GB2126029A - Producing narrowband SSB-FM signals - Google Patents

Abstract

A narrowband SSB-FM signal is produced by combining 18 an AM SSB suppressed carrier signal 14 with the carrier signal 12 which has been shifted in phase 16 by 90 DEG . By multiplying-up the frequency of the SSB-FM signal, f(t), e.g. by squaring, 26-30, the deviation index can be increased permitting wideband operation and a better noise performance: The signal may first be frequency translated downwards 22, 24. The signal is finally passed to a SSB filter 34 over a buffer 32. <IMAGE>

Description

SPECIFICATION A method of producing SSB-FM signals The present invention relates to producing SSB-FM (single sideband-frequency modulated) signals, particularly narrowband SSB-FM signals. The invention has application in any suitable subcarrier tone signalling system, for example analogue systems or tone signalling (FSK) systems.

Interest in SSB-FM signalling has grown because of the desire to use the spectrum more efficiently by having 5 kHz wide channels rather than 25 kHz channels which are customary with FM signalling.

J. L. Dubois and J. S. Aagaard in an article entitled "An Experimental SSB-FM System" published in IEEE Transactions on Communications Systems, June 1964, pages 222 to 229, describe a circuit for producing SSB-FM in which a carrier is amplitude modulated by the proper function at the same time as it is being angle modulated. This circuit is implemented by frequency modulating a carrier wave cos(cot) with the modulating signal s(t) and applying the frequency modulated signal to one input of an amplitude modulator. The negative exponential (e (t)) of the Hilbert Transform of the modulating signal is applied to a second input of the amplitude modulator.The output from the amplitude modulator comprises the SSB-FM signal (f(t)) described by the equation f(t) = e#"S(t?cos(wt + s(t)) This equation applies to wideband as well as to narrowband operation. Because of the rapid increase in envelope power with deviation index, SSB angle modulated systems are not attractive for wideband operation. In the case of narrowband operation the known circuit, in theory, is capable of producing the SSB-FM signals described in the above equation. However in implementing this circuit there are a number of practical problems which inhibit the production of these circuits.A Hilbert Transform circuit, which may comprise a transversal filter, is difficult to build in a cost effective way because there has to be a trade-off between the ripple in the amplitude response and the number of delay stages. The synchronisation of the two branches in the known circuit is critical in order to balance the delays in these two branches. Whilst Dubois and Aagaard propose using an amplitude modulator to combine the frequency modulated signal and the negative exponential of the Hilert Transform, in reality a mixer has been used by later workers in this field and there are difficulties due to having to provide a DC coupling and also due to leakage of the input signal affecting the output of the mixer. The overall adverse effect of these problems is that there will be some unwanted sideband components which, if removed, will lead to distortion of the received signal.

An object of the present invention is to overcome the mentioned disadvantages in the known SSB-FM signalling system.

According to one aspect of the present invention there is provided a method of producing an SSB-FM signal, comprising producing an SSB suppressed carrier signal and combining said signal with the carrier signal which has been shifted in phase by 900.

The method in accordance with the present invention enables a low deviation SS B-FM signal to be produced, which signal has good sideband suppression and is far less sensitive to parameter variations compared to the known circuit. Additionally it is simpler to implement practically.

If desired the SSB-FM signal may be multiplied-up whilst preserving the relationship between the amplitude and phase of the SSB-FM signal. By multiplying-up the signal in this way the deviation index is increased permitting wideband operation and a better noise performance. The multiplying-up may be achieved using non-linear devices such as field effect transistors, diodes or squaring circuits which are followed by filter(s) for selecting the desired signal. Additionally it is desirable to filter the output of the final multiplying device in a bandpass filter.

The bandwidth of the filter is such that the basic information without the out-of-band components is available for transmission thus enabling the bandwidth integrity to be maintained.

According to another aspect of the present invention there is provided apparatus for producing an SSBFM signal, comprising a suppressed carrier SSB generator having an output coupled to an input of a signal combining circuit and means for shifting the phase of a carrier signal by substantially 900, said means being coupled to an additional input of the signal combining circuit which has an output on which an SSB-FM signal is produced.

The present invention will now be described, by way of example, with reference to the accompanying drawings, wherein: Figure 1 is a block schematic circuit diagram of one embodiment of the present invention, Figure 2 is a vector diagram showing the phase relationship of an SSB signal with a large amplitude carrier and the Hilbert Transform of the modulating signal, and Figure 3 is a vector diagram showing the phase relationship of Figure 2 but in which the carrier has been phase shifted by 900.

Referring to Figure 1, an audio modulating signal source 10 and a carrier wave source 12 having an output frequency of for example 10.7 MHz are coupled to a conventional SSB suppressed carrier generator 1 4. The amplitude C of the carrier wave cos(cot) is much bigger, say by five times, than that of the audio signal s(t). A 900 phase shifter 1 6 is also connected to the output of the carrier wave source 12. A signal combining circuit 18 has inputs coupled respectively to an output of the SSB generator 14 and, via a variable gain control 20, to an output of the 900 phase shifter 1 6. The output from the combining circuit 18 is the SSB-FM signal f(t).

The signal f(t) is produced as follows: The phase of a conventional SSB signal with a large amplitude carrier approximates to the Hilbert Transform s(t) of the audio signal s(t). This is illustrated graphically in Figure 2 of the accompanying drawings. In Figure 2 the abscissa represents the vector C+s(t) but the vector will not be of constant length as s(t) is varying continually.

Thus FM demodulation of a conventional SSB signal with a large amplitude will produce (t)/C.

However in order to receive the audio signal s(t) instead of s(t), the carrier should be phase shifted by 900. This is illustrated in Figure 3. Thus ~(t) ~ s(t)/C. In Figure 1 the carrier is phase shifted by 900 in the phase shifter 16.

The output f(t) from the combining circuit 1 8 is susceptible to noise because the deviation s(t)/C is small. In order to obtain satisfactory narrowband operation and improved noise performance it is necessary to increase the deviation index.

In Figure 1 this is done by multiplying the signal from the combining circuit 18. Conveniently the multiplying is done by squaring the signal in for example one or more squaring circuits 26, 28 and 30.

As squaring a signal is more conveniently done at low frequencies, the output of the combining circuit 18 is translated to a carrier frequency of 1 75 kHz using a mixer 22 and a local oscillator 24 having a frequency 10.525 MHz. The output of the mixer 22 is filtered to remove the image frequency. Each squaring circuit 26, 28 and 30 includes a tuned amplifier to eliminate the unwanted products of squaring the signal. By squaring f(t), one obtains:

The signal (A) is the amplitude of the audio and is filtered out using the tuned amplifier thus leaving the signal (B) which has a carrier frequency of 350 kHz and has twice the original frequency deviation. The deviation is increased further by repeating the process in the two other squaring circuits 28 and 30 so that the frequency is increased to 1.4 MHz.Each of the squaring circuits conveniently comprises a four-quadrant multiplier such as a linear IC type 1495 or a field effect transistor which has a square law characteristic. As an alternative to a multiplicity of squaring circuits, a single non-linear device, such as a diode, can be used to produce the desired harmonics. Whichever way is used to increase the deviation, the overriding consideration is that the phase/envelope relationship must be preserved. Hence signal clipping as a generator of harmonics is not acceptable because the envelope is lost.

Having achieved the desired signal deviation then the signal is passed to an SSB filter 34 via a buffer 32 which in the present embodiment is provided for impedance matching. The filter 34 is a crystal filter having a bandwidth of the order of 3 kHz which enables the basic information without the out-of-band components to be transmitted. An advantage of frequency converting the SSB-FM signal before multiplying it is that the final frequency can be one at which crystal filters are readily obtainable at reasonable prices.

Although not shown, the receiver is a conventional FM receiver. However when considering Figure 3 it is apparent that the phase of the R.F. signal may be identified with s(t), once the carrier has been phase shifted by 900. An FM detector of a receiver will receive the instantaneous frequency which is the derivative of the phase. Therefore if this detector is to yield s(t) without further processing, some preprocessing is required at the transmitter. This processing comprises integrating s(t), for example in an integrator 36 as shown in broken lines in Figure 1, before modulation. Thus the phase ~ is the integral of s(t), that is ~(t) = J s(t)dt, and the instantaneous frequency is its derivative, that is d~(t) s(t) = dt In implementing the circuit shown in Figure 1 various technologies can be used and it is possible to fabricate the various parts of the circuit using known designs. Accordingly in the interests of brevity a detailed circuit diagram will not be described and illustrated.

If high frequency multipliers are available then it may not be necessary to frequency convert the SSB--FM signal before multiplying-up. Alternatively if the SS B-FM signal is produced at a low enough frequency then frequency translation before multiplying-up may be necessary.

Claims (13)

1. A method of producing an SSB-FM signal, comprising producing an SSB suppressed carrier signal and combining said signal with the carrier signal which has been shifted in phase by 900.

2. A method as claimed in Claim 1, wherein the deviation index of the SSB--FM signal is increased by multiplying-up the SSB--FM signal whilst preserving the relationship between the amplitude and the phase of the SSB--FM signal.

3. A method as claimed in Claim 2, wherein the multiplying-up of the SSB--FM signal is by squaring said signal and eliminating the unwanted products of squaring.

4. A method as claimed in Claim 2, wherein the multiplying-up of the SSB-FM signal is carried out using a non-linear device and selecting the desired harmonic.

5. A method as claimed in Claim 2, 3, or 4, wherein the SSB-FM is frequency translated to a lower frequency prior to its being multiplied-up.

6. A method as claimed in any one of Claims 2 to 5, wherein the multiplied-up signal is bandpass filtered to pass the basic information and exclude the out-of-band components.

7. A method of producing an SS B-FM signal, substantially as hereinbefore described with reference to the accompanying drawings.

8. Apparatus for producing an SSB-FM signal, comprising a suppressed carrier SSB generator having an output coupled to an input of a signal combining circuit and means for shifting the phase of a carrier signal by substantially 900, said means being coupled to an additional input of the signal combining circuit which has an output on which an SSB-FM signal is produced.

9. Apparatus as claimed in Claim 8, further comprising means for increasing the deviation index of the SSB-FM signal.

10. Apparatus as claimed in Claim 9, wherein the deviation index increasing means comprises signal squaring means and means for eliminating the unwanted products of squaring.

11. Apparatus as claimed in Claim 9, wherein the deviation index increasing means comprises a non-linear device which produces harmonics of a signal applied to the device and means for selecting a desired one of the harmonics.

12. Apparatus as claimed in Claim 9, 10 or 1 further comprising a bandpass filter coupled to the output deviation index increasing means, the filter having a bandwidth to pass the basic information without the out-of-band components.

13. Apparatus for producing an SSB-FM signal, constructed and arranged to operate substantially as hereinbefore described with reference to and as shown in the accompanying drawings.