Quasi-peak detector

Quasi-peak means 'not quite peak', or 'aiming towards peak but not actually peak'. The term is commonly used when referring to electronic detectors or rectifiers. Despite the above definition, the term quasi-peak should not be interpreted as vague in any way. Actual quasi-peak detectors for specific purposes usually have precisely defined dynamic characteristics, in tems of 'attack time', 'integration time', and 'decay time' or 'fall-back' time.

Quasi-peak detectors play an important role in Electromagnetic compatibility (EMC) testing of electronic equipment, where allowed levels of electromagnetic interference (EMI), also called radio frequency interference (RFI), are given with reference to measurement by a specified quasi-peak detector. This was originally done because the quasi-peak detector was believed to better indicate the subjective annoyance level experienced by a listener hearing impulsive interference to an AM radio station . Over time standards incorporating quasi-peak detectors as the measurement device were extended to frequencies up to 1 GHz , although there may not be any justification beyond previous practice for using the quasi-peak detector to measure interference to signals other than AM radio . The CISPR does not currently limit EMI at frequencies above 1 GHz, and the FCC specifies allowed EMI above 1 GHz with reference to an average power detector . The quasi-peak detector parameters to be used for EMC testing vary with frequency .

Conceptually, a quasi-peak detector works like a peak detector followed by a lossy integrator. A voltage impulse entering a narrow-band receiver produces a short-duration burst oscillating at the receiver center frequency. The peak detector is a rectifier followed by a low-pass filter to extract a baseband signal consisting of the slowly (relative to the receiver center frequency) time-varying amplitude of the impulsive oscillation. The following lossy integrator has a rapid rise time and longer fall time, so the measured output for a sequence of impulses is higher when the pulse repetition rate is higher. The quasi-peak detector is calibrated to produce the same output level as an average power detector when the input is a narrow-band signal in the receiver bandwidth

In Audio quality measurement quasi-peak rectifiers are specified in several standards. For example ITU-R 468 noise weighting uses a special rectifier incorporating two cascaded charging time constants. The PPM or Peak programme meter used to measure programme levels is actually a quasi-peak reading meter, again with precisely defined dynamics. Flutter measurement also involves a standardised quasi-peak reading meter. In every case the dyamics are chosen to reflect the sensitivity of human hearing to brief sounds, ignoring those so brief that we do not perceive them, and weighting those of intermediate duration according to audibility.