Introduction
If 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.
Detectors
Peak, quasi-peak, EMI average, and root-mean square (RMS) average detectors, the characteristics for each are different for each frequency band.
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 A: 45ms charge time-constant; 500 ms discharge time-constant.
Band B: 1ms charge time-constant; 160 ms discharge time-constant.
Band C/D: 1ms charge time-constant; 550 ms discharge time-constant.
The RMS-average detector is a new type of weighting detector described in CISPR 16-1-1 that combines an RMS detector for pulse repetition frequencies above a corner frequency (fc) and the average detector for pulse repetition frequencies below fc. With this combination of characteristics, this type of detector is able to achieve a pulse response curve with 10 dB/decade above fc and 20 dB/decade below fc. The author is currently unaware of any product standards which utilize this detector type. The bandwidth requirements for measuring receivers with an RMS-average detector are the same as those for other types of detectors as noted later in this article.
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.
Input Impedance
Nominally 50 Ω with a voltage standing wave ratio (VSWR) not to exceed 2.0:1, unbalanced. VSWR of 1.2:1 for RF input attenuation ≥ 10 dB.
Sinewave Voltage Accuracy Measurement
Better than ±2 dB when the instrument measures a sine-wave signal with 50 Ω resistive source impedance.
Resolution Bandwidth (6 dB roll-off)
A fundamental characteristic of any measuring receiver is its resolution bandwidth’s (RBW’s). CISPR 16-1-1 specifies the following RBW’s for each band:
Band A: 200 Hz
Band B: 9 kHz
Band C: 120 kHz
Band D: 120 kHz
Tests conducted using other bandwidths are possible (for instance during troubleshooting), but not when performing compliance tests to show due diligence with product standards. As the bandwidth size (i.e. window) increases, so does the amount of energy measured by the receiver. The opposite is also true. Just like how a smaller window lets in less light, a smaller RBW lets in less RF energy producing a lower reading on the receiver’s display. Just something to think about if your current receiver doesn’t utilize the correct CISPR resolution bandwidths.
Overload Factor
Overloading a receiver is a big deal that could result in inaccurate readings at the best, and a damaged piece of equipment at the worst.
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 peak measuring receivers, the overload factor does not need to be as high as it is for other types of measuring receivers. For most direct-reading detectors, the overload factor is slightly larger than unity and is adequate for the time-constants used.
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.
References and Further Reading
- 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.
