It works by having a voltage-controlled oscillator (VCO) inside, and driving the VCO with a voltage signal that depends on the frequency difference ... S&H oscillator goes low, the pitch holds ...

055TSMC_VCO_01 is voltage controlled oscillator (VCO) designed to generate a high frequency signal in the range from 3.5 to 7 GHz. The VCO consist of four LC-cores with frequency ranges: 3.5 – 4.15GHz ...

But the design goals are pretty clear: build an oscillator with as little distortion as possible across the audio frequency ... gain control (AGC) that uses a homebrew voltage-controlled resistor ...