Tricked by a spur and nobody to blame


28 Jul 2013: Replaced spec sheets with larger versions.


In RF language, spurs refer to spurious signals or “birdies” as some would call them. These could be caused by unfiltered/ poorly filtered mixing products or unfiltered/inadequately screened high order harmonics of signals or a combination of both.

Last week, my sleuthing around for signals around the 433 MHz was sidetracked when I observed a constant 433.3333 MHz signal on my Anritsu MS2712E Spectrum Master. Around the frequency range of interest, this signal was the most consistent.

One thing that I usually do when I see a small signal is to tune in my handheld and listen to it. That signal is within the 70 cm band of my little BaoFeng handheld. With the squelch set to zero i.e. permanently unsquelched, the audio will change from “much noise” to “less noise” as the RF signal amplitude increases.

Picture this situation:

You are looking for a certain signal from your car, so you put the spectrum analyzer in the front seat. And then you see this consistent signal at 433.3333 MHz on the SA. Now, with the handie’s squelch fully open, the audio gets “less noisy” and sometimes even partly “quieted” as you approach the front seat. You then try removing the whip antenna from the SA – signal disappears from the screen, but the handie still detects the signal. The natural conclusion is that the signal is from somewhere near the front seat, right?

Well, not really, as I was to find out.

The following day, using only the handie, I could no more detect the signal. How could it disappear just like that? So I brought in the SA to have another look. As soon as I turned on the SA, my handie picked up the signal. Could the SA itself be seeing its own signal?

Well, the answer is YES, and you don’t even have a bad product? Well, at least the specs say that the product meets specs. How?

First, I determined that the signal is indeed from the SA:

Then I had a look at how spurs are typically specified in Spectrum Analyzer data sheets.

Spec for my Anritsu MS2712E:MS2712E

Spec for my Agilent E4411B:A04_6364_E4411BNotice how in both cases the spurs are specd with the input to the SA terminated in 50 ohms.

In my case, my efforts were being thwarted by the radiated spurs. So as far as the spec sheets are concerned, my SA is OK.

All for now, and hope you enjoyed my little story.

Wishing you good DX,

Jeff 9V1As

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