Verified answer

Ok by your rules.

suppose we use a phase locked loop (PLL) as a demonstration. A pll is a circuit that is fed an outside waveform and forces a local oscillator to stay locked at some multiple of the that signals frequency.

At the heart of a PLL is acircuit known as a voltage controlled oscillator(VCO). The frequency of the oscillator can be described as

F = Fo + Vcontrol x F1

in other words, the control voltage varies in the same way as the input frequency as long as the pll stays locked.

Breaking your rules

if you want to find out more about any of these things or related ones, google the following:

PLL

VCO

Armstrong detector

ratio detector

phase detector

An FM detector (or demodulator) works by setting a tuned network to a frequency near the carrier. As the carrier frequency varies the voltage across the tuned circuit varies. So in effect the tuned circuit translates frequency modulation to amplitude modulation which can then be rectified and amplified.

A detector is a device that recovers information of interest contained in a modulated wave. The term dates from the early days of radio when all transmissions were in Morse code, and it was only necessary to detect the presence (or absence) of a radio wave using a device such as a coherer without necessarily making it audible. A more up-to-date term is "demodulator", but "detector" has a history of many decades of use, even if it is a misnomer.