GB2066008A - Angular modulators - Google Patents

Abstract

A narrow-band, theoretically distortionless, carrier modulated in accordance with individual values occurring with a periodic interval T is generated by using a pulse-position modulator (2) to produce short pulses whose positions are related to successive values, and feeding the latter pulses to a bandpass filter (4) of bandwidth F centred on a frequency f, where F=1/T and f>>F. The F. The modulated carrier, represented by the frequency f and having a bandwidth F, is generated by the impulse-response of the filter to the short pulses fed thereto. The values may be amplitude samples, sampled at periodic intervals T, of an analogue signal of bandwidth F/2 or less, the PPM pulse positions corresponding to the sample amplitudes. With digital data, the input sampling is not required. At the receiver, after demodulation, the signal is sampled at corresponding points in the original modulation signal of the transmitter and then filtered to reproduce the original modulation signal. <IMAGE>

Description

SPECIFICATION Improvements in or relating to signal modulation methods and apparatus therefor This invention relates to signal modulation methods and apparatus therefor.

The advantages of frequency modulation (FM) over amplitude modulation (AM) in radio transmission are substantial and well known, providing a higher degree of immunity from interference and avoiding the need for linear amplifiers. The main disadvantage however, is that the bandwidth occupied by an FM signal is usually much greater than that required for the corresponding AM signal, and it is sometimes necessaary, for example when operating in a crowded radio spectrum, to reduce the bandwidth of an FM signal at the expense of a reduction in the signal-to-noise ratio.

One compromise which has been adopted is to use so-called small-deviation FM. If the frequency deviation of the FM signal is sufficiently small, the side-bands of the signal tend to those of a doubleside-band AM signal with a phase shift, and so the small-deviation FM signal can be contained within the same bandwidth as an AM signal, ie twice that of the modulating signal.

However, in order to reduce the power in the outer side-bands to an acceptable level, it is necessary to make the deviation very small. This results in a poor signal-to-noise ratio, which is indicated by the fact that the bulk of the transmitted power is in the carrier. For example, in an HF radio channel using small-deviation FM to transmit 3kHz speech in a 6kHz band with a maximum deviation of 150 Hz, the maximum sideband power would be I 6dB down relative to that of the carrier.

A further possible method of reducing the bandwidth of an FM signal is to use widedeviation FM and to then filter out the higher sidebands. This technique is generally unsuitable because it introduces unacceptable distortion.

It is an object of the present invention to provide a signal modulation technique which largely retains the advantages of conventional FM techniques, and which enables the abovementioned disadvantages to be overcome or at least substantially reduced.

According to the present invention a method of generating a signal modulated in accordance with individual values occurring with a periodic interval T, comprises feeding short pulses related to successive values to band-pass filter means having a bandwidth of F cycles, where T=1/f, said band including a frequency f where f F, whereby to derive from the output of said filter means a resulting modulated signal of frequency f and bandwidth F.

The modulated signal of frequency f may be frequency-shifted to a higher frequency.

The phase difference of the modulated signal f between successive intervals T is preferably made not greater than 7t/2.

The short pulses fed to the filter means may be generated as pulse-position modulated (PPM) pulses of unmodulated repetition period T whereof the position of each is related to successive values and wherein said PPM pulses are fed to a bandpass filter of bandwidth F including the frequency f and preferably substantially centred thereon whereof the output is said modulated signal.

The values may be derived by sampling at periodic intervals T the amplitude of an analogue signal having a bandwith less than or equal to F/2, in which case the short pulses fed to the filter means may be generated as PPM pulses of constant amplitude and unmodulated repetition period T whereof the position of each corresponds to the amplitudes of successive samples. In order to regenerate samples corresponding to the initial samples of the analogue signal, the modulated signal may be demodulated and thereafter sampled at periodic intervals T; the resulting samples may be passed through a low-pass filter having a cut-off frequency F/2 to regenerate the analogue signal in a known manner. However regeneration of the samples is unnecessary if the aforesaid phase difference is not greater than 'r/2.

The present method is to be contrasted with normal methods of phase or frequency modulation, in which the modulation is applied to a separately generated carrier. In the present method there is no separately generated carrier; as can be shown, the present modulated carrier represented by the frequency f is generated as the superimposed impulse-responses of the bandpass filter of bandwidth F to the successive short pulses fed thereto, by virtue of the response of such a filter to an impulse, in the time domain, being ideally zero at intervals of T(=i/F) except at the peak, which response will be familiar to those skilled in the filter art.

As applied to analogue signals, the present modulating method is based on a so-called analytic form of the sampling theorem. Any signal can be expressed in the analytic form acos#, where a and , respectively amplitude and phase, are functions of time. The expression is unique because the Hilbert transform of the signal is a sin) giving two equations and two unknowns. The sampling theorem states that if a signal is known to be contained in a certain frequency range of bandwidth F=1/T, and the values of a and 0 are known at intervals of T, then the signal is uniquely defined. This form of the sampling theorem is more useful at frequency bands displaced from zero, while the normal form is generally only applicable to base-band signals.

Consider this now in relation to the present modulation method, in the case of a base-band analogue modulating signal S of bandwidth F/2 with which it is required to modulate a carrier so that the resulting modulated signal is contained within a bandwidth F. If the modulating signal S is sampled in accordance with the normal form of the sampling theorem at intervals of T, the resulting samples uniquely define the signal S as its maximum frequency is less than or equal to F/2.If the set of samples is then multiplied by a suitable constant to produce a set of phases each proportional to the amplitude value of a respective one of the samples, and these phases are effectively added to the phase of the carrier at intervals of T, the resulting phase values can be used as values of 0 in the analytic form of the sampling theorem to define the modulating signal.

By setting a constant at some suitable value, the resulting known values of a and X at intervals of T will uniquely define a signal a cos 0 of bandwidth F in a particular frequency range. This signal will have a constant amplitude at the intervals T and at these intervals the instantaneous phase modulation corresponds to the amplitude of the samples of the modulating signal S. To regain the signal S, the phase of the modulated signal is demodulated at the intervals of T, and the resulting samples applied through a low-pass filter having a cut-off frequency of F/2.

The only theoretical limit to the excursion of the phase F of the modulated signal is that, in order to avoid any ambiguity, the phase difference between successive intervals T must be less than n, although in practice, it is preferable limit the rate of change of phase between intervals T to nor/2.

Usually, the modulating signal S will be a baseband signal having a frequency range extending from 0 to F/2, requiring a minimum sampling rate of F to retain all the signal information. However, where the modulating signal band is displaced from zero, a higher rate of sampling will normally be required. For simplicity, the sampling rate may simply be set at twice the highest in-band frequency of the modulating signal S, although lower rates of sampling, which retain all the signal information, may be used in accordance with known principles. Alternatively, an above-baseband modulating signal may be translated down into a base-band signal before sampling, and retranslated to its original frequency range after demodulation to regain the original signal.

As hereinbefore stated, in one method of generating a modulated signal in accordance with the invention, amplitude samples of an analogue modulating signal are first applied to pulseposition modulate (PPM) short pulses having an unmodulated pulse repetition period T, and the PPM pulses are then fed through a band-pass filter having a bandwidth F including the desired carrier frequency and preferably substantially centred thereon.

As will be seen by those familiar with the impulse-response of a band-pass filter, the carrier frequency of the resulting modulated signal is an integral multiple of F and only one such multiple is within the pass-band; this one will be the carrier.

Normally and preferably this multiple will be made substantially the centre of the filter pass-band.

The amount of position-modulation of the pulses of the pulse-position modulated signal should correspond to the required phase-shift in the resulting modulated carrier. Thus, to produce a phase-shift of Q in a carrier of frequency f, on which the pass-band of the filter is centred, the time-shift applied to the pulses of the signal should correspond to 0/2of relative to their unmodulated time of occurrence.

The invention also extends to apparatus for carrying out modulation and demodulation methods as aforesaid.

The invention will now be described in greater detail, by way of example only, with reference to the accompanying drawings of which: Figure 1 is a block schematic diagram of signal modulation apparatus in accordance with the present invention, and Figure 2 is a block schematic diagram of signal demodulation apparatus for demodulating a signal modulated in accordance with the present invention.

Referring to the drawings, the apparatus shown in Figure 1 comprises a sampling circuit 1 arranged to receive an analogue modulating signal S having a maximum frequency F/2 and to sample the amplitude of the signal S at periodic intervals T=1/F. The amplitude samples of period T produced by the sampling circuit 1 uniquely define the signal S because its maximum frequency is less than F/2, and these samples are applied to a pulse-position modulator 2 arranged to modulate in known manner the position of a set of short pulses having an unmodulated pulse repetition interval of T, by an amount proportional to the amplitude of respective ones of the amplitude samples of the modulating signal S.

The short pulse-position modulated pulses produced by the pulse-position modulator 2 are then applied to a band-pass filter 4 having a bandwidth F a centre frequency f (F fl. There is thereby produced, in effect, a carrier signal of frequency f modulated in accordance with the invention by the signal S, and having a bandwidth of F extending from f-F/2 to f+F/2. The factor of proportionality relating the amount of displacement of each pulse introduced in the pulse-position modulation (PPM) process to the amplitude of the corresponding sample of the signal S, is determined in dependence upon the centre frequency f of the band-pass filter 4, such that the amount of pulse displacement introduced corresponds to the required instantaneous phaseshift of this carrier frequency f. Thus, to produce a phase shift ,b in the carrier of frequency f, the corresponding pulse of the PPM signal should have a time displacement of 0/27if. To avoid any ambiguities, the maximum excursion in the phase 4i of the modulated carrier signal in any time interval T should not exceed rr, which in turn limits the maximum difference in the time displacement of successive pulses in the pulse position modulated signal to 1/2f.

The modulated output signal produced by the filter 4 may then, if required, be frequency-shifted to any suitable frequency, such as RF for radio transmission in a frequency-shift circuit 6 followed by RF amplifier and output band-pass filter stages 7, 8 respectively.

Superficially, a modulated signal produced by the modulation technique in accordance with the present invention resembles a normal phasemodulated (PM) signal - the only apparent difference being a mild amplitude modulation superposed on the modulated signal. However, it is not in fact simply a normal phase-modulated signal which has its bandwidth reduced by an additional amplitude modulation; the spectral structure is quite different from the primary spectrum of a normal PM signal and the phasestructure between the sampling points can also be substantially different.

The temporal pattern of the present modulated signal is that it has the same amplitude and phase as a normal PM signal at the sampling points, but the phase varies in a smooth manner between them. The superimposed amplitude modulation is in most cases fairly small, although it may go right down to zero in extreme cases. However, the differences between the present form of modulation and normal phase-modulation, both in amplitude and phase, increase with the deviation of the modulation.

Unlike normal phase (and frequency) modulated signals, there is no simple relationship between the spectrum of the modulating signal and that of the modulated signal produced by the present modulation technique. If there is no simple integral relationship between the period of the modulating signal and the sempling period T, then the spectrum of the modulated signal is continuous, even if the modulating signal is a simple sinusoid.

It will be noted that the phase of the modulated signal produced by the present modulation technique is only correct at the sampling points, and that the discrepancy in the intervals between sampling points increases with the deviation or depth of modulation. However, it has been found that if the rate of change of phase between sampling points is restricted to less than 7r/2, which is half the theoretical maximum imposed by the need to avoid ambiguities caused by phase differences of more than 7G, then this discrepancy becomes very small. It should be noted however, that the total excursion of phase in the modulated signal need not be restricted, only the rate of change of phase at any time.

As discussed earlier, the amplitude of a modulated signal produced in accordance with the present invention is not constant, and if clipped to make it so will not still be band-limited. This may present difficulties where the transmitted signal is required to have a constant amplitude, eg for engineering reasons.

This problem may be overcome at the expense of introducing substantially negligible distortion by allowing flattening of the modulated signal, an effect which will normally be produced by the RF amplifier stage 7, and using the output band-pass filter stage 8 to remove the resultant frequency spread.Although this is not theoretically correct as such filtering will not fully restore the original signal and will thus introduce some degree of distortion, it has been found that if the rate of change of phase of the modulated signal is restricted to 7r/2 between sampling points as discussed above, the distortion is negFigible. -# Limiting the phase excursion in this way will also reduce the amount of phase discrepancy at points intermediate the sampling points (the phase of the modulating signal being correct only at the sampling points) to only a few percent which avoids synchronising problems in a demodulation of the signal, as will be discussed below, and will potentially improve the signal-to-noise ratio by enabling more of the signal to be used.

Figure 2 shows a suitable arrangement for receiving and demodulating modulated RF signals produced by the circuit of Figure 1, and comprising a radio receiver 10 for receiving the modulated signal, and where necessary arranged to down-shift the signal from radio frequency to a suitable lower frequency for demodulation. The output of the receiver 10 is connected to a PM demodulator 11 , which operates in conventional manner to produce an output signal the amplitude of which varies with the instantaneous phase of the input signal.The output of the PM demodulator 1 1 is then sampled by a sampling circuit 12, phase-synchronised with the sampling circuit 1 (Figure 1) to sample the PM demodulated signal at points corresponding to the sampling points of the sampling circuit 1, whereby to regenerate samples of the original modulating signal S. These samples are than applied to a lowpass filter 13 having a cut-off frequency of F/2 to reproduce the original input signal S.

Synchronisation of the demodulation process with the original sampling of the modulating signal may be effected by any suitable known technique, such as a reference tone transmitted with the modulated signal, or an accurate clock may be use at each end. However, if the rate of change of phase between the sampling points in the modulated signal is restricted to less than 7r/2 as described earlier, no such synchronisation is necessary; in fact the signal may be received simply as normal phase-modulated signal without sampling, ie the sampling circuit 12 and filter 13 can be omitted.

Where the modulating signal S is a base-band signal having a frequency band extending from 0 to F/2, the input signal sampling rate win normally correspond to twice the maximum in-band frequency, ie F/2. However, where the modulating signal S has a bandwidth F/2 extending from F1 to F1 + F/2, the sampling rate may again simply correspond to twice the maximum in-band frequency, ie 2(F1 + F/2), although all the information contained in the modulating signal may be captured without loss using a maximum sampling rate of 2F2/m, where F2 is the maximum in-band frequency (ie F1 + F/2), and m is the largest integral value of in-band frequency divided by bandwidth not exceeding the value F2/(F/2).

Alternatively, of course, such above-base-band modulating signals may be frequency translated to the range 0 to F/2 before sampling, followed by restoration to the original range by an inverse translation after demodulation.

Although the invention has been largely described in terms of phase-modulation of a carrier signal at periodic intervals T in accordance with the amplitude of samples of a modulating signal S taken at intervals T, a corresponding frequency-modulated signal can be produced by integrating the signal S before it is fed to the sampling circuit 1 (Fig 1), and subsequently differentiating the output of the filter 13 (Fig 2), as will be apparent to those of ordinary skill in the art.

In the foregoing detailed description, the modulating information is an analogue signal which is sampled at intervals T to obtain successive individual values representing its amplitude. Where the modulating information is already in the form of digital data occurring at periodic intervals T (ie effectively already in the form of samples), no additional sampling action is required, and with reference to Fig 1 for example, the sampling circuit 1 is omitted. Where each successive digital data value is to be represented by one of serveral standard carrier phases optionally combined with one of several standard carrier amplitudes, depending on the coding adopted, eg as in conventional quadrature amplitude modulation (QAM), the modulated carrier may be generated in accordance with the present invention.

For example, referring to Fig 1, each pulse fed to the filter 4 may be generated as a PPM pulse, from PPM modulator 2, which occupies one of several standard positions (each position corresponding to a given standard carrier phase) and additionally has one of several standard amplitudes. Such a method corresponds to normal CAM except that the modulated carrier is derived from the impulse-response of the filter 4 instead of by modulating a separately generated carrier.

The bandwidth of the thus-generated CAM signal is F as in the analogue case, and is the minimum theoretically possible with this form of transmission without distortion. The transmitted signal will not be of constant amplitude, although this will be approximated if there is no amplitude component in the coding (ie simply phasemodulated digital transmission), as in the sampled analogue signal embodiment described earlier.

Unlike the latter, however, a CAM signal having an amplitude component in its coding clearly cannot be clipped as earlier described, without loss of information. Reception of a thus-generated CAM signal may be performed exactly as for a normally generated CAM signal. Likewise the generated carrier phases can be either absolute or differential, as with normal methods of generating CAM signals.

Although in the foregoing description the generation of the modulated signal involves the step of generating PPM pulses which are fed to a band-pass filter, and this system is preferred for its simplicity, in principle other techniques are possible, eg by taking components cos 0 and sin sss and using orthogonal band-pass filters. However these are likely to be more complex and difficult.

Claims (19)

1. A method of generating a signal modulated in accordance with individual values occuring with a periodic interval T, comprising feeding short pulses related to successive values to band-pass filter means having a bandwidth of F cycles, where T=1/F, said band including a frequency f where f F, whereby to derive from the output of said filter means a resulting modulated signal of frequency f and of bandwidth F.

2. A method as claimed in claim 1 wherein said modulated signal of frequency f is frequencyshifted to a higher frequency.

3. A method as claimed in claim 1 or claim 2 wherein the phase difference of the modulated signal f between successive intervals T is made not greater than 7t/2.

4. A method as claimed in any of claims 1 to 3 wherein the short pulses are generated as pulseposition modulated (PPM) pulses of unmodulated repetition period T whereof the position of each is related to successive values and wherein said PPM pulses are fed to a band-pass filter of bandwidth F including the frequency f whereof the output is said modulated signal.

5. A method as claimed in any of claims 1 to 3 wherein the values are derived by sampling at periodic intervals T the amplitude of an analogue signal having a bandwidth less than or equal to F/2.

6. A method as claimed in claim 5 wherein the short pulses are generated as pulse-position modulated (PPM) pulses of constant amplitude and unmodulated repetition period T whereof the position of each corresponds to the amplitude of successive samples of the analogue signal and said PPM pulses are fed to a band-pass filter of bandwidth F including the frequency f whereof the output is said modulated signal.

7. A method as claimed in claim 4 or claim 6 wherein the filter is substantially centred on the frequency f.

8. A method as claimed in any of claims 5, 6 or 7 as dependent on claim 6 wherein said modulated signal is subsequently demodulated and thereafter sampled at periodic intervals T to regenerate samples corresponding to the initial samples of the analogue signal.

9. A method as claimed in any of claims 1 to 4 and claim 7 as dependent on claim 4 wherein the values are digital data values occurring at periodic intervals T and there is generated by the method a said modulated signal wherein each value is represented by one of a plurality of standard phases and, optionally, by one of a plurality of standard amplitudes, of the modulated signal.

10. Apparatus for generating a signal modulated in accordance with individual values occuring with a periodic interval T comprising means for feeding short pulses related to successive values to band-pass filter means having a bandwidth of F cycles, where T=1/F, said band including a frequency f where f F, whereby the output of said filter means is a modulated signal of frequency f and of bandwidth F.

1 Apparatus as claimed in claim 10 comprising means for frequency-shifting the frequency f to a higher frequency.

12. Apparatus as claimed in claim 10 or claim 11 comprising means for generating said short pulses as pulse-position modulated (PPM) pulses of unmodulated repetition period T whereof the position of each is related to successive values, and a band-pass filter including frequency f and of bandwidth F connected to receive said PPM pulses, whereby the filter output is said modulated signal.

13. Apparatus as claimed in claim 10 or claim 11 including means for sampling the amplitude of an analogue signal at periodic intervals T to derive said values.

14. Apparatus as claimed in claim 13 comprising means for generating from said samples short pulse-position modulated (PPM) pulses of constant amplitude and unmodulated repetition period T whereof the position corresponds to the amplitude of successive samples, and a band-pass filter including frequency f and of bandwidth F connected to receive said PPM pulses, whereby the filter output is said modulated signal.

15. Apparatus as claimed in claim 12 or claim 14 wherein the filter is substantially centred on the frequency f.

16. Apparatus as claimed in claim 13, 14 or 15 as dependent on claim 14 comprising means for subsequently demodulating said modulated signal and for thereafter sampling the demodulated signal at periodic intervals T to regenerate samples corresponding to the initial samples of the analogue signal.

17. A method of generating a signal modulated in accordance with individual values corresponding with a periodic interval T, comprising generating ab initio a reference or carrier frequency f limited to a bandwidth F(=1,T) whose phase and amplitude are uniquely defined at intervals of T by said individual values.

1 8. A method of, or apparatus for, generating a modulated signal substantially as hereinbefore described with reference to Fig 1 of the accompanying drawings.

19. A method of, or apparatus for, modulating and demodulating a modulated signal and regenerating therefrom amplitude samples of an analogue signal substantially as herebefore described with reference to Figs 1 and 2 of the accompanying drawings.