Always perform a quick pre-check

Low-cost benchtop spectrum analysers are particularly popular with small-to-medium companies that often don’t have an EMC test engineer. But these EMI spectrum analysers can often be “abused” in the following ways:

•When performing an EMI scan, the spectrum analyser is connected to the LISN when the unit is turned on and off. This can be a problem because an inductive load’s “kickback” voltage could introduce a significantly high level of transient voltage that may potentially damage the RF front of a spectrum analyser. This could also happen when the spectrum analyser is connected to an RF current probe clamped on a motor winding.

•An EMI spectrum analyser is normally used to measure conducted emissions from 150 kHz. Engineers notice the “ADC overload” measurement error during the measurement, but the measured signals often seem to be within the limit. This is most likely due to the high spurious levels of noise at relatively low frequencies (50Hz and its harmonics). If the analyser is left on during the measurement, there is a risk of damaging the analyser.

The components that often get damaged in low-cost benchtop spectrum analysers are ESD-protective devices and the GaAs switches. The problem with a damaged front end is that the spectrum analyser will appear to function okay because there will be readings across the spectrum. But the readings will be wrong (in terms of amplitude), and wrong harmonics also appear if the front-end ESD suppressors get destroyed.

Therefore, a quick pre-check of the spectrum analyser is necessary. The procedure is simple and quick to perform. There are two ways of performing a pre-check.

•If the spectrum analyser has enabled the tracking generator (TG) function, simply connect the TG output and the spectrum input using a coaxial cable and perform a TG scan. One should see a flat, straight line across the whole frequency range at the supplied TG power level (often between -20 dBm and 0 dBm). This is shown in Figure 1.

•If the spectrum analyser does not feature the TG function, simply connect a function generator/signal generator to test a few selected frequency points. This requires a high-performance generator (to ensure the output waveform is sinusoidal).

Use transient limiters/high pass filters/external attenuators

Knowing the potential risks of damaging the RF input stage of a spectrum analyser, engineers often need to protect the equipment using some passive devices. The popular device choices are transient limiters, high pass filters, and external attenuators. Effective as they are, consider the following issues when using these passive devices.

•Transient limiters and high-pass filters can introduce measurement errors and should be used cautiously.

•External attenuators do not have diode clipping issues, but they raise the spectrum analyser’s noise floor, reducing the system’s sensitivity.

•When using these passive devices, a “software” compensation value must be applied to the final measurement results. A common mistake is that the external factor is not scaled into the final result, causing a 10 dB or more difference in measurement.

“Zoom in” and reduce the RBW

An oscilloscope allows us to zoom in on the details of a time-domain signal. A spectrum analyser can also be “zoomed-in,” so a great detail hidden in the spectrum plot can be reviewed.

A typical case involves a radiated emission test, in which a resonance peak is observed in the frequency range of 60 and 80MHz in the plot of 30 – 300 MHz. To find the noise source that resonates at this frequency range, we can reduce the measurement range to this narrow frequency region and reduce the resolution bandwidth (RBW), as shown in Figure 2.  In this case, a switching frequency of 1.8 MHz was observed, shown as the difference frequency between individual harmonics (for example, 61.25,63.05, 64.85MHz, etc) which identifies the source - a switched-mode power supply on the board under test.

Be aware of the noise generated by the spectrum analyser

Inexpensive benchtop spectrum analysers are infrequently used in an anechoic chamber, so the measured results are always subject to ambient noise. Ambient noise is generated by nearby equipment, lighting in the room, and cables connected to the mains power supply.

There are effective ways of reducing ambient noise, which we’ll discuss in detail in our next “Troubleshooting” column. But, in brief, engineers should always be aware that spectrum analysers produce RF noise. The mains cable of the spectrum analyser conducts and radiates RF noise in the frequency range of 1 MHz and 300 MHz. This can be measured by connecting an RF current probe on the mains cable.

Engineers who perform the test should differentiate the noise from the DUT and the test equipment used. In one case, when the author was testing a motor drive cable noise, the RF noise from the spectrum analyser was coupled to the motor cable being measured, which led to misleading information.