GB2133648A - Amplitude-modulated radio transmitter - Google Patents

Abstract

The transmitter output is progressively compressed as the level of modulation increases; the a.g.c. mechanism of a conventional receiver provides complementary expansion. The control may be such that the peak level of the output envelope is substantially constant. A first modulator 14 modulates a carrier from an oscillator 18 with an audio input signal. A second modulator 20 modulates the output from the first modulator with a control signal Vc which is derived from the output of the second modulator 20 by a control circuit 22 effecting peak detection and smoothing. Other arrangements for high power transmitters and with feed- forward open-loop control are described. The invention reduces mean transmitter power consumption for a given service performance. <IMAGE>

Description

SPECIFICATION Amplitude-modulated radio transmitter This invention is concerned with improving the utilisation of amplitude-modulation (a.m.) radio transmitters.

The object of the invention is to make it possible to achieve either a reduction of mean transmitter power consumption while maintaining a substantially unchanged service or an improved service for the same mean transmitter power or a measure of reduced power consumption with some degree of improvement of service.

According to the present invention, there is provided an amplitude-modulation radio transmitter including control means arranged in operation to reduce the mean level of the envelope of the modulated signal with increasing modulation depth.

The proposed means of improving a.m. transmitter utilisation is thus based on compression of the radiated signal (carrier and sidebands) at the transmitter, and invokes the automatic gain control (a.g.c.) mechanism, provided in virtually all receivers to counteract fluctuations in the level of received signal due to fading in the propagation path, as a complementary expander. The transmitter/receiver combination thus acts in the well known "compander" manner to reproduce the appropriate levels of demodulated signal at the receiver output.

At low levels of modulation the proposed system can, for example, be arranged to produce substantially the same radiated power as does a conventional system. Normal signal-to-noise relationships therefore apply at the receiver output under these conditions.

At higher depths of modulation the level of radiated carrier is reduced. The receiver a.g.c. then acts to restore the carrier level at the demodulator, and hence the level of reproduced programme. Under these conditions of high modulation level there is also some masking of noise and interference which tend to be more apparent with a reduced carrier level.

The invention will now be described in more detail by way of example with reference to the accompanying drawings, in which: Figure 1 shows waveforms for conventional amplitude modulation, Figure 2 shows waveforms for one example of amplitude modulation in accordance with the invention, Figure 3 shows comparative curves of relative radiated power, Figure 4 is a block diagram ofafirstembodiment of the invention, Figure5 is a block diagram of a circuitforderiving a control signal in a second embodiment, Figure 6 illustrates various control laws for the circuit of Figure 5, Figure 7 illustrates use of the control signal from Figure 5 to effect division, Figure 8 is a block diagram of a high power amplitude-modulation transmitting pulse duration modulation techniques and employing the circuits of Figures 5 and 7, Figure 9 is a block diagram of a further circuit for deriving a control signal, Figure 10 is a block diagram of a high power anode-modulated tetrode transmitter using the control circuit of Figure 9, and Figure 1 is a block diagram of an embodiment in a high-power impedance modulated amplifier.

Figure 1 shows modulation envelope waveforms, amplitude versus frequency, illustrative of conventional double-sideband amplitude modulation. Fig ure 1(a) shows the unmodulated carrier and Figures 1(b), 1(c) and 1(d) show the envelope for sinusoidal modulation of approximately 10%, 50% and 100% respectively.

Radiated power increases as the depth of modulation is increased, reaching, at 100% modulation, 12 times that of the unmodulated carrier.

Receiver automatic gain control circuits most commonly respond to the mean level of the modu lated signal (i.e. to the mean carrier level) and so are not influenced by variations of modulation depth.

Thus the demodulated signal should be a faithful reproduction of the audio signal applied to the transmitter.

An alternative, but less common, a.g.c. arrangement responds to the peak level of the modulated waveform. A receiver with this form of a.g.c., imposes compression on its demodulated output from a conventionally modulated transmission, and will demodulate a compressed radiation signal (having the parameters used here for illustration) without expansion.

Figure 2 shows the modulation envelope waveforms, amplitude versus frequency, illustrative of the radiation compression technique proposed.

The law of compression can, of course, be chosen to meet particular requirements: that used for sake of illustration in Figure 2 is such as to keep the steadystate peak amplitude of the radiated signal within the amplitude of the unmodulated carrier. Figure 2(a) shows this unmodulated carrier and Figures 2(b), 2(c) and 2(d) showthe envelope waveform ofthe radiated signal for approximately 10%, 50% and 100% sinusoidal modulation.

Comparing Figures 1 and 2 it will be evident that, whereas under conventional conditions the radiated power increases with percentage modulation (Figure 1), radiated power actually falls as percentage modulation is increased when using the particular radiation compression system illustrated in Figure 2.

Figure 3 shows, for a range of levels of sinusoidal modulation, the relative radiated power of a conventional system (curve 10) and of a radiation compression system using, for illustration, the compression law outlined above (curve 12). Relative radiated power = 1 is the power of the unmodulated carrier. It will be seen that under these conditions there is a progressively increasing economy of power as the percentage modulation is increased, reaching a 75% saving at 100% modulation.

Static considerations Receiver a.g.c. circuits are generally designed to maintain substantially constant mean carrier level at the demodulator, independent of modulation depth, (and therefore very closely to produce the apropriate level of demodulated audio output signal) over a large fading range. In practice, therefore, the com plementing of intentional radiation compression of no more than a few dB, as discussed above, will be virtually perfect under normal static conditions.

As indicated earlier, the level of reproduced noise and interference is related to the operation of the receiver a.g.c. The level of reproduced noise/interference thus varies with the level of the audio signal.

(This kind of noise is therefore commonly referred to as programme-modulated noise.) However, where the maximum amountofcompression and hence the maximum increase of noise level with increasing modulation is only a few decibels, and considering that when the noise is greatestthe programme signal is large, giving enhanced masking, the subjective degradation is small relative to a non-compressed transmission having equal carrier power at zero modulation level.

Dynamic considerations So far only static considerations have been discussed. Somewhat different considerations apply in respect both of the effects of extraneous noise and of the complementary nature of the compression/ expansion operation during transitional periods when the level of modulation, and therefore the amounts of compression and of expansion, are changing. However, provided that appropriate dynamic characteristics are adopted, and particularly where the range of control effected is no more than a few decibels experimental work has shown that su bjective impairment from these sources is negligible under practical operating conditions.

Where the transmitter concerned can accept 100% modulation, operating conventionally, the transition in the compression mode from a low level of modula tionto a high level of modulation - in the limit from no modulation to full modulation, i.e. from the state represented by Figure 2(a) to that of Figure 2(d) - can be made via the condition illustrated in Figure 1 (d). The peak level ofthe radiated signal is allowed momentarily to exceed the quiescent carrier level - but it is thereafter rapidly reduced to that level as compression is imposed.

If, however, this momentary peak of modulation amplitude cannot be allowed, the necessary characteristics can be achieved by means of non-overshoot, delay-line limiter techniques described previously in British Patent Specification#No. 1 108 413. This method of operation could give some cost saving in new transmitters because, using delay-line nonovershoot control techniques at the audio input when effecting the radiation compression, the peak voltage of the modulated carrier can be reduced relative to that of a conventional transmitter with the same quiescent carrier level.- Further, this method may have application in adapting existing transmitters to improve utilisation by allowing transmitter operation to be optimised to take advantage of the reduction in the maximum outputs signal required.

It might, moreover, be used in an adapted existing transmitter - regulatory and other technical considerations permitting-to enable the radiated power actually to be increased at low and medium modulation levels, yet restraining the maximum output signal to whatever limit may be set by, say consideration of absolute maximum power or of peak voltage.

Afurther consideration, especially at stations employing a number of high-power transmitters, is the variation at syllabic rate ofthe power consumption of a transmitter. This can lead to difficulties, particularly if two or more transmitters carry a common programme, or a common progrmme element such as a time signal. By adjusting the degree of compression to a lower value than in the example given previously (which maintained the peak carrier level at a constant value) it is possible to arrive at a condition of substantially constant radiated power, or of substantially constant transmitter power consumption taking into account any variation of transmitter efficiency with modulation depth.Because the condition to achieve either ofthese represents a small degree of compression loss of effective coverage should be negligible relative to that of a conventionally designed transmitter with the same carrier power at low modulation.

Some economy of mean power consumption and useful reduction of peak voltage can still be achieved together with the property of constant power consumption.

Figure 4 shows a low-power embodiment of the invention, using an output-controlled compressor arrangement, by way of illustration. It comprises essentially two modulators in cascade. The first modulator 14 operates conventionally to modulate an audio input signal, bandwidth limited by low-pass filter 16, on to the carrier generated by an oscillator 18. The output of the modulator 14 is an amplitudemodulatd signal of the usual form (Figure 1) which is fed to one input of a second modulator 20. The output of modulator 20, in addition to providing the signal to be radiated, feeds a control chain 22 in which the signal is peak detected, gated with a "threshold" level reference and passed through attack- and recoveryrate control circuits back to the second input of the modulator 20.The form of feedback control used is such as to restrict the peak amplitude of the radiated signal to that of the unmodulated carrier, giving the performance characteristics indicated in Figure 2.

As an example of the rates of change that give subjectively acceptable results in a practical system the attack time of the compressor may be about 0.3 ms and the recovery time about 250 ms. Attack time is here defined as the time taken, on sudden application of 100% modulation, for the momentary output overshoot to recover halfway (on linear scale) to its final steady state. Recovery time is defined as the time taken for the carrier amplitude to recover halfway (on linear scale) to its final steady state following sudden and complete removal of a 100% modulation signal.

Radiation compression as illustrated in Figure 4 is output controlled, with the control signal Ve derived directly from the radiated signal. Compression can equally well be input controlled with Ve derived from the audio input signal, with a DC component added, as appropriate, to represent the quiescent carrier level.

Figures 5 and 7 show means of generating and applying a control voltage Vc derived from the audio input signal, in an input-controlled open-loop radiation-compression system. Figure 5 indicates a method of deriving a voltage Vpk related to the peak amplitude ofthe input audio signal and of combining it with a control threshold voltage Vth to obtain the necessary control signal Vc. A peak rectification and smoothing circuit 24 provides a signal m from the audio input m cost. An adjustable gain stage 26 provides Vpk = k m wherek is a constant set by the stage 26 Vth is provided by a potentiometer 28 and an adder 30 provides Vc = Vth + Vpk = 1 + k m if quantities are normalized relative to Vth.

The resulting control voltage is shown by curve (i) in Figure 6 for the case k = 1.

Figure 7 shows a conventional form of voltage controlled gain arrangement, comprising a multiplier 32 operating in the negative feedback path of an operational amplifier 34 providing an output signal Vofrom an input signal Vi which is applied to the noninverting input. The gain Vo/Vi varies with 1/where Ve is the control voltage applied to the multiplier, i.e.

the circuit acts as a divider. If a conventionally modulated signal is passed through such a circuit and Vc is derived from the audio input as described, the overall performance indicated in Figure 2 can be achieved by imposing the law of variation of control voltage indicated in curve (i) of Figure 6.

Laws of compression other than that represented by Figure 6 curve (i) can also be achieved. For example, laws such as that represented by curve (ii) of Figure 6 - again representing a linear compression law, but with different slope - can be obtained by altering the value of k in Figure 5. Segmented laws, for example as illustrated by curve (iii) of Figure 6 which indicates no compression below a selected modulation depth, can be obtained by imposing # voltage delay on Vth in Figure 5 and adjusting the value of k as required. Curved compression characteristics can be achieved if required by introducing appropriate non-linear elements in the controlvoltage generating circuit.

Figure 8 illustrates an embodiment of a.m. radiation compression in a high power amplitude modulation transmitter using a pulse-duration-modulation technique. For illustration the control voltage Vc is derived as indicated in Figure 5. The audio input signal is added by an adder36 to a DC potential from a potentiometer 38 determining the quiescent carrier level, and regulated in level by a voltage controlled divider, VCA 40, (as indicated in Figure 7) controlled by Ve (derived as in Figure 5). The VCA output signal, VO is combined in an adder42 with a ram signal from a generator 44 and the combination used to generate a pulse-duration modulating signal for subsequent amplification in conventional manner.

Further variations in control voltage derivation technique are, of course, possible. Figure 9 shows an alternative means of deriving a control voltage Vc from the audio input signal, using a local "servo" loop including a first voltage controlled amplifier (VCR) 46. In a radiation compression application Ve is applied to one or more open-loop VCAs 48 arranged to regulate the radiated signal in the required manner. All the VCAs 46, 48 are matched. The input signal Ve to the control circuit comprises the audio input signal pulse a DC potential setting the level of quiescent control voltage Vi. The feedback loop around the VCA 46 comprises a control chain 22 providing the same functions as in Figure 4 and acts to keep the peak level of the output signal Vo constant.Thus the control voltage Ve is precisely that which has to be applied to the further, matched VCAs 48 introduced approriately in a transmitter circuit to regulate carrier and modulation levels. The arrangement is essentially the same as that of Figure 4, though in the one case the servo VCA acts on the audio input plus a DC component representing the quiescent carrier level, whereas in the other it acts on a modulated carrier waveform.

It may be noted that in the application ofthe control system the control laws of the VCAs are not critical in an absolute sense, but must be adequately matched.

Figure 10 shows means by which the control voltage derived as described (Figure 9) may be applied to produce radiation compression in a high power Anode-Modulated-Tetrode transmitter where the quiescent carrier level is independent of the anode modulation process.

Control of the modulating signal is provided by a VCA48A regulating the input to a high level amplifier 50 which effects the anode modulation. The required dynamic carrier control is achieved by appropriate screen modulation using a second, matched VCA48B - with necessary amplification by a d.c. power amplifier 52 - to regulate the screen potential of the tetrode 54 which is connected conventionally with an anode supply inductor 56 and load 58 and a screen inductor 60. The in putto the VCA 488 is from a potentiometer 62 which sets the quiescent carrier level.

A further embodiment is illustrated in Figure 11, which shows radiation compression as applied to an Impedance-Modulated Amplifier. Here control of operation of the "carrier valve" 64 and the "peak valve" 66 is imposed by matched VCAs 48A, 48B controlled by a common control signal Vc.

The audio signal is added by adder 68 to a d.c.

signal setting quiescent carrier level, from a potentiometer 70, and applied through one VCA 48A and a d.c. power amplifier 72 to the screen of the carrier valve 64. The audio signal is also applied through the otherVCA 48B and a power amplifier 74to the screen of the peak valve 66. The r.f. carrier is applied to the grid of valve 64 and through a quarter wave phase shifter 76 to the grid of the valve 66. The anode load 78 is connected direct to the valve 66 and through a quarter wave phase shifter 80 to the valve 64.

It will be appreciated that the a.m. transmitter radiation compression technique may be implemented to achieve, for example one or more of the following advantages.

(a) to reduce mean transmitter power consumption for a given output power at low modulation depths; (b) to increase transmitter output power at low modulation depths without increasing mean power consumption; (c) to further increase transmitter output power at low modulation depths with some increase of power consumption; (d) to improve transmitter utilisation where maximum output signal is set by a particular limiting parameter - for example, a given maximum peak output voltage; (e) to achieve such other law of output voltage or power versus modulation depth as may be desired for example, substantially constant output power or substantially constant power consumption, irrespective of modulation depth.

It will further be appreciated that, though some of the above may not readily be applicable to existing transmitters (for technical or for regulatory reasons) they may none-the-less have application in the economic design of new transmitters.

The various embodiments described may be modified to use different ones of the techniques described for deriving and making use of the control signal,

Claims (12)

1. An amplitude-modulation radio transmitter including control means arranged in operation to reduce the mean level of the envelope of the modulated signal with increasing modulation depth.

2. Atransmitter according to-claim 1, wherein the said mean level decreases linearly with increase in modulation depth.

3. A transmitter according to claim 2, wherein the rate of decrease ofthe said mean level is such that the peak level ofthe envelope remains substantially constant.

4. A transmitter according to claim 2, wherein the decrease ofthe said mean level commences above a predetermined threshold level of modulation depth.

5. Atransmitter according to any of claims 1 to 4, comprising a first modulator modulating a carrier with a modulating signal, followed by a second modulator which modulates the output of the first modulator with a control signal representing the level of the output of the second modulator.

6. A transmitter according to any of claims 1 to 4, comprising a modulator effecting modulation by a modulating signal, a circuit providing a control-signal representing the level ofthe modulating signal, and a divider circuit operating on the modulating signal or the modulated signal to effect division by the control signal.

7. A transmitter according to claim 6, wherein the divider circuit operates on the modulating signal and the modulator is a pulse duration modulator.

8. Atransmitter according to any of claims 1 to 4, comprising a modulator, a plurality of voltage controlled amplifiers controlled by a common control signal which represents the level of the output of a first one of the voltage controlled amplifiers, the first amplifier receiving the modulating signal for the modulator and the other amplifier(s) being connected in the modulator.

9. A transmitter according to claim 8, wherein the modulator is an anode modulated tetrode with the modulating signal applied to its anode by way of a second of the voltage controlled amplifiers and a screen bias voltage applied to its screen by way of a third of the voltage controlled amplifiers.

10. A transmitter according to claim 8, wherein the modulator is an impedance modulated amplifier comprising two tetrodes with their grids and anodes connected by respective quarter wave phase shifters, the modulating signals being applied to the screens of the tetrodes by way of second and third of the voltage controlled amplifiers.

11. A transmitter according to any of claims 5 to 10, wherein the modulating signal is an audiofrequency signal.

12. An amplitude-modulation radio transmitter substantially as described with reference to and using a control signal derivation circuit substantially as described with reference to any of Figures 4, 5 and 9 of the accompanying drawings.