Analyzers
f you’re performing commercial radiated and conducted emissions measurements strictly by the book, you will want to utilize a measurement receiver or spectrum analyzer that fully complies with CISPR 16-1-1. Briefly, this is what this means:
Note 1: The specifications in CISPR 16-1-1 apply to EMI receivers and spectrum analyzers. The term “measuring receiver” is also used in the EMC community and refers to both EMI receivers and spectrum analyzers.
Note 2: Often, the receiver specification under question depends on the frequency range of operation. In CISPR 16-1-1, there is one receiver specification covering the frequency range 9 kHz to 150 kHz (Band A), one covering 150 kHz to 30 MHz (Band B), one covering 30 MHz to 300 MHz (Band C), and finally one covering 300 MHz to 1,000 MHz (Band D). This article covers the 9 kHz to 1 GHz frequency range of operation and Bands A through D. Band E covers the 1 to 18 GHz frequency range of operation and has its own unique requirements.
Note 3: Specific requirements for measuring receivers used for military testing are found in MIL‑STD-461G and DEF STAN 59-411.
Depending on the type of signal being measured (amplitude modulated, pulsed, frequency modulated, etc.), the measured level will vary, depending on the detector used for testing.
The peak detector, also known as the envelope detector, is the easiest to understand. It has a fast response and its output follows the envelope of the measured signal. When using this type of detector, testing goes much faster than when using one of the other types of detectors. In order to save a lot of time, savvy EMC engineers and managers will obtain most of their engineering emissions data using a peak detector and later, after they are sure their product will pass, utilize the other detector types for full compliance testing.
Next up is the average detector. The average detector simply measures the average value of the signal. The benefit of using this detector type is that any non-continuous (i.e., pulsed) signals that are measured with it will record a lower average value than if measuring continuous types of signals.
As the name implies, the quasi-peak detector is not truly a peak detector. It utilizes the following weighted charge and discharge times, which understate peak responses for signals with low pulse repetition rates.
Band B: 1ms charge time-constant; 160 ms discharge time-constant.
Note 4: The corner frequency (fc) is the pulse repetition frequency above which the RMS-average detector behaves like an RMS-detector and below which the RMS-average detector has the slope of a linear average detector.
Band B: 9 kHz
Band D: 120 kHz
To obtain the correct magnitude of a signal, the signal under measurement presented to the detector must be undistorted at much higher levels than the output of the detector. One way to specify this issue is with the overload factor.
The overload factor is defined as the maximum level at which the steady-state response of a circuit does not depart by more than 1 dB from the ideal linearity. This characteristic defines the range of the practical linear function of the measurement circuit.
For receivers employing an average detector, the receiver is not allowed to overload for pulse rates equal to or greater than 25 Hz for Band A, 500 Hz for Band B, and 5,000 Hz for Bands C and D.
Note 5: At very low pulse rates, it’s not possible to provide a sufficient overload factor to prevent non-linear operation of the receiver, a big challenge for measurement receiver designers.
What this all means is that the RF and IF stages of a quasi-peak measuring receiver must be prepared to be overloaded by up to 24 dB for Band A, 30 dB for Band B, and 43.5 dB for Bands C and D.
This was a quick run-through CISPR 16-1-1 for measuring receivers. For more information on this important subject, please see the following.
- Specification for radio disturbance and immunity measuring apparatus and methods – Part 1-1: Radio disturbance and immunity measuring apparatus – Measuring apparatus (CISPR 16-1-1:2010).
- Williams, T., EMC for Product Designers, Fifth Edition, Newnes.