Discrimination of a Synchronized Oscillator against Interfering Tones and Noise

An account is given of the discrimination provided by a synchronized oscillator against unwanted signals accompanying the synchronizing tone; the synchronized oscillator is more specifically a non-linear regenerative tuned circuit. The discrimination is partly due to the frequency response of the system, but has also an important contribution from the non-linear behaviour of the circuit, provided that the wanted (sychronizing) signal has a greater amplitude, after allowing for frequency response, than the unwanted (interfering) signals. Section 2 gives a general description of the phenomena of synchronization and non-linear discrimination, illustrated by experimental observations of frequency response and non-linear discrimination in resistance-capacitance-tuned oscillators. Both single-frequency and noise interferences are considered. It is concluded that very large amounts of discrimination can be obtained when the natural frequency of the oscillator is very close to the synchronizing frequency, so that only very small amplitudes of the latter are needed to maintain synchronism. In more practical cases, where reasonable amounts of variation of both natural and synchronizing frequencies must be allowed for, only smaller amounts of discrimination due to nonlinearity can be obtained, but 10 dB or more is quite feasible. This amount will be important when the interfering frequencies are so close to the synchronized frequency that the amount of frequency discrimination is negligible. For instance, use is made of these effects in the homodyne and synchrodyne demodulators. It is shown that when the synchronizing tone is not dominant or is absent no suppression of the interference or narrowing of its spectrum is obtained. Section 3 gives a mathematical analysis of the effects, on the assumption that the non-linear law of the system has no terms above the cubic, and that the feedback loop has uniform amplitude and phase response over the frequency band covered by the appli- ed signals. Equations are developed for the improvement of signal/noise ratio and suppression of single-tone interference. Interference due to amplitude modulation and phase modulation of the synchronizing tone is also dealt with, and it is shown that the suppression of amplitude modulation is greater than that of signals merely accompanying the synchronizing tone, but that phase modulation is not suppressed at all.