What is Pulse Repetition Frequency?

Pulse Repetition Frequency (PRF) is the number of pulses of a repeating signal in a specified unit of time. The unit of time is usually one second. The PRF tells us how many pulses of a signal are repeated in one second. This term is particularly associated with radar applications. In radars, there are two types of frequencies, one is the carrier frequency on which the radio signal is modulated. The other is the Pulse Repetition Frequency (PRF) which is the number of times the radar pulse is sent out. Both are measured in cycles per second or hertz (Hz), but PRF is usually much lower than carrier frequency.

As radar systems usually consist of a powerful transmitter and sensitive receiver connected to the same antenna, PRF is one of the defining characteristics of radar technology. After producing a short radio pulse, the transmitter has to be turned off so that the receiver can hear the reflections of that signal off distant targets. Since the radio signal has to travel out to the target and back again, the radar’s desired range of operation is dependent on the quiet-period (transmitter ‘off’ state) between 2 transmission pulses which is defined by PRF. Longer range signals require longer quiet-periods or lower PRFs and similarly higher PRFs produce shorter maximum ranges but broadcast more pulses and radio energy in a given time. Therefore, higher PRFs create stronger reflections that make detection easier. Radar systems balance the two competing requirements of range and pulse energy for optimum usage of the technology for various purposes.

Where,

PRF = Pulse Repetition Frequency (Hz)

PRT = Pulse Repetition Time

c= 3 x 108 m/s (Speed of Light)

In older radar electronics, PRFs were generally fixed to a particular value or maybe switchable among a limited set of values. Hence, older radar systems had a characteristic PRF which was used to identify the type or class of a particular platform such as a ship or aircraft, or in some cases, a particular unit. Radar warning receivers in aircrafts included a library of common PRFs which could identify not only the type of radar but in some cases the mode of operation too. This allowed pilots to be warned when they were being targeted by enemies. Modern radar systems have more advanced electronics which allow them to change their PRF, pulse width and carrier frequency, making identification much more difficult.

Other Pulsed systems like sonar and LiDAR also have PRFs. In sonars, the term pulse repetition rate (PRR) is more commonly used, although it has the same concept as PRF.