Spectrum Analyzer Span: The Hidden Measurement Trap

Spectrum analyzer span can make a measurement look convincing while quietly giving you the wrong answer. In this Elektor TV excerpt, Sebastian Westerhold of Baltic Lab demonstrates a trap that often catches RF users: a signal appears at the wrong frequency, not because the transmitter or analyzer is inaccurate, but because the analyzer settings are being interpreted too casually.

Watch the Spectrum Analyzer Span Demo

The instrument in the demo is a Rigol RSA-series spectrum analyzer. Sebastian begins with a deliberately awkward-looking measurement: the fundamental appears at 143.75 MHz, even though the actual signal should be around 145 MHz. The useful part of the demo is that the error is not blamed on reference accuracy, transmitter drift, GPS locking, or some mysterious RF gremlin hiding behind the bench. The issue is more basic and more annoying: the spectrum analyzer span is too wide for the measurement being made.

The analyzer is set to a span of 1.4 GHz with a resolution bandwidth of 100 kHz. That sounds reasonable if the goal is simply to see a large chunk of spectrum. But the analyzer in this case has 801 points across that displayed span. Those display points are not magic; they are discrete positions across the sweep. If the signal falls between them, the analyzer can still show something, but the marker can report a peak at the wrong frequency and with the wrong amplitude.

Why Display Points Matter

Sebastian explains that the 100 kHz RBW refers to the 3 dB bandwidth of the analyzer’s Gaussian-shaped filter. A signal just outside that center can still come through the filter skirt. The result is a believable-looking trace that is not actually telling you where the signal is. In his words, both the frequency reading and the amplitude reading are wrong.

This is the part that should make anyone doing EMC checks, amateur-radio transmitter measurements, or RF debugging sit up a bit. A wide span is useful for hunting, but it is a poor default for measuring. If you want to see everything at once, you are also spreading the display points over a much larger range. The trace may look tidy, but the measurement may be coarse enough to mislead you.

Fixing the Spectrum Analyzer Span

The fix in the demonstration is to reduce the span. Sebastian uses a rule of thumb from EMC practice: keep the spacing between points to around twice the resolution bandwidth. With 801 points and a 100 kHz RBW, that gives a usable span of about 40.5 MHz. He then sets the analyzer to a 40.5 MHz span, keeps the center frequency at 145 MHz, and the frequency reading lands where it should.

The practical lesson is not “never use a wide span.” Use a wide span to search. Then narrow the span when you want to measure. Elektor readers who work with RF will have seen the same broader problem in other bench situations: the instrument is not lying, but it is doing exactly what you asked rather than what you meant. Our recent RF measurement coverage has made a similar point around spurious emissions and analyzer settings.

For anyone learning to use a spectrum analyzer, this is a good reminder to treat span, RBW, and sweep points as a set. If they do not match the measurement you are trying to make, the display can still look professional while the result is nonsense. That is a particularly cruel feature of test equipment: it can be wrong in a very well-rendered font.