I am transmitting a QPSK signal via GRC and when I look at it on the spectrum analyzer, the signal is shifted away from my center frequency. This shift increases the higher the center frequency (i.e. 1 MHz = almost no shift, 10 MHz = 1kHz shift, 50 kHz = 2.3kHz shift, 900 MHz = 43.5 kHz shift). Why does this shift occur? I have attached my flowgraph screenshot below. enter image description here

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    $\begingroup$ The most likely cause is that the USRP and the SA don't completely agree on what "one second" is. $\endgroup$ – MBaz Mar 10 at 19:48
  • $\begingroup$ also, PSK Mod is deprecated. Don't use it. It's mostly broken. $\endgroup$ – Marcus Müller Mar 10 at 21:31
  • $\begingroup$ @MarcusMüller Is there another option to use? $\endgroup$ – lceans Mar 11 at 14:02
  • $\begingroup$ Since you're starting a new thing here, definitely use GNU Radio 3.8 or 3.9 instead of 3.7. The digital blocks are undergoing changes, and I really don't think it pays to support the 3.7 way of doing things, now that nobody's going to use that. $\endgroup$ – Marcus Müller Mar 11 at 14:04
  • $\begingroup$ In 3.8 and later, there's nice examples in the examples/digital subfolder. Orient on these. $\endgroup$ – Marcus Müller Mar 11 at 14:05

Well, there are answers in the comments, but I am just going to put it down here.
Most probably the Local Oscilators (LOs) of your USRP and the Analyzer are not tuned/synchronized.
You could try to provide a common, external clock signal source to both of these devices.


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