A power supply in burst mode is measured twice on the same bench. With the analyser's average detector it clears the conducted average limit by 6 dB. With the receiver's CISPR-average detector it fails by 5 dB. Nothing drifted between the two runs. Both detectors did exactly what they are defined to do, and the definitions are 11 dB apart for that signal. The limit was written for one of them.
- The CISPR-average detector is defined in CISPR 16-1-1 clause 7: a linear average of the signal envelope, indicated through a meter with a specified time constant, and the reading is the highest meter indication during the measurement time.
- A spectrum analyser's average is a sample average over a dwell time, in linear, RMS or logarithmic units, with no meter time constant. Trace averaging and video filtering are different things again.
- CISPR limits mean the CISPR-average. FCC limits define their own average, a 100 ms window under 47 CFR 15.35(b), and call up CISPR receivers through ANSI C63.4 and C63.10.
What the CISPR-average detector is
CISPR 16-1-1 defines the average detector for the measurement of the average value of the envelope of the signal through the reference bandwidth: 200 Hz in band A, 9 kHz in band B, 120 kHz in bands C and D, and a 1 MHz impulse bandwidth above 1 GHz. Two things make it more than an averager. First, the average is linear, of the envelope voltage, not of its logarithm. Second, the detector output is indicated through a meter with a defined time constant, 160 ms in bands A and B and 100 ms in bands C, D and E, and the value recorded is the maximum of the meter indication during the measurement time. The meter is emulated in software in any modern receiver, but the time constant is still there, and it is what the standard verifies.
The verification is in clause 7.3.3, intermittent, unsteady and drifting narrowband disturbances. A sine wave gated on for 160 ms every 1.6 s in bands A and B, or 100 ms in bands C, D and E, shall indicate 9 dB below the same sine wave left on, within 1 dB. That number is the whole difference in one figure. A plain average of the envelope over the 1.6 s period would read 20 dB down, because the signal is on for a tenth of the time. The CISPR meter, with its 160 ms time constant, charges most of the way up during the burst and the receiver records that maximum. The detector is built to make a short, repeating narrowband disturbance count for close to what it does to a receiver, not for its duty cycle.
What "average" means on a spectrum analyser
An analyser offers three things under the word, and none of them has a meter time constant.
- The average detector. Each displayed point is the average of the samples taken during the dwell time for that point. The average can be of the voltage, of the power (RMS) or of the logarithm, depending on the setting, and the result depends on how long the dwell is compared with the signal's cycle.
- Trace averaging. Successive sweeps are averaged point by point. This reduces noise on the display. It is not a detector, and a signal that is on in one sweep and off in the next is simply diluted.
- Video filtering. A video bandwidth narrower than the resolution bandwidth smooths the detected envelope. In linear display mode this approximates a linear average; in logarithmic mode it averages decibels, which is a different quantity for any signal that is not steady.
For a steady sine wave all of these agree with the CISPR-average, because averaging a constant returns the constant. The differences appear as soon as the signal varies within the measurement time, which is exactly the case the CISPR detector was specified for.
| Signal | CISPR-average, CISPR 16-1-1 clause 7 | Analyser average, linear voltage | Analyser average, logarithmic |
|---|---|---|---|
| Steady narrowband | Same reading | Same reading | Same reading |
| Intermittent narrowband, 160 ms on every 1.6 s | −9 dB, by requirement | −20 dB if the dwell covers a whole period, otherwise anything between 0 and −20 dB depending on where the dwell falls | Well below −20 dB, dominated by the noise floor during the off time |
| Broadband pulses, repetition frequency well below the bandwidth | Indication proportional to repetition frequency, verified by clause 7.3.2 | Similar, if linear and the dwell spans many pulses | Much lower; the log of a mostly-zero envelope |
| Drifting or modulated narrowband | Meter follows the variation; the maximum is recorded | Variation smoothed away over the dwell | Smoothed away, biased low |
Which limit means which
An average limit is only meaningful with the detector it was written for. The two families of standards answer differently.
CISPR limits
Every CISPR product standard measures with a receiver conforming to CISPR 16-1-1, through the methods of CISPR 16-2-1 and 16-2-3. When CISPR 32, CISPR 11, CISPR 14-1, CISPR 15 or CISPR 25 print an average limit, the detector is the CISPR-average of clause 7, with the reference bandwidth of the band and the meter time constant. Below 30 MHz that is the 9 kHz bandwidth and the 160 ms meter. Above 1 GHz it is the 1 MHz impulse bandwidth, the 100 ms meter and a linear average; CISPR 16-1-1 states the logarithmic-average requirement above 1 GHz as under consideration, which means a log-average reading is not the specified quantity. CISPR 16-1-1 also defines an RMS-average detector, with its own weighting, but the product standards in common use do not yet set limits against it.
FCC limits
The FCC defines its own average in 47 CFR 15.35(b). When an emission limit is expressed as an average and the emission is pulsed, the field strength is determined by averaging over one complete pulse train, including the blanking intervals, provided the train does not exceed 100 ms. Where it does, or as an alternative, the measurement is the average absolute voltage over the 100 ms interval in which the field strength is at its maximum. The same clause sets a peak limit 20 dB above the average limit. ANSI C63.4 and C63.10, which the FCC references for the methods, call up a CISPR 16-1-1 receiver, and C63.10 additionally allows the average of some transmitters to be derived from a peak measurement with a duty-cycle factor. So an FCC average is a 100 ms window, in the worst 100 ms, made with a CISPR receiver. For most signals it lands close to the CISPR-average with its 100 ms meter, but it is a separate definition, and the certificate and the report should say which one was applied.
The other thing to know about Part 15 is where the average applies at all. Section 15.209 states that its radiated limits are based on a CISPR quasi-peak detector except in 9 kHz to 90 kHz, 110 kHz to 490 kHz and above 1 GHz, where the limits are based on an average detector. Below 1 GHz outside those two bands, an average reading against a 15.209 limit is the wrong detector, however it was averaged.
| Limit | Range | Detector the limit is written for |
|---|---|---|
| CISPR 32, 11, 14-1, 15 conducted | 150 kHz to 30 MHz | Quasi-peak and CISPR-average, 9 kHz bandwidth, 160 ms meter |
| CISPR 14-1, CISPR 15 conducted | 9 kHz to 150 kHz | Quasi-peak and CISPR-average, 200 Hz bandwidth, 160 ms meter |
| CISPR 32 radiated | 1 GHz to 6 GHz | Peak and CISPR-average, 1 MHz impulse bandwidth, 100 ms meter, linear |
| CISPR 25 conducted and radiated | 150 kHz to 2.5 GHz | Peak or quasi-peak, and CISPR-average, per the class tables |
| FCC 15.209 radiated | Below 1 GHz | CISPR quasi-peak, except 9 to 90 kHz and 110 to 490 kHz, which are average |
| FCC 15.209 radiated | Above 1 GHz | Average per 15.35(b), 100 ms window, with peak 20 dB above |
| FCC 15.107 conducted | 150 kHz to 30 MHz | Quasi-peak and average, measured with a CISPR receiver per ANSI C63.4; 15.35(b) governs pulsed emissions |
| FCC 15.247, 15.407 intentional radiators | Per rule part | Average per 15.35(b) and ANSI C63.10, with the duty-cycle provisions of C63.10 |
Set-up decisions that change the answer
Why this is a calibration question too
The property that separates the CISPR-average from a plain average is the meter time constant, and it is verified by the intermittent test in clause 7.3.3 of CISPR 16-1-1: the 160 ms or 100 ms gated signal that shall read 9 dB down. A receiver whose average detector has never been tested that way has an average detector of unknown type. The sine-wave amplitude test cannot tell them apart, for the same reason the two detectors agree on a steady signal. That is why a receiver calibration to CISPR 16-1-1 includes the pulse and intermittent tests, and why a spectrum analyser calibration does not settle the question. The previous note covers what such a certificate has to contain.
Under the accredited scope we calibrate to, CISPR detector response, pulse response and bandwidths are verified from 9 kHz to 1 GHz, including the average detector's intermittent response, and the certificate shows the measured deviation for each detector and band. The EMI receiver calibration guide lists the parameters. If a product reads differently on two detectors that are both called average, send us the two traces and the standard you are testing to, and we will tell you which reading the limit means.