GB1371914A - Oscillator frequency control circuits - Google Patents

Abstract

1371914 Automatic frequency control; frequency detectors SIEMENS AG 21 Oct 1971 [13 Nov 1970] 48895/71 Heading H3A In an oscillator a.f.c. circuit: the output of a frequency modulated oscillator 02, is compared with a reference frequency from a stable, e.g. crystal, oscillator O1 in a control loop containing two mixers M1, M2, one of the signals being applied to both mixers in phase, and the other being supplied to both mixers in quadrature, e.g. via phase shifters RC, RL; the output of one mixer M1 is fed to an integrator 3G1 ; the output of the other mixer M2 is fed to a differentiator DG; and the outputs from the integrator and differentiator are fed to a circuit AS, SS, GR which compares their phase difference to produce a rectified control voltage for the oscillator 02. The difference frequency outputs of the two mixers are in phase quadrature, and the integrator and differentiator function as low and high pass filters respectively to give a further phase shift to the difference frequency components so that the resulting signals U1 are either in phase or in antiphase. The signals Ul, U2 are then fed to sum and difference circuit AS, SS, the outputs from which are rectified, filtered and combined in amplifier GV to provide the frequency control signal for oscillator 02. In a transistorized circuit form, Fig. 5 (not shown), the sum and difference circuits comprise two transistors (T s 6, T s 7) connected as a long-tailed pair, with a third transistor (T s 5) as the "tail". The signal from the differentiator (C16, R16) is applied to the third transistor and thus arrives at the collectors of the other in the same phase. The signal from the integrator (R13, C14) is applied to the other two in push pull, so that sum and difference components occur at the collectors of these transistors (T s 6, T s 7).