The Synthesis of High-purity Oscillations Suitable for Single-sideband Receivers

An oscillator is controlled by a phase locked loop, its output frequency being mixed with a selected part of a 100 kc/s spectrum to form an i.f. in the 900¿1000 kc/s region. This i.f. feeds one side of a phase detector, the other side being fed by an interpolating signal. The latter may be derived either by another similar stage operating at a lower frequency, or by any other suitable method, the required purity in this signal being only of the order of 40 dB. The a.p.c. loop will lock when the i.f. and the interpolating frequency are the same. In the locked condition an additional filter is switched into the loop by means of a quadrature phase detector, and this filter reduces the amplitude of spurious frequencies and noise in the output. The paper discusses factors affecting the resultant purity.