Quasi-peak detection is a form of detection where a signal level is weighted based on the repetition frequency of the spectral components making up the signal. That is to say; the result of a quasi-peak measurement depends on the repetition rate of the signal.
Signals can be classified into two general categories based upon their repetition rate: narrowband or broadband.
A narrowband signal is a signal that can be resolved by the spectrum analyzer. An example of a narrowband signal is a continuous wave (CW) signal. A CW signal is one signal at a fixed frequency. A broadband signal is a signal that cannot be resolved by the spectrum analyzer. An example of a broadband signal is a pulse signal. Peak, quasi-peak, and average detection will yield the same amplitude level for a narrowband signal. A broadband signal will yield a quasi-peak level lower than the peak level.
The weighting (accounted for through specific charge and discharge time constants in the quasi-peak detector circuit), is a function of the repetition frequency of the signal being measured. The lower the repetition frequency, the lower the quasi-peak level.
Many agencies governing the electromagnetic interference (EMI) from comercial products require quasi-peak detection to be used. Even if the emission from a device is over a test limit when measured with peak detection, the device will be considered to pass if the quasi-peak level is below the test limit.
Because of the charge and discharge time constants used in the quasi-peak detector, the spectrum analyzer must sweep considerably slower when the quasi-peak detector is on. Since the quasi-peak level of a signal is always equal to or less than the peak level of that signal, quasi-peak detection need be employed only when a signal is close to or over the test limit when measured with peak detection.
For initial measurements, peak detection is used since peak detection does not require the slower sweep times.