Patent GB-942337-A (GB)
Title: Improvements in or relating to frequency synthesisers
Inventor: MAITLAND PETER
IPC: H03B21/04
Filed: 1961-03-23
Abstract: 942,337. Frequency synthesizers; automatic frequency control systems. RACAL ELECTRONICS Ltd. May 23, 1962 [March 23, 1961], No. 10565/61. Headings H3A and H3F. A frequency synthesizer comprises a source of harmonics of stable frequency and a plurality of variable frequency oscillators whose respective frequency settings determine different steps of the synthesizer output, each of the oscillators being variable over the same frequency range and being set to a predetermined frequency under the control of a selected one of the said harmonics and, except for the oscillator determining the lowest step in the synthesizer output, under the control also of the oscillator determining the next lower step in the synthesizer output. The source of harmonics comprises a crystal oscillator, Fig. 1, driving a harmonic generator which feeds into a set of filters from which the individual harmonic frequencies may be taken. The arrangement also comprises a number of decade steps each including a stable variable frequency oscillator, the desired output frequency, e.g. 3.1423 mc/s. being taken from the oscillator in the highest frequency step, Fig. 3, which is manually adjusted to approximately this frequency. This oscillator output is also mixed with a selected 100 kc/s. harmonic from the crystal oscillator, in this instance 2.8 mc/s. and the difference frequency of 0.3423 mc/s. extracted by means of a filter for application to a phasecomparator. The phase comparator includes an impedance converter and low-pass filter and in it this difference frequency is compared with an accurate frequency of 0.3423 mc/s. obtained from the previous step of the synthesizer; any difference is fed to a reactance valve which corrects the frequency of the final oscillator so as to reduce the phase comparator output to zero. The previous step of the synthesizer, Fig. 2, is similar to the highest frequency step, but terminates in a frequency divider, so that the desired frequency, e.g. 0.3423 mc/s. required for the last step may be derived from an oscillator working at 3.423 mc/s., i.e. over the same frequency range as the oscillator in the last step. The oscillator in this previous frequency step is so calibrated that it will be set manually to provide the frequency of 3.423 mc/s. , when adjusted to the second figure in the desired frequency, and a harmonic is again selected from the harmonic source so as to give a suitable difference frequency, e.g. 323 kc/s. for application to a phase comparator. Comparison is again made with an accurate frequency from the previous step and the difference used to pull the oscillator to the correct frequency. The preceding figures in the desired frequency are similarly set up in previous steps of the synthesizer, which are similar; in the first step, however, the comparison frequency will be a harmonic-as described 300 kc/s.-of the crystal source and it is derived therefrom by means of a multiplier (see Fig. 1). The Specification also refers to the modification of replacing the oscillator/divider combination in each of the earlier steps of the synthesizer with an oscillator/multiplier combination ; e.g. instead of generating 3-4 mc/s. and dividing to obtain 300-400 kc/s., generating at the latter frequency and obtaining the former by multiplication.