WO2001082468A1

WO2001082468A1 - Reduced distortion digital fm demodulator

Info

Publication number: WO2001082468A1
Application number: PCT/US2001/011983
Authority: WO
Inventors: James Baker; Denise Riemer; Michael Carney
Original assignee: Motorola Inc.
Priority date: 2000-04-20
Filing date: 2001-04-12
Publication date: 2001-11-01
Also published as: AU2001253419A1

Abstract

A reduced distortion digital FM demodulator (324, 600) is designed to accommodate and demodulate frequency signals with a maximum frequency deviation. The demodulator includes a phase-to-frequency converter (402) that converts a digital representation of a phase signal into a frequency signal. The frequency signal has an observable maximum frequency deviation that is higher than the maximum frequency deviation the demodulator must accommodate. A dynamic range reducer (406) receives the frequency signal directly from the phase-to-frequency converter or after the frequency signal is filtered by a filter (404). The dynamic range reducer limits the dynamic range of the frequency signal to have the maximum frequency deviation.

Description

REDUCED DISTORTION DIGITAL FM DEMODULATOR

Field of the Invention The present invention relates generally to frequency demodulation, and in particular, to a digital FM demodulator with reduced distortion.

Background of the Invention

Frequency modulation (FM) is a common method of transmitting analog signals between communications devices over a communications channel. Frequency modulation is used in a myriad of communications applications, including for example wireless telephony communications. In particular, frequency modulation is used in cellular communication systems in the United States, Japan, and Europe.

In an FM communication system, an FM demodulator is required to convert a received FM signal into the informational component of the signal that is modulated thereon. Analog and digital FM demodulators are known. An example FM demodulator is disclosed in U.S. Patent No. 5,661,433, issued to LaRosa et al., and assigned to the same assignee as the present application. Digital FM demodulators provide certain advantages over analog demodulators including reduced size and power consumption.

A known digital FM demodulator 100 is shown in FIG. 1. The demodulator shown in FIG. 1 converts baseband in-phase (I) signal 104 and quadrature (Q) signal 106 to a received audio signal 136, which is further processed and made audible. The demodulator 100 is designed for a maximum frequency deviation. The maximum frequency deviation is the maximum amount the phase can change from one sample to another sample. Demodulator 100 is designed for a maximum frequency deviation of 16 Kilohertz (kHz), to accommodate a cellular telephony standard requiring a maximum deviation of 14 kHz, as specified in the Advanced Mobile Phone System EIA Interim Standard IS-19.

In demodulator 100, the I signal 104 is passed through a low pass IF filter 102. Similarly, Q signal 106 is passed through a low pass IF filter 107. The resulting filtered signals are then converted from an analog representation to a digital representation by analog-to-digital (A/D) converters 108, 110, respectively. An I/Q to phase converter 112 converts the digital I/Q signals into a digital phase signal. A phase-to-frequency converter 114 then converts the digital phase signal into a frequency representation. More specifically, phase-to-frequency converter 114 employs differentiation by subtraction using subtractor 118 and two delay elements 116, 117. The output of the phase-to-frequency converter is limited to the maximum frequency deviation, in this case 16 kHz. Then an integrator 120 is used to improve the signal-to-noise ratio of the frequency signals from phase-to-frequency converter 114. More specifically, integrator 120 uses a delay and add circuit with adder 124 and two delay elements 122, 123. The times-two decimate 126 receives the integrated signal from integrator 120 and produces a signal that is further integrated by integrator 130, having an adder 130 and two delay elements 132, 133, to produce received audio signal 136.

One problem with the digital demodulator shown in FIG. 1 is that high deviation signals, that is signals near the maximum frequency deviation, are distorted by filters 102, 107, and other components, including a radio frequency front end. This distortion then causes corresponding distortion in the demodulated audio signal. At least some of the distortion is comparable to a signal experiencing wraparound due to fixed-point overflow. FIG. 2 is a graph 200 illustrating the type of distortion produced at high deviation.

The distortion found in known digital demodulators is undesirable and therefore, a need exists for a digital FM demodulator with reduced distortion.

Brief Description of the Drawings FIG. 1 is block diagram of a prior art FM demodulator that produces distortion.

FIG. 2 is a graph illustrating distortion produced by the FM demodulator shown in FIG. 1.

FIG. 3 is a block diagram of a communication system including a communications device with a digital FM demodulator in accordance with a preferred embodiment. FIG. 4 is a block diagram showing further details of the FM digital demodulator shown in FIG. 3.

FIG. 5 is a block diagram showing in further detail certain components of the demodulator shown in FIG. 3 and FIG. 4. FIG. 6 is a block diagram of a digital FM demodulator in accordance with an alternate preferred embodiment.

FIG. 7 is a block diagram of a correction circuit for the demodulator of FIG. 6.

Detailed Description of the Preferred Embodiments In summary, a digital FM demodulator in accordance with the present invention reduces distortion. The digital demodulator is designed for a nominal maximum frequency deviation, which relates to a maximum frequency deviation of signals to be received. In one aspect, a digital phase signal is converted to a frequency signal with an observable maximum frequency deviation greater than the nominal maximum frequency deviation. In a preferred embodiment, a reduced delay differentiator is used to increase the observable maximum frequency deviation beyond the nominal maximum frequency deviation limit. Then the dynamic range of the frequency signal is limited to the nominal maximum frequency deviation. Preferably, the dynamic range is limited by clipping the frequency signal at the nominal frequency deviation. This eliminates any high deviation distortion above the nominal deviation and reduces the required dynamic range of subsequent signal processing circuits. Optionally, a stage of integration is employed to the frequency signal prior to reducing the dynamic range to increase the signal-to-noise ratio.

In another aspect, where the digital demodulator is designed for a nominal maximum frequency deviation, the conversion of a digital phase signal to a frequency signal produces a plurality of digital samples of the frequency signal with the nominal maximum frequency deviation. A correction circuit receives the plurality of digital samples of the frequency signal and filters the samples to produce a plurality of filtered samples of the frequency signal. More specifically, the correction circuit determines a difference between successive samples of the frequency signal by comparing a current sample with a delayed sample. If the difference does not exceed a predetermined threshold, then the delayed sample is made a filtered sample. More specifically, the delayed sample becomes a filtered sample in the plurality of filtered samples and the current sample is retained by the correction circuit to be the delayed sample in the next comparison. On the other hand, if the difference exceeds a predetermined threshold, indicating a large difference between successive samples and possibly distortion, then the current sample is discarded. That is, the current sample is not ever made a filtered sample and the delayed sample is reused in place of the current sample. More specifically, the delayed sample is retained by the correction circuit for use as the delayed sample in the next comparison. And, the delayed sample is repeated in the plurality of filtered samples in a position corresponding to the current sample. Optionally, the plurality of filtered samples are integrated, processed by a times-two decimater, and further integrated to produce a received audio signal.

FIG. 3 is a block diagram of a communication system 300 with an FM demodulator in accordance with a preferred embodiment. Communication system 300 includes a transceiver 301 and a communication device 307. Communication device 307 is, for example, a radiotelephone. Transceiver 301 is, for example, a radio base station. Transceiver 301 and communication device 307 communicate over the air via antenna 314 and antenna 312, respectively. Communication device 307 is alternatively a two-way radio, cellular telephone, radio frequency receiver or any other receiver device.

Communication device 307 preferably includes a transmitter 308 and a receiver 310. A duplexer 311 interfaces transmitter 308 and receiver 310 with antenna 312 for over-the-air communications. Transmitter 308 is coupled to a microphone 313. Transmitter 308 modulates voice received by microphone 313 into a modulated signal. Receiver 310 is coupled to a speaker 320, which renders a demodulated audio signal from an over-the-air communication into an audible signal. A controller 316 is provided on communications device 307 for controlling transmitter 308 and receiver 310.

Receiver 310 includes a RF front end 322, a demodulator 324, and an audio processor 326. RF front end 322 receives a modulated input RF signal from a complementary communication device, such as transceiver 301, via antenna 312 and duplexer 311. RF front 322 produces a baseband I signal and 304 a baseband Q signal 306 from the modulated RF input. Demodulator 324 converts the analog baseband I and Q signals into a digital audio signal. More specifically, demodulator 324 separates the information modulated onto a carrier signal so that the information may be processed. Audio processor 326 processes the received audio signal into an audible representation that is forwarded to speaker 320, which makes the signal audible to a user. Demodulator 324 reduces distortion in accordance with the preferred embodiment.

FIG. 4 is a block diagram of demodulator 324. Demodulator 324 is similar to prior art demodulator 100 described above with respect to FIG. 1, as noted by the corresponding components and reference numbers. However, demodulator 324 does not include phase-to-frequency converter 114, which produces a distortion corresponding to a distortion introduced by IF filter 102, IF filter 107, and RF front end 322. Rather, demodulator 324 includes a new phase-to-frequency converter 402, a filter 404, and dynamic range reducer 406, all being intermediate or between the I/Q-to-phase converter 112 and integrator 120.

Phase-to-frequency converter 402 increases the observable maximum frequency deviation over the phase-to-frequency converter 114 shown in FIG. 1. Preferably, phase-to-frequency converter 402 increases the observable maximum frequency deviation by a factor of 2^N over the nominal maximum frequency deviation, where N is a non-zero integer. Phase-to-frequency converter 402 receives a digital phase signal comprised of digital phase samples from I/Q-to-phase converter 112. Phase-to-frequency converter 402 produces a digital frequency signal comprised of digital frequency samples. The digital frequency signal has the increased observable maximum deviation. For example, in a preferred embodiment where the nominal maximum deviation for the demodulator is 16 kHz (for a 14 kHz requirement), phase-to frequency converter 402 produces a digital frequency signal comprising 8-bit samples at an observable maximum frequency deviation of 32 kHz full scale.

Filter 404 is optional, but preferred. Filter 404 increases or restores the signal- to-noise ratio that is effectively reduced by phase-to-frequency converter 402. In the preferred embodiment, filter 404 receives 8-bit samples at an observable maximum frequency deviation of 32 kHz and a signal-to-noise ratio of 7 bits, and produces 9 bit samples at an observable maximum frequency deviation of 32 kHz and a signal-to- noise ratio of 8 bits. Filter 404 produces a filtered frequency signal comprised of filtered frequency samples.

Dynamic range reducer 406 reduces the dynamic range of the signal received. Where filter 404 is employed, dynamic range reducer 406 limits the dynamic range of the filtered frequency signal to the nominal maximum frequency deviation. On the other hand, where filter 404 is not employed, dynamic range reducer 406 limits the dynamic range of the digital frequency signal received directly from phase-to- frequency converter 402. Preferably, dynamic range reducer 406 limits the observable maximum deviation to the nominal maximum frequency deviation to produce a limited frequency signal. In the preferred embodiment that employs filter 404, the filtered frequency samples with an observable maximum frequency deviation of 32 kHz produced by filter 404 are limited to a peak deviation of 16 kHz. Dynamic range reducer 406 eliminates high deviation signals and produces a digital frequency signal comprising 8-bit samples at an observable maximum deviation of 16 kHz full scale.

FIG. 5 shows in further detail phase-to-frequency converter 402, filter 404, and dynamic range reducer 406. In the preferred embodiment, phase-to-frequency converter 402 performs an approximation to differentiation using a delay and subtract circuit. More specifically, a delay element 500, in conjunction with subtractor 502, is used to subtract a current digital sample from a delayed digital sample to approximate the differentiation of the phase, which produces a frequency representation.

Filter 404 performs an approximation to integration using a delay and add circuit. More specifically, an adder 506 is used to add a current sample and a delayed sample produced by delay element 504. Filter 404 restores or improves the signal-to-noise ratio that is reduced by the delay and subtract circuit of phase-to- frequency converter 402. The delay elements 500 and 504 are preferably D-type flip- flops. Dynamic range reducer 406 is preferably implemented as a multiplication by

2 with clipping. The multiplication is most preferably performed as a shift. It is also possible to multiply by a factor other than two by using a digital multiplier circuit or other methods. For example, a multiply by 1.5 is accomplished by adding the original signal to a signal scaled by one half (shifted right). Methods of clipping are well known in the art and generally involve monitoring the sign bit of the original signal and overflow detection, which occurs when the resultant sign bit changes. FIG. 6 is a block diagram of a digital FM demodulator 600 in accordance with an alternate preferred embodiment. Demodulator 600 includes the components of demodulator 100, described above with respect to FIG. 1. In addition, demodulator 600 includes a correction circuit 602 that reduces distortion. Correction circuit 602 is located between phase-to-frequency converter 114 and integrator 120. Correction circuit 602 detects large differences between successive frequency samples, which large differences may represent undesired distortion. The input signal to correction circuit 602 is filtered to eliminate large differences in successive samples, as discussed further below.

FIG. 7 is a block diagram of a preferred embodiment of the correction circuit 602 for the demodulator of FIG. 6. Correction circuit 602 includes a subtractor 702, a comparator 704, a multiplexer 706, and a delay element 708. Correction circuit 602 receives digital samples of a frequency signal 710 and produces filtered samples of the frequency signal 712. As a current digital sample of the frequency signal is received, the signal is compared with the output of delay element 708. In particular, subtractor 702 determines a difference between the current digital sample and the output of delay element 708. Nominally, the output of delay element 708 is the prior digital sample of the frequency signal. More precisely, the output of delay element 708 is a filtered sample of frequency signal 712, which is a previous sample of the frequency signal, but may not be the immediate prior sample of the frequency signal. The difference between the current digital sample and the output of delay element 708 is compared to a predetermined threshold, which may be hardwired or programmable. In particular, comparator 704 compares the difference from subtractor 702 with the predetermined threshold. In a preferred embodiment, comparator 704 determines the absolute value of the difference and determines whether the absolute value of the difference is greater than the predetermined threshold. The output of the comparator determines whether the delay element 708 stores a current digital sample or a previously stored output of delay element 708. More specifically, the output of comparator 704 serves as the select input to a multiplexer 706 that has as inputs, the current digital sample and the output of delay element 708. The output of multiplexer 706 is the input to delay element 708.

A reduced distortion FM demodulator, as described above, improves performance of a receiver by eliminating undesirable distortion. In particular, the reduced distortion demodulator reduces distortion near peak frequency deviation.

Whereas the present invention has been described with respect to specific embodiments thereof, it will be understood that various changes and modifications will be suggested to one skilled in the art and it is intended that the invention encompass such changes and modifications as fall within the scope of the appended claims.

We claim:

Claims

1. A communications device comprising: a receiver that includes a digital frequency modulation (FM) demodulator, the demodulator being designed for a nominal maximum frequency deviation; the demodulator comprising: a phase-to-frequency converter for converting a digital phase signal to a frequency signal, the frequency signal having an observable maximum frequency deviation that is higher than the nominal maximum frequency deviation; and a dynamic range reducer coupled to the phase-to-frequency converter to receive the frequency signal and limit the frequency signal to the nominal maximum frequency deviation to produce a limited frequency signal.

2. The device of claim 1 wherein the phase-to-frequency converter approximates differentiation of the digital phase signal.

3. The device of claim 2 wherein the phase-to-frequency converter comprises: a delay element; and a subtraction circuit coupled to receive an input to the delay element and an output of the delay element and produce a difference between the input to the delay element and the output of the delay element.

4. The device of claim 1 wherein the dynamic range reducer multiplies the frequency signal by two to produce a product that is clipped.

5. The device of claim 4 wherein the dynamic range reducer multiplies the frequency signal by two using a shift operation.

6. A digital FM demodulator comprising: a phase-to-frequency converter that converts a digital phase signal into a digital frequency signal, the digital frequency signal being a plurality of digital frequency samples; a circuit coupled to the phase-to-frequency converter to receive the digital frequency signal and produce a filtered frequency signal, the filtered frequency signal being a plurality of filtered frequency samples; wherein the circuit compares a current digital frequency sample with a filtered frequency sample to produce a difference between the current digital frequency sample and the filtered frequency sample; wherein if the difference is greater than a predetermined threshold, a next filtered frequency sample is the filtered frequency sample; and wherein if the difference is not greater than the predetermined threshold the next filtered frequency sample is the current digital frequency sample.

7. The demodulator of claim 6 wherein the phase-to-frequency converter approximates differentiation of the digital phase signal.

8. The demodulator of claim 7 wherein the phase-to-frequency converter comprises: a first delay element; a second delay element having an input coupled to an output of the first delay element; and a subtraction circuit coupled to receive an input of the first delay element and an output from the second delay element and produce a difference between the input of the first delay element and the output from the second delay element.

9. The demodulator of claim 6 wherein the circuit further comprises: a subtraction circuit that produces the difference between the current digital frequency sample and the filtered frequency sample; a comparator that determines whether the difference between the current digital frequency sample and the filtered frequency sample exceeds the predetermined threshold; and a delay element that stores the filtered frequency sample as the next filtered frequency sample if the difference exceeds the predetermined threshold and stores the current digital frequency sample as the next filtered frequency sample if the difference does not exceed the predetermined threshold.

10. A communications device comprising: a receiver that includes a demodulator: the demodulator comprising: a phase-to-frequency converter that converts a digital phase signal into a digital frequency signal, the digital frequency signal being a plurality of digital frequency samples; a circuit coupled to the phase-to-frequency converter to receive the digital frequency signal and produce a filtered frequency signal, the filtered frequency signal being a plurality of filtered frequency samples; wherein the circuit compares a current digital frequency sample with a filtered frequency sample to produce a difference between the current digital frequency sample and the filtered frequency sample; wherein if the difference is greater that a predetermined threshold, a next filtered frequency sample is the filtered frequency sample; and wherein if the difference is not greater than the predetermined threshold, the next filtered frequency sample is the current digital frequency sample.

11. The device of claim 10 wherein the phase-to-frequency converter approximates differentiation of the digital phase signal.

12. The device of claim 11 wherein the phase-to-frequency converter comprises: a first delay element; a second delay element having an input coupled to an output of the first delay element; and a subtraction circuit coupled to receive an input of the first delay element and an output from the second delay element and produce a difference between the input of the first delay element and the output from the second delay element.

13. The demodulator of claim 10 comprising: a subtraction circuit that produces the difference between the current digital frequency sample and the filtered frequency sample; a comparator that determines whether the difference between the current digital frequency sample and the filtered frequency sample exceeds the predetermined threshold; and a delay element that stores the filtered frequency sample as the next filtered frequency sample if the difference exceeds the predetermined threshold and stores the current digital frequency sample as the next filtered frequency sample if the difference does not exceed the predetermined threshold.

14. A method for demodulating an FM signal with a demodulator designed for a nominal maximum frequency deviation, the method comprising the steps of: A) converting an analog baseband signal to a digital phase signal; B) converting the digital phase signal into a digital frequency signal, the digital frequency signal having an observable maximum frequency deviation that is higher than the nominal maximum frequency deviation; and C) reducing a dynamic range of the digital frequency signal to the nominal maximum frequency deviation.

15. The method of claim 14 wherein the observable maximum frequency deviation is 2^N times the nominal maximum frequency deviation and N is a non-zero integer.

16. A frequency demodulator that demodulates frequency modulated signals, the demodulator being designed for a nominal maximum frequency deviation, the demodulator comprising: a phase-to-frequency converter for converting a digital phase signal to a frequency signal, the frequency signal having an observable maximum frequency deviation that is higher than the nominal maximum frequency deviation; a filter that receives the frequency signal and produces a filtered frequency signal, the filtered frequency signal being an approximation of an integration of the frequency signal; and a dynamic range reducer coupled to the filter to receive the filtered frequency signal and limit the filtered frequency signal to the nominal maximum frequency deviation to produce a limited frequency signal.

17. The demodulator of claim 16 wherein the filter comprises: a delay element; and an adder coupled to receive an input to the delay element and an output from the delay element and produce a sum of the input to the delay element and the output from the delay element.

18. A method for demodulating an FM signal comprising the steps of: converting an analog baseband signal to a digital phase signal; converting the digital phase signal into a plurality of digital frequency samples; filtering the plurality of digital frequency samples to produce a plurality of filtered frequency samples; comparing a current digital frequency sample with a filtered frequency sample to produce a difference between the current digital frequency sample and the filtered frequency sample; if the difference is greater than a predetermined threshold, setting a next filtered frequency sample as the filtered frequency sample; and if the difference is not greater than the predetermined threshold, setting the next filtered frequency sample as the current digital frequency sample.