When selecting EMC test equipment, specifications are more than numbers on a datasheet. They are often the only information available to compare different brands. If a specification does not reflect the actual capabilities of an instrument, one might end up with a suboptimal choice. This can harm the quality of your setup and the results of your measurements. Below are several traps to avoid.
Requirements
Quality standards require instruments that match their intended use. For example, ISO 9001 states that an organisation must decide where and when quality checks are needed. It also requires the use of instruments that are “suitable for the specific type of monitoring and measurement activities being carried out”. ISO/IEC 17025 includes a similar requirement in paragraph 6.4.
“Typical” specifications
Sometimes specifications are marked as “typical”. It is not always clear how to interpret this. Does it mean an average, a best case, or a worst case? One may wonder why “typical” is used at all. Does the manufacturer not know the exact specification? Or is it that the worst-case specifications are not good enough to be specified?
Specifications Over the Full Range
When searching for an EMC measurement instrument it is important to first define the requirements like frequency range & test level. Although these specifications seem mostly straight forward, one should pay attention whether the specification is at one or more specific points or the whole range. In some cases, instrument performance can degrade near the lower and upper limits of its specified range. When such behaviour is specified, it is important to evaluate whether these edge characteristics are acceptable. Otherwise, you might end up with an instrument that has much worse specifications at some part of the range.
For example, an RF power amplifier might be sold as a 100 W amplifier, yet only delivers an output power of 47 dBm at 1 dB compression over the full range, which is equivalent to 50 W. If consistent 100 W performance is needed across the entire frequency range, particularly at lower frequencies where higher power is typically required, such an amplifier would not be suitable for an immunity test setup.
Relevant points
The same principle applies to the uncertainty of E-field probes. According to one of our white papers, the largest contributor to measurement uncertainty is isotropic behaviour, or more precisely, the anisotropy specification of the E-field probe (measured in accordance with IEEE-1309). It is well known that uncertainty increases with frequency, sometimes leading to errors of up to 12 dB. Therefore, a field probe’s specification stated only at 100 MHz, may show an excellent anisotropy value, while it may be far worse at higher frequencies.
In short, specifications should always be reviewed carefully, and the manufacturer or reseller should be consulted if any doubts arise. Specifications can occasionally be incomplete or even be misleading, and direct comparisons between instruments are not always as simple as matching single specifications in data sheets.