposed by Darabi et al, however, requires filtering of the mixer output signal before the signal is amplified. If no filter were applied before further amplification, the mixer output signal would be an 800 MHz square wave instead of the desired 2400 MHz signal. This results from the harmonic relation between the two mixer input signals. Hence, this approach requires more precise filtering. The approach proposed by Darabi is illustrated in FIG. 1 and comprises a local oscillator 100 for producing output signals LOI 107 and LOQ 108 having a desired frequency of 2400 MHz. Local oscillator 100 includes a VCO circuit 101, a buffer 109, a frequency divider 102 for producing 800 MHz in-phase LO component I 103 and quadrature component Q 104 signals that are half the output of the VCO, and two mixers 105 106. VCO circuit 101 produces a 1600 MHz signal, which is then provided via buffer 109 to frequency divider circuit 102, which produces an in-phase signal I 103, and a quadrature signal Q 104 having a frequency that is 800 MHz, or half the frequency output by the VCO 101. Thus, a solution to the LO pulling of the prior art is needed. The present invention provides a system and method for generating an LO signal for a transmitter without the risk of pulling the VCO. The system and method of the present invention employs a filtering technique to select a desired harmonic of the VCO signal. The required in-phase and quadrature LO signals can be derived by means of dividers. The system and method of the present invention avoids any harmonic relation between the transmit signal and the VCO frequency, without generation of intermediate signals close to the VCO frequency. Hence the system and method of the present invention avoids pulling of the VCO. A VCO generates a sinusoidal input signal that is subsequently converted to a square wave, divided and filtered to obtain a desired harmonic in-phase I and quadrature Q signal of a pre-determined output frequency. According to one aspect of the present invention a local oscillator is provided for generating signals at a desired output frequency. The local oscillator includes an oscillatory circuit configured to generate a first in-phase signal and a first quadrature signal at a first frequency that is lower than the desired output frequency, the first quadrature signal having a phase shift relative to the first in-phase signal; a pair of harmonic filters one coupled to receive the first in-phase signal and the other to receive the first quadrature signal and configured to obtain a pre-determined harmonic of each of these first signals; first and second transmit mixers coupled respectively to be driven by said obtained harmonic of each of said first signals. In one embodiment a desired harmonic of a square wave input signal is obtained and an in-phase and a quadrature signal are generated therefrom such that the quadrature signal is phase-shifted relative to the in-phase signal, the I and Q signals are used to drive transmit mixers. In one embodiment the oscillatory circuit is configured using an LC tank having a high Q and a small area. The desired harmonic is preferably the third harmonic. Alternative embodiments employ one filter to select the desired harmonic.

FIG. 1 illustrates a prior art approach to avoiding LO-pulling; FIG. 2 illustrates a first preferred embodiment of pulling-free LO generation, according to the present invention; FIG. 3 illustrates a second preferred embodiment of pulling-free LO generation, according to the present invention; FIG. 4 illustrates a third preferred embodiment of pulling-free LO generation, according to the present invention; FIG. 5 illustrates an architecture of a wireless communication system whereto embodiments of the present invention are to be applied; and FIG. 6 illustrates a simplified block diagram of a wireless devices of the communication system of FIG. 5 according to an embodiment of the present invention.

It is to be understood by persons of ordinary skill in the art that the following descriptions are provided for purposes of illustration and not for limitation. An artisan understands that there are many variations that lie within the spirit of the invention and the scope of the appended claims. Unnecessary detail of known functions and operations may be omitted from the current description so as not to obscure the present invention. In a first embodiment, a pulling-free LO is generated as illustrated in FIG. 2. The VCO 201, running at 1600 MHz provides and input 212 to an IQ-generation circuit 200 that is divided in half by divider 202 to generate an in-phase LO component I 203 and a quadrature LO component Q 204, containing odd-order harmonics of 800 MHz. As illustrated in FIG. 2, the output signal of the divider 202, preferably a divide-by-2, is a square wave 203 204, containing these odd-order harmonics. In this first preferred embodiment, the system and method of the present invention applies filters 211 to the square waves 203 204 to obtain the third harmonic of these signals, and thus derives a 2400 MHz in-phase signal LOI 207 and a quadrature signal LOQ 208. LOI 207 and LOQ 208 are then used for driving transmit and receive mixers. In the first embodiment, an LC-tank Q factor of 10 is assumed and the fundamental 800 MHz component is suppressed by 28 dB. Since the 3rd harmonic has a 9.5 dB lower signal level as compared to the fundamental 800 MHz component (Fourier coefficients of a square wave: 1, 1/3, 1/5, 1/7...) the resulting LO signal of 2400 MHz contains an 800 MHz spurious signal of -20 dBc. After mixing, in the first embodiment, this unwanted signal is removed by an antenna filter. In order to achieve good image rejection in the transmit path, the I and Q signal must have a perfect quadrature and equal amplitude. The first embodiment includes at least one filtering amplifier that accomplishes this matching. A poly-phase (LC) filter is preferred to meet the matching requirements. In a second embodiment, a pulling-free LO is generated as illustrated in FIG. 3, such that the stringent matching requirements of the first embodiment are circumvented. The VCO 201, running at 1600 MHz provides an input 312 to an IQ-generation circuit 300 such that in the second embodiment, the third harmonic filtering 211 is done prior to the divider, preferably a divide-by-2 202. The quadrature relation of the I and Q signal is now determined by the divider 202, which ensures perfect quadrature. The 3rd harmonic filtering 211 is performed directly on a clipped (amplified) version of the VCO signal, so the LC-tank is tuned to 4800 MHz in the second embodiment. The higher frequency allows for a higher tank Q factor, which, as a side benefit, has a much smaller area. In addition, only one LC-tank is needed in this second embodiment, resulting in further saving of area. Due to the regenerative nature of the divide-by-2 202 the input sinusoidal signal is converted into a square wave, hence extra amplification of the filtered third harmonic signal or the desired I and Q LO signals is not necessary. Amplification (or clipping) is only required before the filter in order to generate a 1600 MHz signal with sufficient third harmonic signal content. The output spectrum of the harmonic filter still contains a residual 1600 MHz component, but this signal is suppressed by 6 dB extra by the divider 202. Hence the LO spectrum contains an unwanted signal component at 1600 MHz which is less then 26 dBc with respect to the LO at 2400 MHz, still assuming a Q=10 (there is also a spectral component at 3200 MHz of -26dBc due to the 'symmetrization' effect of the divider 202). In the second embodiment, the unwanted spectrum is filtered by one of an antenna filter or a filter located in the transmit chain (both not shown). In a third embodiment, the regenerative properties of a divide-by-2 413 are used to generate a 1600 MHz square wave input to the third harmonic filter 211 of the second embodiment, as illustrated in FIG. 4. In the third embodiment, a pulling-free LO is generated by a VCO 201 that is tuned to 3200 MHz, which allows for a higher Q and a smaller area of the LC-tank, and the 3200 MHz signal is input to an IQ-generation circuit 400 in which the 3200 MHz signal is first divided and a resulting 1600 MHz square wave is input to a circuit substantially identical to the second embodiment. The output spectra of the I 407 and Q 408 signals are comparable to I 307 and Q 308 signals of the second embodiment, containing residual spectral components at 1600 MHz and 3200 MHz at least 26 dB down with respect to the desired 2400 MHz signal. The 1600 and 3200 MHz spectra are suppressed further by an antenna filter (not shown). In alternative embodiments, further filtering is applied before the divider 202 to each of the first through third embodiments to suppress the 1600 MHz fundamental signal. This further filtering can be accomplished by any of: AC-coupling with a corner frequency of, say, 4 GHz between the divider output and the filter input, adding a 2nd or even a 3rd stage of filtering, and applying a 1600 MHz notch filter. Filtering is done prior to the final divide-by-2, since division of the signal creates symmetrical sidebands. The third embodiment has an advantage that the signal component that pulls the VCO is only present after the last divide-by-2. (Note that this 3200 MHz signal is 100% correlated to the VCO signal itself, so only 'DC pulling', i.e., a phase shift, can occur). The apparatus and method of the pulling-free local oscillator of the present invention can be used for wireless personal area networks (WPANs) and local area networks (WLANs) in which an RF transmitter comprises a mixer configured to modulate the output signal of the LO with a data signal. Networks to which the present invention applies may further include GSM, Bluetooth, and DECT devices. FIG. 5 illustrates a representative wireless network whereto embodiments of the present invention are to be applied. According to the principle of the present invention, there is provided a mixer configured to modulate the output signal of an LO with a data signal such that the pulling of the LO is avoided. It should be noted that the network illustrated in FIG. 5 is small for purposes of illustration only. In practice, most WLANs would include a much larger number of mobile transceivers incorporating the present invention. The low-power circuit and architecture of the LO generator of the present invention has application to a WPANs and WLANs and enables wireless devices thereof to reduce both cost and power consumption. To this end, the present invention introduces a filtering technique to select a desired harmonic of a VCO signal with a required in-phase and quadrature LO signal being derived by means of dividers. Any harmonic relation between the transmit signal and the VCO is avoided as well as generation of intermediate signals close to the VCO frequency. Referring now to FIG. 6, each device within the WPAN/WLAN shown in FIG. 5 may include a transceiver with an architecture that is illustrated in the block diagram of FIG. 6. Each device may include a controller 602 coupled to at least a transmitter 601, an LO generator 603 (200 300 400) according to the present invention, and a receiver 604. The transmitter 601 and the receiver 604 are coupled to an antenna 605. All may be integrated into a single chip along with other components such as the antenna. The controller 602 can provide adaptive programming such that, for example, the transceiver is adapted to different modulation schemes and data rates specific to various communication protocols including IEEE 802.11, Bluetooth, and any other protocol known in the art. The controller can program the LO generator to select a specific harmonic, it not being limited to the third harmonic. While the preferred embodiments of the present invention have been illustrated and described, it will be understood by those skilled in the art that the local oscillator as described herein is illustrative and various changes and modifications may be made to the local oscillator and equivalents may be substituted for elements thereof without departing from the true scope of the present invention. In addition, many modifications may be made to adapt the teachings of the present invention to a particular situation without departing from its central scope, e.g., the VCO may be operated at different output frequencies. Therefore, it is intended that the present invention not be limited to the particular embodiments disclosed as the best mode contemplated for carrying out the present invention, but that the present invention include all embodiments falling with the scope of the appended claims.

Claims

CLAIMS:

1. An oscillator circuit for generating an in-phase and a quadrature component of a transmit signal having a desired frequency, said circuit comprising: an oscillator to output a first sinusoidal signal having a first frequency; and an IQ-generation circuit comprising a final divider, said IQ-generation circuit to receive the first sinusoidal signal and derive therefrom an in-phase signal component and a quadrature-phase signal component of the transmit signal, each said component having the desired frequency such that there is no harmonic relation between the desired frequency of the transmit signal and the first frequency, wherein pulling of the oscillator is avoided.

2. The oscillator circuit of claim 1, wherein the IQ-generation circuit comprises: means for converting the first sinusoidal signal to a first square wave signal; a frequency divider as the final divider to receive and divide the first square wave to generate a first in-phase signal component and a first quadrature-phase signal component thereof each said first component being phase-shifted relative to the other, and each said first component including odd-order harmonics of the divided first sinusoidal signal; and first and second filtering means to respectively receive, amplify, and filter the first in-phase signal component and first quadrature-phase signal component from the frequency divider to respectively obtain a predetermined harmonic thereof as the in-phase signal component and the quadrature-phase signal component, respectively, of the transmit signal.

3. The oscillator circuit of claim 2, wherein the means for converting comprises an amplifier.

4. The oscillator circuit of claim 3, wherein the frequency divider is a divide- by-2 divider and the predetermined harmonic is the third harmonic.

5. The oscillator circuit of claim 4, wherein the in-phase signal component and quadrature-phase signal component of the transmit signal have a perfect quadrature and equal amplitude.

6. The oscillator circuit of claim 5, wherein each of the first and second filtering means comprises an LC tank and a poly-phase (LC) filter.

7. The oscillator circuit of claim 6, wherein said first frequency is 1600 MHz, said desired frequency is 2400 MHz, and said LC tank has a Q factor equal to 10.

8. The oscillator circuit of claim 7, wherein said IQ-generation circuit further comprises a fundamental filtering means to suppress a 1600 MHz fundamental signal, said fundamental filtering means being operatively coupled to said IQ-generation circuit prior to the final divider to receive an input signal to the final divider and provide instead a suppressed signal as the input signal to the final divider.

9. The oscillator circuit of claim 8, wherein the fundamental filtering means is selected from the group consisting of a 1600 MHz notch filter, a third filtering means, and a third and a fourth filtering means.

10. The oscillator circuit of claim 1, wherein the IQ-generation circuit comprises: means for converting the first sinusoidal signal to a first square wave; filtering means to receive, amplify and filter the first square wave to obtain a predetermined harmonic thereof as a second sinusoidal signal having a second frequency; and a frequency divider as the final divider to receive and divide the second sinusoidal signal and generate therefrom the in-phase signal component and the quadrature-phase signal component, respectively, of the transmit signal.

11. The oscillator circuit of claim 10, wherein the means for converting comprises an amplifier.

12. The oscillator circuit of claim 11, wherein the frequency divider is a divide- by-2 and the predetermined harmonic is the third harmonic.

13. The oscillator circuit of claim 12, wherein: the first frequency is 1600 MHz; the second frequency is 4800 MHz; the desired frequency is 2400 MHz; and the first filtering means further comprises an LC tank having a Q factor equal to 10, said LC tank being tuned to 4800 MHz.

14. The oscillator circuit of claim 13, wherein said IQ-generation circuit further comprises a fundamental filtering means to suppress a 1600 MHz fundamental signal, said fundamental filtering means being operatively coupled to said IQ-generation circuit prior to the final divider to receive an input signal to the final divider and provide instead a suppressed signal as the input signal to the final divider.

15. The oscillator circuit of claim 14, wherein the fundamental filtering means is selected from the group consisting of a 1600 MHz notch filter, a second filtering means, and a second and a third filtering means.

16. The oscillator circuit of claim 1, wherein the IQ-generation circuit comprises: a first frequency divider to receive, divide and convert to a first square wave the first sinusoidal signal; filtering means to receive, amplify and filter the first square wave to obtain a predetermined harmonic thereof as a second sinusoidal wave having a second frequency; and a second frequency divider as the final divider to receive and divide the second sinusoidal signal to generate therefrom the in-phase signal component and the quadrature- phase component, respectively, of the transmit signal.

17. The oscillator circuit of claim 16, wherein the first and second frequency divider is a divide-by-2 and the predetermined harmonic is the third harmonic.

18. ' The oscillator circuit of claim 17, wherein: the first frequency is 3200 MHz; the first square wave has a frequency of 1600 MHz; the second frequency is 4800 MHz; the desired frequency is 2400 MHz; and the filtering means further comprises an LC tank having a Q factor greater than 10.

19. The oscillator circuit of claim 18, wherein said IQ-generation circuit further comprises a fundamental filtering means to suppress a 1600 MHz fundamental signal, said fundamental filtering means being operatively coupled to said IQ-generation circuit prior to the final divider to receive an input signal to the final divider and provide instead a signal with a suppressed 1600 MHz component as the input signal to the final divider.

20. The oscillator circuit of claim 19, wherein the fundamental filtering means is selected from the group consisting of a 1600 MHz notch filter, a second filtering means, a second and a third filtering means, and an AC-coupling with a corner frequency of 4 GHz being further operatively coupled to said IQ-generation circuit prior to the first filtering means.

21. A method for generating an in-phase signal component I and a quadrature- phase signal component Q of a transmit signal each component having a same desired frequency, comprising the steps of: outputting by an oscillator a first sinusoidal signal having a first frequency; and deriving from the first sinusoidal signal an in-phase signal component I and a quadrature-phase signal component Q of the transmit signal, each said component being phase shifted relative to the other, each said component having the desired frequency such that there is no harmonic relation between the desired frequency of the transmit signal and the first frequency, thereby avoiding pulling of the oscillator.

22. The method of claim 21, wherein the deriving step further comprises the steps of: converting the first sinusoidal signal to a first square wave; dividing the first square wave by 2 to obtain a first in-phase square wave component and a first quadrature-phase square wave component such that each said first component has a phase shift relative to the other and includes odd-order harmonics of the divided first frequency; amplifying (clipping) each said first component to obtain a second in-phase signal and a second quadrature-phase signal ; and filtering the second in-phase signal and the second quadrature-phase signal to obtain a predetermined harmonic thereof as the I and Q signal, respectively.

23. The method of claim 22, wherein the step of converting comprises amplifying (clipping) the first sinusoidal signal.

24. The method of claim 23, wherein the predetermined harmonic is the third harmonic.

' 25. The method of claim 24, wherein the I and Q signals have a perfect quadrature and equal amplitude.

26. The method of claim 25, wherein the desired frequency is 2400 MHz and the first frequency is 1600 MHz.

27. The method of claim 21, wherein the deriving step further comprises the steps of: converting the first sinusoidal signal to a first square wave; amplifying (clipping) the first square wave to a first frequency; filtering the amplified square wave to obtain a predetermined harmonic thereof as a second sinusoidal wave having a second frequency; and dividing by 2 the second sinusoidal wave to obtain the I and Q having the desired frequency.

28. The method of claim 27, wherein the step of converting comprises amplifying (clipping) the first sinusoidal signal.

29. The method of claim 28, wherein the predetermined harmonic is the second harmonic.

30. The method of claim 29, wherein the first frequency is 1600 MHz, the third frequency is 4800 MHz, the desired frequency is 2400 MHz.

31. The method of claim 30, further comprising the step of: prior to the dividing step performing the step of filtering the second sinusoidal signal to suppress a 1600 MHz fundamental signal contained therein.

32. The method of claim 21, wherein the deriving step further comprises the steps of: dividing by 2 the first sinusoidal signal to obtain a first square wave; filtering and amplifying the first square wave to obtain a predetermined harmonic thereof as a second sinusoidal signal having a second frequency; and dividing by 2 the second sinusoidal signal to obtain the I and Q having the desired frequency.

33. The method of claim 32, wherein the first frequency is 3200 MHz; the first square wave has a frequency of 1600 MHz; the second frequency is 4800 MHz; and the desired frequency is 2400 MHz.

34. The method of claim 33, further comprising the step of: prior to the step of dividing by 2 the second sinusoidal wave, performing the step of filtering one of the second square wave and the second sinusoidal signal to suppress a 1600 MHz fundament signal contained therein.

35. A transceiver, comprising: a local oscillator generator having an oscillator to output a first signal having a first predetermined frequency, an IQ-generation circuit comprising a divider to receive the sinusoidal signal and derive therefrom an in-phase square wave and a quadrature square wave component of a transmit signal, each square wave component having a predetermined transmit frequency; and a controller to program the first predetermined frequency of the oscillator and a predetermined transmit frequency of the divider such that there is no harmonic relation between the predetermined transmit frequency and the first predetermined frequency, wherein pulling of the oscillator is avoided.

36. The transceiver of claim 35, wherein the oscillator comprises a voltage controlled oscillator.