3-31D2 B
A circuit that compares the output of a voltage-controlled oscillator (VCO) to a frequency standard and produces an error voltage that is then used to adjust the capacitance of a varactor diode used to control frequency in that same VCO is called what?
A. Doubly balanced mixer.
B. Phase-locked loop.
C. Differential voltage amplifier.
D. Variable frequency oscillator.
