GB2097227A - FM rebroadcast transceiver - Google Patents

Abstract

An FM common channel duplex rebroadcast transceiver comprises a transmitter including a transmitter aerial 1 arranged to be fed via a power amplifier 3 from a modulator 2, and a receiver which includes a single port mixer 8 arranged to be fed from a receiver aerial 6 with a received wanted signal and with the signal which is radiated locally from the transmitter aerial which is utilised by the single port mixer as a local oscillator signal. The transceiver also includes a single side band suppressed carrier generator Fig. 3 (not shown) which includes an oscillator (21) and which is arranged to be fed from the single port mixer so as to provide a resultant signal in which frequency modulation of the received wanted signal is superimposed on the oscillator frequency, and FM discriminator or detector means (28) arranged to receive the resultant signal and to provide an AM output signal corresponding to the received FM modulation which is fed back to control the modulator. <IMAGE>

Description

SPECIFICATION FM rebroadcast transceiver This invention relates to common channel duplex FM rebroadcast transceivers.

A common channel duplex transmitter/receiver is described in our UK. Patent No.

1577514 which permits of contemporaneous transmission and reception of the same channel and in which the received signal can be fed back to modulate the signal transmitted thereby to provide a rebroadcast or radio relay facility. This transmitter/receiver system as described in our U.K. Patent referred to above, utilises a receiver including two quadrature related signal paths to facilitate suppression of the locally generated transmission, each signal path including a mixer. This transmitter/receiver system although suitable for many applications has problems which limit its dynamic range. That is to say suppression in the receiver of the locally generated transmission is not complete and thus the amplitude of the locally generated transmission must be limited.

The most serious problem limiting the performance of this system is differential phase shift variation with frequency modulation between the signals at the two input ports of the mixers. Suppression in the receiver of the locally generated transmission is dependent on the mixer local oscillator signals and the unwanted signals, which pass from the transmitter via the path between the transmitter and receiver aerials, maintaining a fixed phase relationship to one another. In practice the unwanted signal is also fed through R.F.

filtering and cables etc. In one embodiment of the system described in our patent hereinbefore referred to, a delay device is included in the local oscillator line to the mixers to equalise the signal path delay characteristics. However, it is not practicable to completely equalise a complex delay variation over the modulation cycle. There is therefore always a significant amount of unwanted signals present at the mixer output ports. Since the unwanted signal is substantially correlated with the modulation of the transmitter it is possible to reduce it with a simple audio cancellation circuit as explained in our U.K. Patent referred to above but the degree of cancellation possible is dependent on the modulation linearity and other factors and is not therefore complete. The residual unwanted signal represents a limitation in the dynamic range of the equipment.

According to the present invention an FM common channel duplex rebroadcast transceiver comprises a transmitter including a transmitter aerial arranged to be fed via a power amplifier from a modulator, and a receiver which includes a single port mixer arranged to be fed from a receiver aerial with a received wanted signal and with the signal which is radiated locally from the transmitter aerial which is utilised by the single port mixer as a local oscillator signal, a single side band suppressed carrier generator which includes an oscillator and which is arranged to be fed from the single port mixer so as to provide a resultant signal in which frequency modulation of the received wanted signal is superimposed on the oscillator frequency, and FM discriminator or detector means arranged to receive the resultant signal and to provide an AM output signal corresponding to the received FM modulation which is fed back to control the modulator.

The A.M. output signal may be fed back via gate means to provide a modulating signal for the modulator and to level detector means responsive to the level of the AM output signal for causing the gate to be operated when signals above a predetermined level are detected whereby feedback of the signals to the modulator is momentarily inhibited.

The single side band suppressed carrier generator (SSBSCG) may comprise a wide band audio frequency quadrature phase shift network, which is arranged to be fed from the single port mixer, a pair of multiplicative mixers coupled to the network so as to receive quadrature related signals therefrom, the oscillator being set to a frequency outside the audio frequency range and being arranged to feed the multiplicative mixers with phase quadrature related oscillator signals, and an additive combiner arranged to be fed from the mixers and providing the resultant signal.

The (SSBSCG) may include a 90 phase shift network via which the oscillator is arranged to feed the mixers whereby the mixers are arranged to be fed with phase quadrature related signals.

The gate means may comprise a gate which is arranged to be fed from the level detector via a monostable device operative to 'open' the gate for a predetermined period in response to the detection of an output signal which exceeds the predetermined level.

The single port mixer may comprise a diode having operatively associated with it R.F. decoupling means.

The R.F. decoupling means may comprise at least one choke and at least one capacitor.

Thus in a transceiver according to the present invention the problem of variable differential phase shift between the local oscillator signal and the wanted signal is eliminated because they follow a common signal path.

Another problem associated with common channel duplex are unwanted signals which are due to variations at the receiver in the amplitude of the locally transmitted signal over the modulation cycle due to multiple reflections in the signal path and FM to AM conversion in the transmitter.

According to one aspect of the invention these effects are considerably reduced by the inclusion of a variable tuned circuit in the signal path.

The tuned circuit may form a part of the receiver RF filtering.

In operation of the system, the centre frequency of the tuned circuit is adjusted to produce an AM to FM conversion character istic which cancels the unwanted AM and thereby substantially reduces this source of unwanted signals.

An embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which; Figure 1, is a block schematic diagram of a common channel duplex rebroadcast transceiver; Figure 2, is a block schematic diagram of single port mixer arrangement and, Figure 3, is a detailed block schematic diagram of the demodulator discriminator arrangement shown as a single block in Fig. 1.

Referring now to Fig. 1, an F.M. common channel duplex transceiver comprises a transmitter aerial 1 fed from a modulator 2 via a power amplifier 3. The modulator 2 is frequency modulated by a control signal which is fed to the modulator on line 4 from a demodulator discriminator 5 which operates to demoduiate a signal received by a receiver aerial 6.The signal received at the receiver aerial 6 is fed via a tuned circuit 7 to a single port mixer 8 which as well as receiving the wanted R.F. signal receives via the tuned circuit 7 a sample of the locally transmitted signal which is radiated from the transmitter aerial 1. The single port mixer 8 thus utilises the locally transmitted signal which is radiated from the transmitter aerial 1 as a local oscillator signal and thus feeds an F.M. beat frequency signal which is at audio frequency via a channel filter 9 to the demodulator discriminator 5.

The single port mixer 8 is shown in greater detail in Fig. 2 and comprises a mixer diode 10 which is fed from the tuned circuit 7, the diode 10 being shunted by R.F. decoupling components comprising chokes 11 and 12 and a capacitor 1 3. In operation of the mixer 8 the wanted R.F. frequency is received together with a sample of the locally transmitted signal which is radiated from the transmitter aerial 1. The two signals beat together and are mixed in the diode 10 to provide a beat frequency audio output signai on line 14 which is fed to the channel filter 9.

The output signal from the channel filter 9 is thus the frequency modulation on the received signal and is referenced to zero frequency. In this form the output signal from the filter 9 is difficult to demoduiate with a conventional F.M. discriminator or a ratio detector and so a circuit is included in the demociulator discriminator 5 which is effective to shift the frequency modulation from zero to be centred on a higher frequency so that a conventional F.M. discriminator or a ratio detector can be utilised to detect it. The circuit which is used to do this is shown schemati cally in Fig. 3 within broken line 14a and comprises a single side band suppressed car rier (SSBSC) generator.Signals from the chan- nel filter 9 are fed to the SSBSC generator on line 1 5 to a wide band audio frequency phase shift network 1 6 which serves to provide twc output signals on lines 1 7 and 1 8 respectively which are phase quadrature related. The phase quadrature related signals are fed to a pair of multiplicative mixers S 9 and 20, the mixers being fed also from a local oscillator 21 via a phase quadrature network 22 with phase quadrature related local oscillator sig nals. The local oscillator frequency Fx is chosen so as to be outside the audio frequency range and the precise frequency is not of special importance.Output signals on lines 23 and 24 from the multiplicative mixers 1 9 and 20 respectively are fed to an additive combiner 25. It will be appreciated that output signals on line 26 from the additive combiner 25 correspond to the signals fed to the SSBSC generator on line 1 5 but are shifted in frequency by the frequency FX of the local oscillator 21. Thus the SSBSC generator serves to provide a signal which can be demodulated by a conventional FM discriminator or a ratio detector.

Output signals from the additive combiner 25 ars fed via line 26 and a filter 27 to a conventional FM discriminator 28. The discriminator 28 is tuned to be centered on the frequency FX of the local oscillator 21 and thus it provides an audio frequency output signal on line 29 which corresponds to the received modulation.

It will be appreciated that if the mixer 8 were to receive a locally generated local oscillator signal from the aerial 1 which corresponded exactly with the centre frequency of the received wanted frequency modulated signal, then the signal on line 29 would be a full wave rectified version of the wanted FM modulation since it would be folded. In order to avoid this situation it is arranged for the frequency of the transmitted signal to be slightly offset with respect to the frequency of the received signal and this is achieved by maintaining a d.c. offset negative feedback signal on the line 4 which feeds the transmitter modulator 2. In spite of tnis otfset signal, it can happen under noisy signal conditions that the negative feedback signal which is applied from the demodulator discriminator to the modulator 2 via line 4 can go positive so as to cause a positive feedback situation and a corresponding positive feedback spike. In order to prevent this situation from developing, the level of the signal on line 29 is monitored with a level detector 30 and in the event of the level exceeding a predetermined threshold level, a signal is applied to a monostable device 31 which opens a gate 32 for a predetermined period, the period being determined by the monostable device.Thus if during operation, the received and transmitted signals transiently cross over each other in frequency due to noise and/or the effects of delay in the low pass filters etc. at high modulation frequencies, the leading edge of the transient pulse generated at the output of the discriminator due to the loop changing from a negative to a positive feedback condition is detected by the threshold detector and utilised to operate the gate 32 so that for the few microseconds during which the transient pulse occurs, the loop is open circuited.

It will be appreciated that with the transceiver as just before described, the problems arising from differential phase variation are eliminated because the local oscillator and wanted signal paths are one and the same.

Although the system just before described is immune to the effects of differential phase shift, a further problem can arise due to variation in the amplitude of the unwanted signal over the modulation cycle due to multiple reflections in the signal path and FM to AM conversion in the transmitter. These effects can be considerably reduced by the inclusion of a variable tuned circuit in the receiver signal path. Thus as shown in Fig. 1 and Fig. 2, the input tuned circuit can be made variable so that the centre frequency of the tuned circuit can be adjusted to produce an FM to AM conversion characteristic which cancels the unwanted AM. As shown in the drawing, the variable tuned circuit conveniently forms a part of the receiver RF filtering and might be made accessible to the operator so that the tuned circuit can be adjusted in operation.

Claims (9)

1. An FM common channel duplex rebroadcast transceiver comprising a transmitter including a transmitter aerial arranged to be fed via a power amplifier from a modulator, and a receiver which includes a single port mixer arranged to be fed from a receiver aerial with a received wanted signal and with the signal which is radiated locally from the transmitter aerial which is utilised by the single port mixer as a local oscillator signal, a single side band suppressed carrier generator which includes an oscillator and which is arranged to be fed from the single port mixer so as to provide a resultant signal in which frequency modulation of the received wanted signal is superimposed on the oscillator frequency, and FM discriminator or detector means arranged to receive the resultant signal and to provide an AM output signal corresponding to the received FM modulation which is fed back to control the modulator.

2. A transceiver as claimed in claim 1 wherein the A.M. output signal is fed back via gate means to provide a modulating signal for the modulator and to level detector means responsive to the level of the AM output signal for causing the gate to be operated when signals above a predetermined level are detected whereby feedback of the signal to the modulator is momentarily inhibited.

3. A transceiver as claimed in claim 1 or claim 2 wherein the single side band suppressed carrier generator (SSBSCG) comprises a wide band audio frequency quadrature phase shift network, which is arranged to be fed from the single port mixer, a pair of multiplicative mixers coupled to the network so as to receive quadrature related signals therefrom, the oscillator being set to a frequency outside the audio frequency range and being arranged to feed the mutliplicative mixers with phase quadrature related oscillator signals, and an additive combiner arranged to be fed from the mixers and providing the resultant signal.

4. A transceiver as claimed in claim 3 wherein the (SSBSCG) includes a 90 phase shift network via which the oscillator is arranged to feed the mixers whereby the mixers are arranged to be fed with phase quadrature related signals.

5. A transceiver as claimed in claim 2, claim 3, or claim 4, wherein the gate means comprises a gate which is arranged to be fed from the level detector via a monostable device operative to 'open' the gate for a predetermined period in response to the detection of an output signal which exceeds the predetermined level.

6. A transceiver as claimed in any preceding claim wherein the single port mixer comprises a diode having operatively associated with it R.F. decoupling means.

7. A transceiver as claimed in claim 6 wherein the R.F. decoupling means comprises at least one choke and at least one capacitor.

8. A transceiver as claimed in any preceding claim wherein a variable tuned circuit is included in the signal path.

9. A transceiver as claimed in claim 8 wherein the tuned circuit forms part of the receiver RF filtering.

1 0. A transceiver substantially as hereinbefore described with reference to the accompanying drawings.