Patent GB-961028-A (GB)
Title: Improvements in frequency synthesis apparatus
Inventor: HERMAN RALPH BERTRAND
IPC: H03B21/02
Filed: 1960-01-14
Abstract: 961,028. Frequency synthesising arrangements. PLESSEY CO. Ltd. Jan. 26,1961 [Jan. 14, 1960], No. 1352/60. Heading H3F. A frequency-synthesising arrangement comprises a generator of a basic frequency, means for deriving from its output a series of harmonics, and a number of sections each comprising two or more balanced mixers, at least one band-pass filter and a selector switch. The mixers in the first section derive one input from a harmonic of the basic frequency, while those in each succeeding section derive one input from the preceding section. The second input of each of the mixers comprises a harmonic of the basic frequency, the corresponding band-pass filter being in each case such as to pass the sum frequency and reject the difference. In one embodiment the harmonic 3f is fed to the two mixers 3, 4, Fig. 1, of the first section which are also fed with the harmonics 4f and 3f respectively. Each mixer is followed by a band-pass filter 8, 9 respectively, passing the sum frequency 7f, 6/ respectively to the selector switch 13. A selected frequency is passed to the two mixers 15, 16 of the second section, which are also fed with the harmonics 8f, 7f respectively. Each mixer is followed by a band-pass filter 20, 21 respectively passing the frequencies 14f-15f and 12f-13f respectively to the second selector switch 25. A selected frequency is passed to the two mixers 27, 28 of the third section, which are also fed with the harmonics 16f, 14f respectively. Each mixer is followed by a band-pass filter 32, 33 respectively passing the frequencies 28f-31f and 24f-27f respectively to the third selector switch 37. The arrangement is such that any integral frequency in the range 24f-31f may be selected: if the frequency channels are numbered consecutively in increasing order and according to the binary system, then any required frequency may be selected by setting the switches so that the number, also in the binary system, represented by the set of switch positions is the same as the number of the frequency channel. A large ratio of maximum to minimum frequency may be obtained by mixing the final output signal with a stable frequency source and extracting the difference frequency. In a modification (Fig. 2, not shown) a single filter is used in each section, being placed on the output side of the selector switch, while in a further modification of this arrangement (Fig. 3, not shown) the filter is in each case followed by a frequency halving circuit. In this case all the sections are made identical and the minimum output frequency of the arrangement is the same as the input frequency, the frequency spacing between channels being halved for each additiinal section added. A suitable halving circuit comprises a balanced mixer 2, Fig. 4, followed by a low-pass filter 3 and amplifier 4, the output of which is at half the input frequency and constitutes the second input to the mixer 2. The frequency synthesising arrangement may be extended to a radix greater than two by using further balanced mixers in each section, a switch with a corresponding number of positions and a corresponding dividing circuit (Fig. 5, not shown). To obtain a sufficient rejection against unwanted frequency components in the output signal, phase locked oscillators may be introduced, locking on to the output signal of the synthesizer or a section of it; alternatively, the frequency dividers may be designed to serve this purpose.