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Hello. I tried checking a 50MHZ 0dBm signal on my spectrum analyzers, and I see a difference in BB60D’s representation of the signal in real time mode, that I can’t say whether it’s a problem with BB60D, or actually a spur in my signal:
In BB60D’s sweep mode, or in SA124B or in other sweep spectrum analyzers, I can’t see additional spurs, but in BB60D’s real time mode, I see a spur at around 51.39MHZ.
Now, if I would also see this in BB60D’s sweep mode, then I could attribute it to lower phase noise or dynamic range in SA124B, but considering that even BB60D’s sweep mode doesn’t show it, I’m starting to be suspicious of BB60D’s real time mode here.
I also tried amplifying the signal (to also amplify the spur) but still couldn’t see it with other spectrum analyzers.I used same settings everywhere: 10MHZ span, 10KHZ RBW.
Also, the signal is not short-lived. It’s a 50MHZ CW coming out of an Agilent power meter, as the reference for calibrating the power sensors.Any idea how I can find the root cause? (could this be due to the way BB60D’s FFT works? Is this a bug?)
Mehdi,
I think this is a spur from the real-time mode. Sweep mode lets us choose optimal LO frequencies, but real-time mode does not. This can result in additional low-level spurs, like this -77 dBc spur you found.
If you suspect a spur in real-time mode, set your span to, say 20 MHz, and step your center frequency by 5 MHz. This will configure the LO for a new frequency and result in a different set of potential spurs. If the spur doesn’t move, it is much more likely to be real than if it does.Thanks Justin. It actually moves, as I move around the center frequency.
Does this happen on BB60D only, or SP145 and SM family could also show similar behaviour?
EDIT:
I played a bit with span settings, and now I can see other spurs (depending on the span, and center freq)
It’s a bit concerning, because then from now on, every time I have to check if the spurs I see are artifacts of the spectrum analyzer itself, or exist in the signal.
I had never noticed this (maybe because I rarely work with frequencies this low).
Could it be something more likely to happen in the lower frequencies? According to BB60D’s diagram, this specific signal I tested, pass through a different signal path (<130MHZ) than what I usually use.Each product line is a bit different. But for all of them we have a “spur reject” mode (under settings) where two different LO frequency results are masked together to eliminate most spurs. For measuring CW signals this is a good approach, but you lose the real-time nature of measurements in this mode.
The BB60D, being a double conversion superhet, will probably have the most consistent behavior across frequency.
Also keep in mind the BB60D spur shown is -77 dBm. Your bottom picture has a spur of -78 dBm, you just don’t notice it because of the noise.
Edit: I should add that all of our spectrum analyzers, in real-time mode, use a larger set of LO frequencies than sweep mode.
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