Quasi-Peak Detector: Preparation, Placement and Process Control

A pre-scan with a peak detector shows a point above the limit line, and the conclusion that the product has failed follows immediately. It is often the wrong conclusion. The peak detector is a fast and deliberately conservative screening tool, and the formal limit for the measurement is usually expressed for a different detector, with its own time constants and measurement conditions. A marginal peak reading is a place to investigate, not a verdict.

Understanding the difference between detectors changes how a test result is read and, more importantly, how much margin a design should carry. Two products with identical peak readings can produce quite different quasi-peak results, and a design that is judged only by a peak sweep can be either unnecessarily over-engineered or surprisingly close to the limit.

What Peak Detection Is For

Measuring every frequency point with the final detector and the specified dwell time would take an impractical amount of time across a full band. The practical approach is a fast sweep with a detector that never reads below the final result, so that any frequency whose peak is comfortably below the limit can be dismissed without further measurement. Peak detection serves that purpose because the peak reading of a signal is never lower than its quasi-peak reading.

The frequencies that are close to or above the limit are then selected for detailed measurement. At that point the detector, the resolution bandwidth and the dwell time are chosen according to the product category and the frequency range being assessed, and the result may be several decibels below the peak value that triggered the investigation. The screening reading is a pointer, and the formal value comes from the specified measurement.

How the Quasi-Peak Detector Weights Pulses

The quasi-peak detector is not a peak reading with a fixed offset subtracted. It charges quickly when a pulse arrives and discharges slowly between pulses, using time constants defined by the standard for the measurement category. The reading therefore depends on how often pulses arrive. A rapidly repeating disturbance does not give the detector time to discharge, so the reading approaches the peak value. A sparse disturbance allows the charge to fall between pulses, so the reading is lower even though the individual pulses are just as large.

This weighting is the reason two products with the same peak amplitude can produce different quasi-peak results, and it is also why a broadband oscilloscope measurement of the maximum value cannot be substituted for a compliant measurement. The instrument bandwidth, the detector time constants and the reading method are all part of the definition, and a value taken outside that definition is not comparable with the limit.

Why Two Products With the Same Peak Can Differ

The pulse repetition rate of the disturbance is the variable that explains most of the difference. A switching converter running at a fixed frequency produces pulses at a steady rate, and if that rate is high the quasi-peak reading approaches the peak. A microcontroller that wakes periodically, or a device that transmits in bursts, produces pulses separated by long intervals, and the detector has time to discharge between them, so the quasi-peak reading sits well below the peak.

The spectral character of the emission adds a second dimension. Broadband emissions spread energy across many frequencies, while narrowband emissions concentrate it at discrete points. The measurement bandwidth interacts with both differently, which is why a peak sweep can identify a frequency as critical and the formal measurement at that frequency can show margin. Reading the two results as if they were the same quantity leads to decisions that either waste money or leave the product exposed.

What This Means for Design Margin

The practical lesson is to carry margin against the detector that the product will actually be tested with, and to know which detector that is. A design that is judged only by a peak sweep may be pushed to add filtering it does not need, while a design that is judged only by a favourable quasi-peak result may have very little margin against production variation. The right target is a documented margin against the applicable limit, measured with the applicable detector.

Diagnosis should also separate the two contributors. If a frequency is high in the peak sweep but much lower in the formal measurement, the source is likely an intermittent or low repetition rate disturbance, and the design work belongs in the timing or the shielding of that source rather than in additional filtering. If the two readings are close, the disturbance is dense and the filtering approach is appropriate; the techniques are described in EMI suppression principles and in the layout guidance for DC-DC converter routing, and the grounding mechanisms are covered in ground current and harmonic distortion.

Whatever the result, the measurement should be recorded with the conditions that produced it, including the detector, the bandwidth and the dwell time used. A number without those details cannot be compared with a later measurement, and the comparison is the only way to know whether a design change made things better. Keeping measurement discipline as part of PCB design quality characteristics prevents a test result from being treated as a single pass or fail mark.

Additional Considerations for This Build

Practical attention to emission margin pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating emission margin explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.

Process Control and Verification

Reviewing the design before the data is released is far cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.

The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule.

Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.

Setting a Realistic Emission Margin

A design should be released with an emission margin rather than a pass. The margin has to cover three things: the variation between units of the same design, the change in behaviour when the product is installed in its enclosure with its cables attached, and the degradation that occurs over the service life as components age and capacitors lose capacitance. None of those appear in a measurement made on one sample on a bench in an open configuration, so a result that is only a decibel below the limit is not a pass but a warning.

Quantifying the margin means measuring more than one unit and measuring the assembled product in a representative installation. Where the formal result is comfortably inside the limit, the remaining work is documentation: recording the detector and settings used, the configuration measured, and the frequencies where the margin is smallest, so that a later design change can be evaluated against the same conditions. Treating the emission margin as a design parameter rather than a test outcome is what makes the result repeatable across revisions.

FAQ

Is a peak reading always higher than the quasi-peak reading? Yes. The peak detector responds to the largest excursion, while the quasi-peak detector has a defined discharge between pulses, so its reading cannot exceed the peak for the same signal.

Does a peak failure mean the product fails? Not necessarily. It means those frequencies need measurement with the specified detector and settings. A formal result may be several decibels lower, and the margin is what decides compliance.

Why not use a spectrum analyser maximum hold as a substitute? Because the detector time constants, the bandwidth and the reading method are part of the standard definition. An instrument value collected outside that definition cannot be compared with the limit.