WO1998013943A1

WO1998013943A1 - Method and apparatus for detecting and cancelling unwanted transmitter modulation

Info

Publication number: WO1998013943A1
Application number: PCT/US1997/016974
Authority: WO
Inventors: Mark A. Jones
Original assignee: Ericsson Inc.
Priority date: 1996-09-26
Filing date: 1997-09-24
Publication date: 1998-04-02
Also published as: AU4737297A

Abstract

Unwanted transmitter modulation which would otherwise have an undesirable impact at a radio receiving the transmitted signal is detected and cancelled. The radio transmitter includes a phase-locked loop that receives an input signal and a feedback signal. An amplifier stage amplifies an output from the phase-locked loop, and an antenna transmits the amplified signal. A detector detects the transmitted signal and generates the feedback signal. Inclusion of the amplifier stage in the phase-locked loop compensates for undesirable transmitter modulation introduced at the amplifier stage. One example application of the present invention is to vehicular-based radio systems, where the radio transmitter is susceptible to the noise generated by the vehicle electrical system which ultimately results in an audible whine at the radio receiver. The whine heard at the receiver is greatly reduced/eliminated.

Description

METHOD AND APPARATUS FOR DETECTING

AND CANCELLING UNWANTED

TRANSMITTER MODULATION

FIELD OF THE INVENTION

The present invention address the problem of unwanted radio transmitter modulations that adversely impact radio reception of transmitted information, and in particular, to a method and apparatus for detecting and greatly reducing/eliminating such unwanted transmitter modulation.

BACKGROUND AND SUMMARY OF THE PRESENT INVENTION

In real world radio transmission applications, a radio transmitter is susceptible to a number of external disturbances and influences which ultimately generate unwanted modulations in the transmitted signal, including unwanted phase modulations, frequency modulations, and/or amplitude modulations.

One example of this problem is found in vehicular-based radio systems like land mobile radio and cellular radio systems. In an automobile, the radio transmitter is susceptible to fluctuations in the automobile's electrical system. In particular, the automobile's alternating current (AC) alternator is usually unregulated in frequency and amplitude. As a result, the actual voltage supplied at the vehicle's battery terminals fluctuates. For example, changes in engine RPM cause periodic power supply fluctuations referred to as "ripple." These amplitude and/or frequency fluctuations can result in undesirable modulation of the transmitted signal, which when demodulated at the radio receiver, cause an audible "whine."

Fig. 1 is a function block diagram of a vehicular-based mobile radio transmitter which includes a conventional 12 volt alternator battery supply 12. Radio transmitter 10 includes synthesizer/modulator circuitry 14 for generating a modulated signal which is preamplified in exciter 16 and then amplified in power amplifier 18 before being transmitted via an antenna. As described above, the automobile power supply system generates unwanted transmitter frequency modulation in the transmitted signal resulting in signal whine at the radio receiver.

Prior art methods to minimize alternator whine employ a filter similar to filter 20 shown in Fig. 1 including a series-connected inductor 22 coupled through capacitor 24 to ground. The filter 20 removes some of the undesired components generated by the power supply 12. Some of the undesired modulation frequency components are not blocked by filter 20 causing fluctuations in power supply to the power amplifier 18 which result in the undesired frequency/phase modulations described above.

In addition to not being particularly effective at preventing undesired transmitter modulations, the filter's inductive coil 22 is both large and expensive. Inductor 22 must be large enough in order to handle the large currents generated by power supply 12. In addition, inductor 22 and capacitor 24 must both be of relatively large value (further adding to the filter's size) to effectively filter the lower frequency range in which alternator whine is generated. As result, a filter that achieves acceptable levels of whine reduction is bulky and expensive. It is an object of the present invention to detect and cancel unwanted transmitter modulations accurately and inexpensively.

It is a further object of the present invention to detect and cancel undesired transmitter modulations accurately and inexpensively and without requiring additional filter components.

A novel radio transmitter architecture is used to detect and cancel or substantially reduce undesired modulations or other disturbances, e.g., alternator whine, on the transmitted signal. The novel transmitter architecture detects and includes in the feedback signal of a phase-locked loop (PLL) of the radio transmitter a small amount of the transmitted signal being routed to the antenna.

More specifically, the radio transmitter in accordance with the present invention includes a phase-locked loop which receives an input signal and a feedback signal. An amplifier stage amplifies the output from the phase-locked loop, and an antenna transmits the amplified signal. A signal detector such as a directional coupler detects the transmitted signal and generates the feedback signal. Inclusion of the amplifier stage in the feedback portion of the phase-locked loop compensates for undesirable transmitter modulation introduced at the amplifier stage. In essence, the phase-locked loop modulates the desired modulation of the RF carrier with the information to be transmitted, and at the same time, also rejects undesired modulation. Additional components such as the bulky and expensive filter components described above are not needed. In one example embodiment, the radio transmitter includes a differencer such as a phase detector which receives an input signal from a frequency or phase modulator as well as the feedback signal to generate a difference or error signal. A loop filter/amplifier couples the output of the phase detector to a voltage-controlled oscillator (VCO) which drives the difference signal to zero. Rather than using the output of the VCO being the feedback signal, the present invention includes the amplifier stage (which may include an exciter and a power amplifier) in the feedback loop. As a negative feedback loop, the VCO is controlled to drive the phase difference or error which effectively includes the undesired modulation to zero thereby eliminating or at least substantially reducing the undesired modulation in the transmitted signal. As applied to the vehicular-based radio system described above, the undesirable modulation may be caused in part by fluctuations in the transmitter power supply.

In another example embodiment of the present invention, the radio transmitter includes a phase-locked loop frequency synthesizer which receives an input signal and a first feedback signal. An amplifier amplifies the output from the phase-locked loop which is then transmitted via an antenna. A first detector detects the amplified signal and generates the firs sti - O '^ feedback signal. Inclusion of the amplifier^*a^*txlthe phase-locked loop compensates for undesired transmitter phase or frequency variations. In addition, a second detector detects the amplitude of the transmitted signal. The difference between the detected amplitude and a predetermined power setting is determined to generate a second feedback signal for controlling the gain of the amplifier. The second feedback loop further compensates for undesirable transmitter amplitude modulations. The present invention also provides a method for operating a radio transmitter that includes a phase-locked loop and an amplifier stage which includes a power amplifier. A phase difference between an input signal and a feedback signal is used to drive the phase-locked loop. The transmitted signal is detected and used to generate the feedback signal. As described above, including the power amplifier in the phase-locked loop compensates for undesirable transmitter variations, and in particular, undesirable transmitter phase or frequency variations. The amplitude of the transmitted signal may also be detected and compared with a predetermined power setting to generate a second feedback signal for controlling the gain of the power amplifier. The second feedback loop further compensates for undesirable transmitter amplitude variations.

BRIEF DESCRIPTION OF THE DRAWINGS These objects and features of the present invention as well as specific example embodiments of the invention will now be described in conjunction with the following drawings in which like reference numerals refer to like elements:

Fig. 1 is a function block diagram of a conventional radio transmitter which employs a discrete filter to reduce power supply alternator whine;

Fig. 2 is a function block diagram of an example radio transmitter employing the present invention;

Fig. 3 is a function block diagram of another example embodiment of the present invention; and Fig. 4 is a function block diagram of still another example embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION In the following description, for purposes of explanation and not limitation, specific details are set forth, such as particular circuits, circuit components, techniques, etc. in order to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well known methods, devices, circuits, and components are omitted so as not to obscure the description of the present invention with unnecessary detail.

Throughout this description, reference is made to the example set forth above involving a vehicular-based radio transmitter. However, this example is intended only for purposes of illustration and is in no way limiting. The present invention may be applied to any radio transmitter application where a disturbance to the radio transmitter generates an undesirable variation/modulation in the transmitted output signal. For example, the present invention may be applied in a radio base station/repeater or omer fixed radio transmitter to eliminate undesirable modulations that might result from a 50/60 Hertz based power supply system providing power to the radio transmitter power amplifier. Another example application of the present invention might be to disturbances to the transmitter other than those generated by a power supply such as radio microphonic disturbances, e.g., a vibrating cooling fan, etc. Reference is now made to the block diagram shown in Fig. 2 in accordance with one embodiment of the present invention in the context of radio transmitter 50. Audio or data information which is ultimately to be transmitted is input to a frequency or phase modulator 52 to modulate an intermediate frequency (IF) local oscillator signal generated by frequency synthesizer 72. The modulated IF signal is then processed in phase detector 54 along with a loop feedback signal to generate a phase difference or phase error signal. A loop filter 56 which may be for example a high gain amplifier with a controlled frequency response, provides an amplified difference/error signal to a voltage-controlled oscillator (VCO) 58. VCO 58 changes the phase/frequency of its output signal to drive the detected phase error/difference to zero. The VCO output is amplified by an amplifier stage 59 which may include for example an exciter 60 connected to a power amplifier 62. The output of power amplifier 62 is transmitted by way of antenna 64. In this example, the power supply to the power amplifier 62 fluctuates or includes some other disturbance, e.g., alternator whine. Such changes in the voltage supply amplitude and frequency ultimately result in undesired frequency or phase modulations in the transmitted signal.

A directional coupler 66 detects the transmitted signal output by the power amplifier 62 and feeds that signal to an optional amplitude limiter 68 and downconverter mixer 70 to the phase detector 54 thereby completing die feedback loop/phase-locked loop. Mixer 70 downconverts the detected RF signal using a RF local oscillator signal generated by frequency synthesizer 72. This is done because phase detectors typically are designed for frequencies lower than RF. However, higher frequency phase detectors are available, and if used, would eliminate the need for downconverter mixer 70 in the feedback path. Limiter 68 although optional is nonetheless preferable because it effectively strips the amplitude from the detected transmitted signal leaving only phase information including phase disturbances in the feedback loop. 5

The frequency or phase modulator 52, although preferred, is not necessary in the transmitter architecture. In other words, the audio or data may be routed directly to the phase detector 54 thereby eliminating the frequency or phase modulator 52 assuming that the phase-locked loop

10 makes the necessary RF upconversion. The main elements of the phase- locked loop are phase detector 54, loop filter 56 (preferable but not absolutely necessary), voltage-controlled oscillator 58, one or more including power amplifier 62,

y- 6> ^"Bidweciional_^ coupler 66, the output of which is coupled back to the phase

15 detector 54. Moreover, while reference is made to a simple phase-locked loop as defined above, other advanced types of feedback loops may also be used. For example, a frequency synthesizer may incorporate a phase- locked loop along with other components like frequency dividers, frequency multipliers, etc.

20

In essence, the present invention exploits the operation of the negative feedback loop to eliminate (or at least substantially minimize) any disturbance within the loop. In conventional phase-locked loops, the amplifier stage, which in the example embodiment in Fig. 2 includes

25 exciter 60 and power amplifier 62, is outside the feedback loop. Therefore, the feedback loop is unable to compensate for disturbances like alternator fluctuations in the power supply to the power amplifier. However, by including external disturbances like power supply fluctuations to the power amplifier in the feedback loop, the loop forces the phase of the feedback signal to match the phase of the input signal, and in that way, eliminates the undesirable phase modulation in the transmitted signal caused by the external disturbance without requiring additional costly and/or bulky filters.

To be most effective, the feedback, phase-locked loop bandwidth needs to be greater than the bandwidth of the disturbance which would otherwise result in undesired transmitter modulation. In other words, the response time of the loop to eliminate or substantially minimize the error/difference detected by phase detector 54 must be faster than the speed with which the disturbance is changing. Therefore, the highest frequency component of the disturbance, (i.e., the part of the disturbance that is changing most rapidly), should be included within the bandwidth characteristic of the feedback phase-locked loop. Disturbance frequency components which exceed the loop bandwidth will end up as undesired modulations in the transmitted signal.

If the loop bandwidth is properly designed so that it is larger than the unwanted disturbance bandwidth, the feedback loop dynamically and immediately cancels or at least substantially cancels unwanted disturbances such as power supply ripple effects in the amplitude of the power amplifier output signal. Both phase and amplitude of the transmitted signal can be detected and controlled to cancel unwanted disturbances like alternator whine. In fact, the inventor of the present invention designed the embodiment of Fig. 2 and tested it to observe the achieved reduction in hum and noise at die antenna output. With simulated alternator noise present, the present invention as implemented in the embodiment of Fig. 2 dramatically reduced such hum and noise from approximately -33 dB, which resulted when the power amplifier was not included in the feedback, phase-locked loop, to -57 dB when the power amplifier was included in the feedback, phase-locked loop.

Another example embodiment of me present invention is now described in conjunction with the block diagram illustrated in Fig. 3 where like reference numerals refer to like elements from Figs. 1 and 2. Fig. 3 is a simplified diagram showing a synthesizer/phase-locked loop block 74 along with a second feedback loop. Similar to how me phase-locked loop (the first feedback loop) in Fig. 2 detects and cancels unwanted phase disturbance in the power amplifier output, the second feedback loop in Fig. 3 detects and cancels amplitude disturbances in me power amplifier output. Reducing amplitude fluctuations is advantageous when transmitting to a companion receiver which exhibits less than ideal immunity to undesired AM (amplitude modulation) components.

The transmitted signal detected by coupler 66 is routed both to limiter 68 as in the embodiment shown in Fig. 2 and also to an RF level detector (e.g., an envelope detector) 74 which detects the amplitude of the transmitted output. The detected amplitude is compared to a predetermined RF power setting in a differential amplifier 76 (for example) which generates a difference signal used to control the gain of exciter 60 to maintain the desired power level. The bandwidth of this second feedback loop should also be greater than the bandwidd of the disturbance. As in the embodiment shown in Fig. 2, the feedback of the phase-locked loop is the transmitted signal output from the power amplifier rather than the output of the synthesizer/phase-locked loop 74. Of course, the second feedback loop may also be connected to control the gain of the power amplifier.

Fig. 4 shows another example embodiment of the present invention. The block diagram of Fig. 4 is similar to that shown in Fig. 3 except that limiter 68 is preferably included in the first feedback loop. The limiter 68 is added to the first feedback loop in Fig. 4 for the reasons set forth above with regard to the preferred embodiment illustrated in Fig. 2. The embodiment shown in Fig. 4 may be advantageous when the phase detector is particularly sensitive to input power levels and thus requires constant input levels. In that situation, the limiter 68 evens out the input power thereby improving the performance of the phase detector. With a less sensitive phase detector, however, the limiter may not be necessary, and the embodiment in Fig. 3 may be more appropriate.

Both embodiments shown in Figs. 3 and 4 have me additional advantage of dynamically cancelling power supply ripple effects or other disturbances in d e amplitude of the power amplifier output. Thus, unwanted phase or frequency disturbances can be detected and compensated for by the first feedback loop and undesired amplitude modulations may be detected and compensated for in the second feedback loop.

While die invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiment, but contrary, is intended to cover various modifications and equivalent arrangements included wimin the spirit and scope of the appended claims.

Claims

What is claimed is:

1. A radio transmitter, comprising: a differencer receiving an input signal and a feedback signal and generating a difference signal; a voltage-controlled oscillator coupled to the differencer for driving the difference signal to zero; an amplifier for amplifying an output from the voltage controlled oscillator; an antenna coupled to the amplifier for transmitting the amplified signal; and a detector detecting the amplified signal and generating the feedback signal, wherein a loop including die differencer, the voltage-controlled oscillator, and amplifier compensates for undesirable transmitter modulation.

2. The radio transmitter in claim 1, further comprising: a power supply for supplying power to the amplifier, and wherein the undesirable modulation is caused at least in part by fluctuations in the power supply.

3. The radio transmitter in claim 1, wherein the undesirable modulation is caused at least in part by a disturbance affecting the amplifier.

4. The radio transmitter in claim 1, wherein the differencer is a phase detector and the input signal is a desired modulating signal, the phase detector detecting a phase difference between d e desired modulating signal and die feedback signal.

5. The radio transmitter in claim 1, wherein the detector is a directional coupler.

6. The radio transmitter in claim 1, further comprising: a loop filter coupled between d e differencer and the voltage- controlled oscillator.

7. The radio transmitter in claim 1, further comprising: a frequency or phase modulator modulating an intermediate frequency (IF) local oscillator wid an audio or data signal to generate the input signal, and a radio frequency (RF) downconverting mixer connected to d e feedback signal for downconverting the feedback signal to an intermediate frequency.

8. The radio transmitter in claim 1, further comprising: a limiter for eliminating amplitude modulations from e feedback signal.

9. The radio transmitter in claim 1, wherein the bandwidd of die loop is greater than the bandwiddi of die undesirable transmitter modulation.

10. The radio transmitter in claim 1, further comprising: an amplitude detector detecting the amplitude of die feedback signal, and means for determining the difference between the detected amplitude and an predetermined power to generate gain control signal for controlling a gain of me amplifier.

11. the radio transmitter in claim 10, wherein the means for determining is a differential amplifier.

12. The radio transmitter in claim 10, wherein e amplifier includes: an exciter coupled to die output from the voltage controlled oscillatpr, and a power amplifier coupled between the exciter and die antenna, wherein the gain control controls the exciter.

13. The radio transmitter in claim 10, wherein die bandwidth of d e amplitude detector is greater man the bandwiddi of die undesirable transmitter modulation.

14. The radio transmitter in claim 10, further comprising: a limiter for eliminating amplitude modulations from me feedback signal.

15. A radio transmitter, comprising: a phase-locked loop receiving an input signal and a feedback signal; an amplifier for amplifying an output from the phase-locked loop; an antenna coupled to die amplifier for transmitting the amplified signal; and a detector detecting die amplified signal and generating the feedback signal, wherein inclusion of die amplifier in die phase-locked loop compensates for undesirable transmitter modulation introduced at die amplifier.

16. The radio transmitter in claim 15, wherein the bandwiddi of a loop including d e phase-locked loop, me amplifier, and d e detector is greater than the bandwidth of die undesirable transmitter modulation.

17. The radio transmitter in claim 15, further comprising: a limiter for eliminating amplitude modulations from the feedback signal.

18. A radio transmitter, comprising: a phase-locked loop receiving an input signal and a first feedback signal; an amplifier for amplifying an output from the phase-locked loop; an antenna coupled to die amplifier for transmitting the amplified signal; a first detector detecting die amplified signal and generating die first feedback signal, wherein inclusion of die amplifier in d e phase-locked loop compensates for undesirable transmitter phase or frequency variations, a second detector for detecting d e amplitude of the first feedback signal; and a differencer determining the difference between d e detected amplitude and an predetermined power setting to generate a second feedback signal for controlling a gain of die amplifier, wherein the second feedback loop compensates for undesirable transmitter amplitude variations.

41 19. The radio transmitter in claim 18, wherein the amplifier includes: an exciter coupled to die output from me voltage controlled oscillator, and a power amplifier coupled between the exciter and the antenna, wherein die second feedback signal controls die exciter.

20. The radio transmitter in claim 18, wherein the bandwiddi of a loop including the phase-locked loop, die amplifier, and die first detector is greater man the bandwiddi of d e undesirable transmitter modulation.

21. The radio transmitter in claim 18, wherein die bandwiddi of a loop including die amplifier, the first detector, and die second detector is greater man the bandwiddi of die undesirable transmitter amplitude modulation.

22. The radio transmitter in claim 18, further comprising: a limiter for eliminating amplitude modulations from the feedback signal.

23. The radio transmitter in claim 18, wherein the undesirable variations are caused at least in part by fluctuations in ie power supply.

24. In a radio transmitter including a phase-locked loop and an amplifier stage including a power amplifier, a method for controlling desired modulation of a transmitted signal and rejecting undesired transmitter modulation caused at least in part by a disturbance externally introduced to die amplifier stage using the phase- locked loop.

25. The method in claim 24, wherein the bandwiddi of the phase- locked loop is greater dian die bandwiddi of d e undesired transmitter modulation.

26. In a radio transmitter including a phase-locked loop and an amplifier stage including a power amplifier, a mediod comprising e steps of: receiving an input signal at the phase-locked loop along with a feedback signal; amplifying an output from die phase-locked loop; transmitting die amplified signal; and detecting die amplified signal and generating the feedback signal, wherein inclusion of the amplifier in die phase-locked loop compensates for undesirable transmitter modulation introduced at least in part at the amplifier stage.

27. The method in claim 26, further comprising: eliminating amplitude modulations from the feedback signal.

28. The method in claim 26, wherein die undesirable modulation is caused at least in part by fluctuations in a power supply to the amplifier stage.

29. In a radio transmitter including a phase-locked loop and an amplifier stage including a power amplifier, a method comprising the steps of: receiving an input signal at die phase-locked loop along widi a first feedback signal; amplifying an output from the phase-locked loop; transmitting die amplified signal; and detecting die amplified signal and generating die first feedback signal, wherein inclusion of die amplifier in the phase-locked loop compensates for undesirable transmitter phase or frequency variations, detecting die amplitude of d e first feedback signal; and determining die difference between die detected amplitude and an predetermined power level and generating a second feedback signal for controlling a gain of the amplifier, wherein the second feedback loop compensates for undesirable transmitter amplitude variations.

30. The method in claim 29, wherein the undesirable variations are caused at least in part by fluctuations in a power supply to die amplifier stage.