Pulse-frequency modulation
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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), in which the magnitude of an analog signal is encoded in the duty cycle of a square wave. Unlike PWM, in which the width of square pulses is varied at a constant
frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...
, PFM fixes the width of square pulses while varying the
frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...
. In other words, the
frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...
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 are kept constant.


Applications

PFM is a method of encoding analog signals into trains of square pulses and therefore has a wide variety of applications. There are practical difficulties in the design of electronics when working with non-fixed frequencies, such as transmission line effects in board layout and magnetic component selection, so generally, PWM mode is preferred. There are, however, select cases in which PFM mode is advantageous.


Buck converters

PFM mode is a common technique for increasing the efficiency of switching step-down DC-DC converters ( buck converters) when driving light loads. In medium to high loads, the DC resistance of buck converter switching elements tends to dominate the overall efficiency of the buck converter. When driving light loads, however, the effects of DC resistances are reduced and AC losses in the inductor, capacitor, and switching elements play a larger role in overall efficiency. This is especially true in discontinuous mode operation, in which the inductor current drops below zero, resulting in the discharging of the output capacitor and even higher . PFM mode operation allows the switching frequency to be reduced and for a control method that prevents the inductor current from dropping below zero during light loads. Rather than applying square pulses of varying widths to the inductor, square pulse trains with a fixed 50% duty cycle are used to charge the inductor to a predefined current limit then discharge the inductor current to, but not below, zero. The frequency of these pulse trains is then varied to produce the desired output voltage with the aid of the output filter capacitor. This allows for a number of switching loss savings. The inductor is given known levels of peak current, which, if chosen carefully in regards to saturation current, can reduce switching losses in its magnetic core. Since the inductor current is never allowed to fall below zero, the output filter capacitor is not discharged and does not have to be recharged with every switching cycle to maintain the proper output voltage. All of this done at the expense of output voltage and current ripple, which increases as a result of the reduction in switching frequency and the gap between pulse trains.


See also

*
Pulse-amplitude modulation Pulse-amplitude modulation (PAM) is a form of signal modulation where the message information is encoded in the amplitude of a series of signal pulses. It is an analog pulse modulation scheme in which the amplitudes of a train of carrier pulse ...
*
Pulse-code modulation Pulse-code modulation (PCM) is a method used to digitally represent sampled analog signals. It is the standard form of digital audio in computers, compact discs, digital telephony and other digital audio applications. In a PCM Stream (comp ...
*
Pulse-density modulation Pulse-density modulation, or PDM, is a form of modulation used to represent an analog signal with a binary signal. In a PDM signal, specific amplitude values are not encoded into codewords of pulses of different weight as they would be in puls ...
*
Pulse-position modulation Pulse-position modulation (PPM) is a form of signal modulation in which ''M'' message bits are encoded by transmitting a single pulse in one of 2^M possible required time shifts. This is repeated every ''T'' seconds, such that the transmitted bi ...
*
Rate coding Neural coding (or Neural representation) is a neuroscience field concerned with characterising the hypothetical relationship between the stimulus and the individual or ensemble neuronal responses and the relationship among the electrical activity ...
, pulse-frequency modulation in living systems


References

*Lenk, John D. (1999). "Circuit Troubleshooting Handbook" p242. McGraw-Hill, New York {{refend


External links


Determining Buck Converter Efficiency in PFM Mode

Pulse Frequency Modulation in Google Patents

Introduction to Buck Converters: Understanding Mode Transitions
Contains a video with a nice description of PFM in the buck converter application. Quantized radio modulation modes de:Pulsfrequenzmodulation ta:துடிப்பு அதிர்வெண் குறிப்பேற்றம்