Patent GB-588974-A (GB)
Title: Signal-translating system
IPC: H04B14/00
Filed: 1944-06-28
Abstract: 588,974. Phase and frequency modulation; multiplex radio signalling. HAZELTINE CORPORATION. June 28, 1944, No. 12341. Convention date, Sept. 20, 1943. [Class 40 (v)] In a signal translating system an angularvelocity modulated carrier signal together with a reference signal indicative of the phase of the carrier component are translated over a single channel. One of the signals comprises a pair of signal components individually spaced from the other signal in the frequency spectrum by equal and opposite values. Either of these signals is frequency or phase modulated in accordance with information to be transmitted, the width of the predominant modulation sidebands being less than one of values referred to. The other (reference) signal may be used to transmit further intelligence, for example, by amplitude modulation thereof. Transmitter, Fig. 1. The transmitter 10, comprises an oscillator 15 supplying a carrier of frequency f0 phase modulated at 17 by the signal input 16 representing the intelligence to be transmitted. The carrier from oscillator 15 is also amplitude modulated in a balanced modulator 19 by a signal of frequency f1 from a source 18. The modulator 19 suppresses the carrier f0 and passes only the amplitude modulation sideband components, f0+f1 and f0 - f1 which form the reference signal components. These reference signal components are amplitude modulated at 20 in accordance with additional information it is desired to transmit from signal source 21. The output circuits of modulators 17, 20 are passed through a combining amplifier 22 to the signal translating channel 12 which may be a radio link or a direct wire connection. As shown in Fig. 2, the predominant modulation components of the phase modulated carrier f0 lie in a band f0 + #f spaced in the frequency spectrum from the reference carriers f0+f1 and fo - f1 and their amplitude modulation component bands a, b to facilitate separation in the receiver. Receiver 11, Fig. 1. The received signals are amplified at 30 to compensate for any non- uniform frequency-response characteristics of the translating channel 12 and separated by frequency selective filters 31, 32, 33. Filters 31, 32 have pass-bands sufficiently wide to pass, respectively, the amplitude modulation components bands a, b associated with the respective reference signal component f0 - f1 and f0+f1. Filter 33 passes the band f0+#f. The phase modulated carrier selected by filter 33 is passed through a frequency doubler 35 to a phase modulation detector 36, for example, as described in Specification 536,917. The outputs from filters 31, 32 are fed to a mixer and limiter stage 34 where the separated reference signal components f0-f1 and f0+f1 are combined. The sum frequency 2f0 is selected, and after all amplitude modulation has been removed by the limiter of unit 34, is used as the reference signal, which bears a predetermined phase relationship to the carrier f0, in the phase modulation detector 36. At the same time the output from the filter 31 (or filter 32) is passed ,to an amplitude modulation detector 37 for deriving the modulation components corresponding to the signals applied at 21 in the transmitter. The outputs from the two detectors 36, 37 are passed to utilizing circuits 38. Modifications. The reference signals may be derived from an auxiliary oscillator instead of the same oscillator 15 which supplies the carrier f0 but synchronized therewith. Amplitude modulation detectors may be provided in the receiver for deriving the amplitude-modulation components of each reference signal component, and their outputs combined to compensate for frequency distortion produced by the transmission channel. The reference signal components from unit 19 in the transmitter may be individually amplitude modulated with additional information to be translated by replacing modulator 20 by pair of amplitude modulators individually coupled to unit 19 through refrequency selective circuits so that the reference signals f0-f1 and f0+f1, are applied separately to these modulators and individually modulated therein with the intelligence to be translated. In this case an amplitude modulation detector must of course be provided in the receiver for each filter 31, 32. Where the amplitude modulation of the reference signal component carriers is less than 100 per cent the signal outputs from filters 31, 32 may be supplied to mixer 34 through limiter stages adjusted to remove all amplitude modulation therefrom, thereby reducing the filtering problem otherwise presented when the signals applied to unit 34 comprise the amplitude-modulated referencesignal components. Transmitter, Fig. 3. In the modified transmitter 10<1>, the carrier frequency f0 is supplied by oscillator 15<1>, phase modulated at 17<1> by the signal input from 16<1> and passed to the combining amplifier 22<1>. The second wave of frequency f1 derived from oscillator 18<1> is first phase modulated at 24 by a signal input 23 in accordance with additional information to be translated, and then used to amplitude modulate the carrier frequency f0 in the balanced amplitude modulator 19<1>. This produces a pair of reference signal components of means frequency f0+f1 phase modulated in synchronism but in opposite senses, which are passed to the combining amplifier and translated together with the phase modulated carrier f0. The carriers f0, and f0+f1 are again so chosen that the bands associated therewith corresponding to the predominant modulation components are spaced in the frequency .spectrum. The receiver may be as shown at 11, Fig. 1, except that the amplitude modulation detector 37 must be replaced by a phase modulation detector and the filters 31, 32 must have band widths to accommodate the predominant modulation components associated with the corresponding reference-signal components. Transmitter, Fig. 4. In a further modification the carrier wave of frequency f0 supplied by oscillator 15" is amplitude modulated in a balanced modulator 19<11> by signals of frequency f1 from source 18<11>. The carriers f0+f1 thus produced are phase modulated in synchronism and in the same sense in phase modulator 17<11>, in accordance with the intelligence to be transmitted supplied from the signal source 16<11> and passed. to the translating channel 12<11> through the combining amplifier 22<11>. The carrier f0 is, in this case, used as the reference signal and is supplied direct through amplifier 22" to the channel 12<11>. The receiver 11 of Fig. 1 may be used. The amplitude modulation detector 37 therein will however not be required and filter 33 may be sharply selective to frequency f0. The two phase modulated signal component carriers f0+f1 after separation in filters 31, 32 are combined in mixer 34 where the sum frequency is taken to produce a carrier of frequency 2f0 modulated in accordance with the transmitted intelligence which is passed to the demodulator 36 together with the reference signal of frequency 2f0 derived from filter 33 through the frequency doubler 35. Modifications. In another modification, Fig. 5 (not shown), the carrier frequency f0 is first phase modulated in accordance with the intelligence to be transmitted, this carrier then being amplitude modulated by the signal frequency f1 to produce a pair of phase-modulated carriers of mean frequency f0+f1, identical with those produced in the arrangement of Fig. 4. The carrier f0 is again used as the reference signal, and the receiving arrangements may also be as for the Fig. 4 arrangement. In the arrangements of both Figs. 4 and 5 the reference signal may be used to transmit additional information by amplitude modulation thereof. The Specification also refers to the F.M. system using " zero-beat " frequency discrimination disclosed in the proceedings of the T.R.E. volume 30, No. 8, August 1942, pages 365-367.