Tuning circuit for HF receiver with local oscillator - has tuning voltage varied so that rate of tuning decreases as required frequency is approached

摘要

Local oscillator is a VCO. In order to generate a tuning voltage a comparator compares a first number which represents the state of a counter which periodically counts the oscillations of the local oscillator, taking into consideration the i.f., with a second number fed through a coder to a store and representing the sender. The two numbers are compared digit by digit, beginning with the highest. A store (10) is provided for storage of the comparison result, and an amplifier (16) generates the tuning voltage from the store (10) outputs through resistors (11-15) of different values, so that the rate of tuning depends on the number of equal digits in the two numbers. This rate is high when all digits are different, and decreases as the number of equal digits increases.