Improvements in or relating to transmitters and receivers for single-sideband signals

Abstract

Abstract 636,467. Radio signalling. PHILIPS LAMPS, Ltd. Oct. 31, 1946, No. 32393. Convention date, Oct. 21, 1943. [Class 40 (v)] In a single sideband suppressed carrier transmission system, one or more auxiliary oscillations are transmitted, and a fixed ratio exists between each of the frequencies of the auxiliary oscillations and the suppressed carrier wave, or, where frequency transformation is introduced, between the differences between each of these frequencies and the suppressed carrier. The system enables oscillations to be produced at the receiver for detection purposes which are synchronized with the suppressed carrier of the transmitter independently of variations of this frequency at the transmitter. The first embodiment assumes that the frequencies of the two auxiliary oscillations n1f and n2f and the suppressed carrier n,f are derived at the transmitter from a common frequency f, and Fig. 1 shows the I.F. and detecting portions of a suitable receiver. The auxiliary oscillations may be located an equal distance (e.g. 1 kc.) above and below the signal frequency band respectively. Local oscillations at frequency t from oscillator 2 are mixed with the incoming signals in stage 1 to produce oscillations at n1f+t and n2f+t, which are selected respectively by filters 4 and 5. Filter 3 selects the signal sideband of the suppressed carrier wave. The outputs from filters 4 and 5 are supplied to mixing stage 6 which supplies the difference frequency (n1-n2)f to frequency detector 7. Oscillations from a control oscillator 8, the frequency of which is as nearly as possible equal to " f," the fundamental frequency at the transmitter, are passed to harmonic producer 9, which provides the (n1 - n2)th harmonic for application to frequency detector 7. Frequency detector 7 supplies a control voltage dependent on the difference between the two applied frequencies, which controls a motor 10 to regulate the tuning of oscillator 8, so that its frequency is made continuously equal to the fundamental frequency f at the transmitter. The frequency t of oscillator 2 is a harmonic of the frequency f of oscillator 8 so that the transformed suppressed carrier frequency n3f/+t is then also a harmonic, whereby the required carrier wave may be supplied from harmonic producer 9 to detector 11. To avoid excessive multiplication, the frequency t of oscillator 2 is controlled by frequency detector 13, which provides a control voltage dependent on the difference between the frequency n1f+t from filter 4, and a corresponding selected harmonic from harmonic producer 9. If this condition is met, then frequency t must also be a harmonic of the frequency f. The second embodiment assumes that the transmitted frequencies of two or more auxiliary oscillations and the suppressed carrier are obtained by sum or difference transformation of oscillations derived from a common frequency f so that the ratios of the frequency differences are fixed. The oscillations from which the transmitted frequencies may be derived are assumed to be 49, 27 and 48 kc/s. -respectively, the single sideband extending from 28 to 48 kc/s. These frequencies, after transmission on short waves are received by an arrangement, as shown in Fig. 2. The input signals at I.F. frequency (289 to 311 kc/s.) are applied to mixing stage 1 together with the output from oscillator 2 to provide frequency transformed signals at 27 to 49 kc/s. The modulated signal side-band (28 to 48 kc/s.) is selected by filters 31, 3<11> and passed to detector 11. Filter 4 selects the first auxiliary oscillation (49 kc/s.) and filter 14 passes all except the first auxiliary oscillation (27 to 48 kc/s.). The output from filter 14 is mixed in stage 15 with the 22nd harmonic of oscillator 8 (which oscillates at 1 kc/s.), and the output of this plus the second auxiliary oscillation (27 kc.) yields a frequency of 49 kc/s., which after selection by filter 5 is applied to frequency detector 7. Detector 7 provides a control voltage to regulate the frequency of oscillator 8, such that the frequency from filter 4 is equal to that from filter 5, whereby oscillator 8 is synchronized with the fundamental transmission frequency f. In this embodiment, to avoid excessive frequency multiplication, the local carrier wave is generated by an independent oscillator 16 (48 kc/s.), means being provided to regulate the frequency of oscillator 2 also in accordance with the frequency of oscillator 16. The output from oscillator 16 is combined with a 1 kc. signal from oscillator 8, to produce a sum frequency (49 kc/s.) which is compared with the 49 kc. auxiliary oscillation from filter 4 in frequency detector 13, which regulates the frequency of oscillator 2 so that the two frequencies are equal. An automatic gain control voltage dependent on the intensity of the auxiliary oscillations may be provided at terminals 20. A simpler arrangement involving only one auxiliary frequency is also described (Fig. 3, not shown), but in this case it is not possible to switch to any desired wavelength. Wavelength may be changed in the Fig. 1 circuit, by altering the selected ratios, but this is not necessary in the Fig. 2 circuit, since in this case the frequency difference factor is independent of the wavelength. The signal bandwidth may be changed in the Fig. 2 circuit by changing one of the auxiliary frequencies and the appropriate filters. Other modifications (e.g. for multi-channel purposes), involving different or additional frequency transformations are described.

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