[USRP-users] Amplitude oscillation on B210

Marcus Müller marcus.mueller at ettus.com
Sun Nov 12 12:25:57 EST 2017


Hello Javier,

the "slantedness" of the constant frequency tone (the fact that it's not
a perfect circle) looks like quite a bit of IQ imbalance to me – how
many samples are those? What frequency did you work with?

The other two graphs, based on the const freq graph, look familiar – one
"swing" has about the shape of the const freq loop, and it slowly
rotates – the effect of the frequency difference between transmitter and
receiver.

So, what worries me is the fact that your constant frequency plot
doesn't have the same dimension in I and Q!

Best regards,

Marcus


On 08.11.2017 12:48, Javier Coronel (Expo Impex, S.L.) via USRP-users wrote:
> Hi all!,
>
> I am using a setup where Tx antenna is in front of Rx antenna, this
> way I can measure all the signal path and environment interferences of
> the transmission.
>
> When I send an AM signal at a constant frequency, everything is right,
> I receive the same signal (the amplitude depends on many factors, but
> it is almost constant).
>
> But if I send a FM signal starting at some frequency and gradually
> increase that freq. in some amount, I get amplitude variations of the
> received signal. This is consistent using a direct cable connection.
> Also I dumped the wave I send into a file to check if everything is
> OK, and it is so.
>
> Attached you can see polar representations of the I vs Q signals
> received for the cases of constant frequency, 10 Hz variation and 50Hz
> variation.
>
> I saw the schematics and the only reason I can figure is due to the
> switch (SKY13335−381LF) or some kind of AGC digital filtering on the
> path. I set the gain manually, but I am not sure if this is also
> affected by any other configuration.
>
> Should I call any function to get the same amplifier response as in
> the case of constant frequency? Thank you in advance!.
>
> Have a nice day!
>
>
>
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