Patent GB-1420271-A (GB)
Title: Electronic frequency changing apparatus
IPC: H03B21/02
Filed: 1973-07-27
Abstract: 1420271 Frequency changing, heterodyne circuits SOC D'ETUDE ET D'APPLICATION DES TECHNIQUES NOUVELLES NEO-TEC 27 July 1973 [27 July 1972] 35849/73 Heading H3F and H3Q A frequency changing circuit comprises a heterodyne frequency source 1, Fig. 1. of high stability, a divider circuit 2-5 capable of furnishing at least one heterodyne signal whose frequency is selected from a plurality of successive multiples of a frequency f, and a mixing circuit 7, the heterodyne signal being so selected that the output F s1  from the mixer 7 has a frequency between f/2 and f. In this embodiment the frequency f is 80 kHz and four counter dividers 2-5 are provided, the outputs from which are selected by means of a switch 6 and the signal path includes a band pass filter 8 for cutting out the image frequency. Alternatively, each divider may feed a separate mixer, the input signal being applied to all the mixers and the output being selected from the mixers by means of a switch. The arrangement of Fig. 1 forms the first stage 10, Fig. 3 of a frequencychanging arrangement which reduces an incoming signal of 240-480 kHz to a frequency of 40-80 kHz. It is followed by a second stage 20 of similar type which produces a further reduction of 8-16 kHz, feeding a final stage 30. The final stage 30 comprises a controlled synthesiser 31 followed by a divider 32; the final output is at 4 Hz and is filtered by filter 50 having a passband of 0À5 Hz. Image frequency rejection circuit.-In an arrangement which rejects the image frequency, the stage 30, Fig. 4, comprises a controlled synthesiser 31 followed by divider 32A dividing x k 2 / 2  and giving the waveform 2 Hz, Fig. 5. Divider 32A is followed by x 2 divider 32B which gives the waveform Hz, Fig. 5. The outputs from dividers 32A, 32B from the two inputs of an exclusive-OR gate 37, the output of which is the waveform H<SP>1</SP>z, Fig. 5, which is the same as the waveform Hz but with a 90 degree phase-shift. The incoming signal f e  is fed via bandpass filter 33 to two mixers 34A, 34B the heterodyne inputs to which comprise the waveforms Hz, Hz<SP>1</SP> respectively. The outputs from the mixers 34A, 34B are combined by an RC network R, C chosen so that where f s  is the output frequency (4 Hz). Synchronous detector.-In a modification for use with a synchronous detector 63, Fig. 6, the stage 30 comprises a controlled synthesiser 31 adjusted to give an output at K 2 f e  where f e  is the signal frequency. The heterodyne frequency source 1 is also coupled to a divider 61 to obtain an output at K 2 f s  where f s  is the desired output frequency from the mixer stage 30. The outputs from synthesiser 31 and divider 61 are applied to subtracting circuit 38 to obtain an output at K 2  (f e -f s ) which is applied to counter divider 32 to obtain the frequency (f e -f s ) which forms the heterodyning input to mixer 34. The output from divider 61 is also applied to a second divider 62 which provides the demodulating input to synchronous detector 63. The Specification also describes an arrangement in which an incoming signal containing two different frequencies is applied simultaneously to two frequency-dividing circuits of the type described with reference to Fig. 3, the outputs of which are combined over switches which permit of the selection of one or other of the component frequencies (Fig. 7, not shown): alternatively, separate outputs may be provided for the two.