Abstract: A circuit for determining frequency response includes a voltage controlled oscillator (VCO) which generates a signal for application to a system under test and a counter which either counts quartz-stabilized pulses during a VCO period or which counts, during a quartz-stabilized counting interval, the VCO output itself. The counts represent with quartz accuracy the test frequency, which corresponds to the mean value within the counting interval, and are associated with respective measurement of signal level returned from the unit under test to thereby establish frequency response. Counts, together with corresponding levels developed by the system under test, are applied to a suitable display.

Abstract: The invention provides a DC-FM phase locked loop for use in transmitters for measuring and control purposes, wherein the modulation signal controls, via an A/D converter, the output frequency of a frequency synthesizer, this output frequency being used as the reference for the phase comparator. This permits modulation with extremely low frequencies or DC without the necessity to interrupt the loop.