Quasi-Peak Explainer
If you think about old-school over-the-air radio (AM/FM), you’ve almost certainly heard static on a weak channel. If you think about the noise that could be coming through your car stereo speakers, which is more annoying, background/hissing static, or a continuous shrill tone?
The idea that a continuous screech is more annoying than background pops and clicks is the rationale behind QP measurements. In this method, you dwell at each frequency step long enough to apply weighting to the signals you find there. If you have a continuous wave (CW) signal, that will come through as a single tone. However if you have transient noise that is bouncing around and only present during part of the dwell time, that would be the popping and hissing type of noise. QP measurements weight CW signals more heavily. Thus for FCC or CISPR 25 testing, you have to show measurements compared to the QP limit line, in order to be less annoying to the public.
In a world where all our noise sources are stable, which is what we’re ideally trying to achieve in testing, then there can’t be any signal “worse” than a CW signal when measuring for QP. However, a peak detector will easily capture the peak of a CW signal, no problem. And a peak sweep is much faster than QP, since you don’t have to dwell at each frequency so long. So what is typically recommended is to do a peak sweep that covers the full frequency range first. If you pass that, it’s accepted that you will obviously pass the QP measurements as well. If there is a peak value over the limit, then you can return to that specific frequency range and do a longer-dwell QP sweep only in that area, saving test time.
TIP:
I’d like to note that the rule that QP is always less-than-or-equal-to Peak only holds when testing conditions are static. If you are testing a more complex system that may have functions turning on and off during testing, it is possible to see QP values higher than Peak at the same frequency. That’s because the measurements are taken at different times, and the vehicle or other DUT may have turned on something in a later test that wasn’t there originally (e.g. a continuously running electric vehicle (EV) turns on its cooling system for thermal management partway through a test). That’s not to say that you should always run a full scan on QP detection; that would take forever. But if you see QP greater-than Peak, that can be an explanation.