Practical Paper

The Quasi-Peak Detector

By Edwin L. Bronaugh, ANSI ASC C63 Historian

Many modern EMC practitioners have asked how the Quasi-Peak detector came about. In this article, I will attempt to provide some historical answers to this question. But first, some EMC history may be appropriate. The science of EMC started out in the 1920s and 1930s as an effort to solve problems with what would later be called RIV (radio influence voltage) and RIF (radio influence field-strength).1 In those days, this "science" dealt entirely with "noises" which interfered with radio broadcast reception and the reception of government and licensed commercial services.

Quoting from [1], "Almost from the beginning of radio broadcasting, the electric utility companies were faced with problems of radio noise. In 1924 the National Electric Light Association appointed a committee to study the subject. The manufacturers of electric power equipment had encountered similar problems, and in 1930, a subcommittee of the NEMA Codes and Standards Committee was set up. The following year, the EEI-NEMA-RMA2 Joint Coordination Committee on Radio Reception was organized."

The EMC efforts addressed mostly unintentionally generated man-made radio noises such as noise from power lines (probably corona and leakage noise), switching transients, electric motor commutator sparking, automobile ignition noise, etc., and some natural phenomena such as atmospheric noise and signal fading. In those days, these efforts were far from a science because the phenomena of concern were not well understood, and solutions for the interference problems were often considered akin to "black magic." The instruments used in those days were relatively simple radio broadcast and communications receivers sometimes accompanied by an external audio frequency voltmeter to provide a somewhat less subjective notion of the amount of radio-noise being received.

During this time, the CISPR3 had been organized and undertook to develop a method of voltage measurement in the frequency range from 150 kHz to 1605 kHz. To develop the method and an instrument, an assessment of interference related to its effect on the reception of sound broadcasting was needed. As mentioned above, much of the interference was impulsive in nature and its effect increased with increasing repetition rate in a way that was shown to be approximated by a quasi-peak detector circuit with appropriate time constants. During this development, engineers and scientists from both Europe and North America were involved in the CISPR, since it was an international organization.

In the 1930s, a board of listeners was formed to decide what characteristics of a radio disturbance caused annoying interference, and the degree of annoyance, for listeners to radio broadcast (sound) reception. The broadcast receiver of the day received signals in the LF or MF bands, and had an IF bandwidth of between 8 kHz and 10 kHz. The desired signal was a carrier with voice or music amplitude modulation. Using a radio broadcast receiver equipped with an audio output voltmeter, the board of listeners rated the annoyance of the interference with its audio output and its particular pulse repetition frequency. Each member of the board of listeners was said to have worked independently, so that the results would be statistically useful. Out of this study came the specifications for the quasi-peak (QP) detector used in the first CISPR Radio-Noise Meter. When radio broadcasting was extended into the HF band, the frequency range of the CISPR Radio-Noise Meter was extended upward from 1605 kHz to 30 MHz. Since the later radio broadcasting services to be protected had about the same characteristics as the earlier ones, the QP detector from the early CISPR Radio-Noise Meter was retained, and did a good job predicting the interference effects of radio disturbances. CISPR Publication 1 was the specification for this radio-noise meter.

The CISPR has extended the quasi-peak technique to a much broader range of frequencies over the years. Currently, CISPR radio-noise meters use the QP detector from 9 kHz to 1 GHz; and, there is discussion of extending it into the GHz frequency range. I have not read anything that indicates that the QP detector, even with different bandwidths and time constants, is really appropriate to measure interference to radio and television broadcasting and radio communications above 30 MHz. As far as I can tell, there was no formal "board of listeners or viewers" to decide that some form of QP detector appropriately predicted the interference effect of various radio disturbances to the radio services operating above 30 MHz. But, right or wrong, quoting from [2], "Instruments using the quasi-peak detector still remain as the basic reference for determining compliance with CISPR limits."
This has been a short historical sketch of the QP detector. Many technical and historical questions remain unanswered. I am therefore extending this invitation to any readers out there who can add to it. Perhaps we can finally put together a really good history of the QP detector and archive it so it won't get lost, again. Some questions that were asked by an anonymous reviewer deserve answers, but require much research. They are (in no particular order):

  1. Who actually designed the first QP detector and why QP?
  2. For many years CISPR QP and ANSI QP were different. How did this happen and why was the ANSI QP dumped in favor of the CISPR QP?
  3. How were the charge and discharge time constants selected for the first QP detectors and why were they changed over the years? How was the bandwidth selected? [Note: the 9 kHz bandwidth was the prevalent bandwidth of radio receivers for sound broadcast reception at the time. EdB]
  4. The dynamic range of the first CISPR meters was less than 15 dB, whereas the first Stoddart meters had a dynamic range of 40 dB. Why did the CISPR meter have this limitation and how did Stoddart get around it?
  5. Who put together this first board of listeners? What were they asked to do and what meters with what detectors were used in this work? Was the QP detector really designed as a result of this work?
  6. When CISPR extended the QP technique to a broader range of frequencies, how did they come up with the time constants in this meter since they are not the same as the time constants for the lower frequency meter?
  7. What about the T&D Committee of the IEEE PES "board of listeners" that evaluated TV reception in the presence of power line interference several years ago?
  8. With the proliferation of communication systems, one could easily ask if any single detector can adequately predict the interference effect of all the various disturbances on all the various communications systems.


  1. ASA4 C63.4-1963, American Standard Methods of Measurement of Radio-Noise Voltage and Radio-Noise Field Strength 0.015 to 25 Megacycles/ Second Low-Voltage Electric Equipment and Nonelectric Equipment, p. 3.
  2. CISPR Publication 16:1987, C.I.S.P.R. specification for radio interference measuring apparatus and measurement methods, Second Edition, pp. 13 & 15.
  3. E. L. Bronaugh, "Introduction to Electromagnetic Compatibility: Review, History and Trends," Proceedings of the 17th Electrical/ Electronics Insulation Conference, IEEE EIS, NEMA and ICWA, Boston, MA, Sept. 30 - Oct. 3, 1985, p. 177.
  4. E. L. Bronaugh and W. S. Lambdin, Electromagnetic Interference Test Methodology and Procedures, Vol. 6, Handbook series on Electromagnetic Interference and Compatibility, Interference Technologies, Inc., State Route 625, P.O. Box D, Gainesville, VA 22065, U.S.A., 1988, pp. 1.22-1.24 & 3.35-3.38.
  5. ANSI C63.4-1981, American National Standard Methods of Measurement of Radio-Noise Emissions from Low-Voltage Electrical and Electronic Equipment in the Range of 10 kHz to 1000 MHz, p. 3.
  6. CISPR Publication 7:1969, Recommendations of the C.I.S.P.R., Amendment No. 1:1973; and supplements. CISPR Publications 7A:1973, First supplement; and, 7B:1975, Second supplement.
  7. CISPR Publication 8:1969, Reports and Study Questions of the C.I.S.P.R., Amendment No. 1:1973; and supplements. CISPR 8A:1973, First supplement; CISPR 8B:1975, Second supplement, Amendment No. 1:1980; CISPR 8C:1980, Third supplement; and, CISPR 8D:1982, Fourth supplement.

Biographical Notes

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Edwin L. (Ed) Bronaugh is a Life Fellow of the IEEE and an Honorary Life Member of the EMC Society. He has often served on the EMC Society Board of Directors, and is a past president of the Society. He is also a distinguished lecturer on EMC topics. He is a member of the EMC Standards Committee and represents the IEEE on ANSI-Accredited Standards Committee C63; of which he is Vice Chairman. He is a member of the US Technical Advisory Groups for CISPR, CISPR/A and CISPR/I. The EMC Society has awarded him several of its highest awards including the Richard R. Stoddart Award, the Lawrence G. Cumming Award and the Standards Medallion, and the IEEE Third Millennium Medal in 2000. He has authored a book on EMI measurements and authored over 150 papers in professional meetings and publications. He is a senior member of the National Association of Radio and Telecommunications Engineers (Certified EMC Engineer). He is listed in Who's Who in America, Who's Who in the World, Who's Who in Science and Engineering, Who's Who in the South and Southwest, and Men of Achievement. Mr. Bronaugh is Principal of EdB EMC Consultants, an independent EMC consulting firm. Previously, he was Lead Engineer for Siemens for Hardware Design Assurance at Siemens Communication Devices, Austin, Texas, Vice President for Engineering at the Electro-Mechanics Company, Technical Director of Electro-Metrics, and Manager of EMC Research at Southwest Research Institute. He may be reached at EMC

1 We did not get around to calling it radio frequency interference (RFI) until much later, and then much, much later we started calling the science Electromagnetic Compatibility (EMC).

2 Edison Electric Institute, National Electrical Manufacturers Association, and Radio Manufacturers Association.

3 CISPR is an abbreviation made up of the initials of the French for "International Special Committee on Radio-Interference."

4 American Standards Association, Inc., forerunner of the American National Standards Institute, Inc. (ANSI).

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