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I am attempting to do a simple record of a CW, 250MHz signal at the Rx port of an Ettus B200 using an Anritsu MG369 signal generator. I am using the 10 MHz ref signal provided by the SG (output @ ~9dBm). The frequency/phase of the signal seems to be too unstable from what I should expect. Is this common for the B200? I am expecting a B210 soon and am just trying to get ahead of the game while awaiting the new hardware, but if this is user error I'd like to figure it out in advance if possible as I eventually will need to consider phase closely when test is in its final form. I've seen in Ettus literature that a square wave is best for reference, could a sine reference be the cause of this?

Could it be the PLL misbehaving or an amp somewhere in ref circuitry? RF front end is confirmed to be working okay and can see a clean, stable signal when running CW from Tx through cable to Rx port.

Including flow chart as well as two pics of spectrum to try to illustrate instability.

EDIT: Including better FFT sink snapshots

Signal Recording flow chart

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You're looking only at the real part. Obviously, if the phase of transmitter and receiver aren't locked (like here), that will mean the sent oscillation will be more or less in-phase.

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  • $\begingroup$ Thanks for your response! Although I'm not sure I understand your point. I am looking at real part for time domain, but i am looking at FFT of IQ pair in frequency sink and am seeing a lot of instability in both plots. Would you mind expounding a bit? $\endgroup$
    – not_fogarty
    Feb 4, 2021 at 19:57
  • $\begingroup$ Could you specify what "a lot of instability" means? (your frequency sink has a menu when you click on it with the middle mouse button, with which you can take screenshots). $\endgroup$
    – Marcus Müller
    Feb 5, 2021 at 10:41
  • $\begingroup$ Absolutely, for some reason save feature wasn't working properly on my dated system, but I changed out screen shots to hopefully give better idea. Have a 200k signal filtered by 100k BPF that is varying wildly in SNR (see the two snapshots) and in time domain I see jitter in both amplitude and phase. However these effects are not seen when I use internal transmitter and loop through to the receive side, so I'm pretty sure it must be with how Sig Gen and Ettus are interacting. $\endgroup$
    – not_fogarty
    Feb 5, 2021 at 14:31
  • $\begingroup$ I may assume the signal we're referring to is between -100 kHz and -300 kHz? I can't quite align what you're describing in this comment with what you're describing in your question: That 200 kHz/filtered to 100 kHz signal is not a CW? $\endgroup$
    – Marcus Müller
    Feb 5, 2021 at 14:34
  • $\begingroup$ Sorry if I'm not being clear, Sig Gen is transmitting a CW at 225 MHz to RF front end of ettus, the ettus is then down converting to a 200kHz signal that i ran through a BPF with a bandwidth of about 100kHz. $\endgroup$
    – not_fogarty
    Feb 5, 2021 at 14:56

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