Quasi-peak detector

Lore & Background

The quasi-peak detector was originally developed because it 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. Both CISPR and the U.S. Federal Communications Commission (FCC) limit EMI at frequencies above 1 GHz with reference to an average-power detector, rather than quasi-peak detector.

Conceptually, a quasi-peak detector for EMC testing 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 centre frequency. The peak detector is a rectifier followed by a low-pass filter to extract a baseband signal consisting of the slowly 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 a peak-power detector when the input is a continuous wave tone.

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 peak programme meter (PPM) 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 dynamics 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.

Reader's Guide

The quasi-peak detector's significance lies in its standardized use for EMC testing, where it provides a measurement that correlates with the subjective annoyance of impulsive interference to AM radio reception. Its defined attack and decay times, varying with frequency band, allow consistent comparison of emissions across different equipment and laboratories. The CISPR quasi-peak detector, defined in Publication 16 of the International Special Committee on Radio Interference (CISPR) of the International Electrotechnical Commission (IEC), is applied to conducted emissions measurements (0.15–30 MHz) with an attack time of 1 ms, decay time of 160 ms, and IF filter bandwidth of 9 kHz, and to radiated emissions measurements (30–1000 MHz) with an attack time of 1 ms, decay time of 550 ms, and IF filter bandwidth of 120 kHz. Its legacy extends beyond EMC into audio measurement, where quasi-peak rectifiers are used in ITU-R 468 noise weighting, peak programme meters, and flutter measurement, all designed to reflect human hearing's sensitivity to brief sounds. Despite its widespread use up to 1 GHz, the notes indicate that there may not be justification beyond previous practice for applying the quasi-peak detector to signals other than AM radio, and above 1 GHz both CISPR and the FCC use an average-power detector instead.

Did You Know?

• The quasi-peak detector was originally believed to better indicate the subjective annoyance of impulsive interference to an AM radio station.
• The CISPR quasi-peak detector for conducted emissions (0.15–30 MHz) has an attack time of 1 ms and a decay time of 160 ms.
• For radiated emissions (30–1000 MHz), the CISPR quasi-peak detector uses an attack time of 1 ms and a decay time of 550 ms.
• The peak programme meter (PPM) is actually a quasi-peak reading meter with precisely defined dynamics.