Abstract: 1. Means for searching across a frequency band and instantly stopping at any received frequency, comprising a detector, means for heterodyning said received frequency being connected to said detector, variable oscillator means, means for periodically varying said oscillator over a range of frequencies, means for injecting the frequencies of said oscillator in discrete steps into said heterodyning means, a pulse generator, a pulse-blocking bistable gate means coupling the output of said pulse generator to said oscillator means, pulse-shaping means associated with said detector to instantly form a pulse upon reception of said any received frequency, and means connecting the output of said pulse-shaping means to said gate means to open it in response to pulses from said pulse-shaping means, whereby the frequency variation of said oscillator means is discontinued and it provides a particular frequency in response to the output of said pulse-shaping means.