Pulse-Frequency Modulation (PFM) is a modulation method for representing an analog signal using only two levels (1 and 0). It is analogous to pulse-width modulation (PWM), to which the reader may refer for more detailed information, as the magnitude of an analog signal is encoded in the duty cycle of a square wave. If a pulse rate is set to 8000 pulses per second at 0 signal voltage then when the signal voltage reaches maximum the pulse rate will step up to 9000 but when the negative maximum voltage is reached the pulse rate will step down to 7000. A stable oscillator that is frequency modulated is used to create the pulse rate because of this PFM is not as widely used.[1] Unlike PWM, in which the width of square pulses is varied at constant frequency, PFM is accomplished using fixed-duration pulses and varying the repetition rate thereof. In other words, the frequency of the pulse train is varied in accordance with the instantaneous amplitude of the modulating signal at sampling intervals. The amplitude and width of the pulses is kept constant. The advantage of pulse frequency modulation is better immunity to noise interference than PAM. The disadvantage is more complex transmitter and receiver design. [2]
It has been proposed that PFM could serve as a suitable retinal prosthesis device. PFM's ability to operate independently and asynchronously promotes the flow of nutrients through the chip which is essential for living cells. Cell stimulation would be accomplished through the output of pulse streams by PFM. A large dynamic range is also practical in the replacement of photoreceptors. [3] PFM could be used to transmit intelligence signals, such as audio signals, by demodulating the signal from the receiving end at the transmitting end.[4]
An example of PFM instrument design can be viewed: http://server.elektro.dtu.dk/ftp/database/Data_CDs/Component_data/Maxim_2001/0012/VIEW_033.HTM
Lenk, John D. (1999). "Circuit Troubleshooting Handbook" p242. McGraw-Hill, New York