Broadcast receiver input stage tuning system - provides at input stage tuning voltage which is derived from storage capacitor and is free of residual ripple

摘要

Previous variable duty cycle tuning systems suffered from ripple; in proposed circuit, a coincidence testing circuit determines whether a received HF signal is a television channel frequency. If so, a high value signal is output by a Schmitt trigger. A logic gate responds to the signal to render non-conductive a field effect transistor in a switching circuit. Thus the tuning voltage output by signal converter in response to coarse and fine tuning signals is no longer passed to the output terminal of the circuit. Instead, a ripple-free voltage stored on the capacitor which has the same amplitude as the tuning voltage is passed to the output terminal. The automatic frequency control circuit acts to vary the voltage stored on the capacitor so that it stores the voltage required to tune the receiver precisely to the required broadcast frequency.