On 2022-04-08 09:56, Tobias Kronauer wrote:
Hi all,
we use a X410 and send 60k samples via localhost. We increase the
frequency within the frame every 6k samples. We observe a frequency
selectivity of the USRP. Can you help us on that? In the attachment
[0], you can find the MWE (we tried to keep it as small as possible).
Let us explain the problem in the following plot:
In the left column, you can find the transmitted signal, in the right
column the received signal. In the lower row, you see the spectrogram.
One can clearly see that the signal gets attenuated, depending on the
frequency. One would expect the amplitude to remain constant.
The transmitted signal changes every 6k signal its frequency. Within
these 6k samples, we simply send complex exponentials. The frequencies
are: [-25, -19.44 -13.9, -8.3, - 2.78 2.78 8.3 13.9, 19.4 25].
Rf config:
```cpp
const double FS = 50e6;
Config conf;
conf.txAnalogFilterBw = 400e6;
conf.rxAnalogFilterBw = 400e6;
conf.txGain = 35;
conf.rxGain = 35;
conf.txCarrierFrequency = 2e9;
conf.rxCarrierFrequency = 2e9;
conf.txSamplingRate = FS;
conf.rxSamplingRate = FS;
```
For different RfConfigs, the results are less distinct.
Kind regards,
Tobias
Attachments:
[0]: usrp_mwe.cpp
Your plots aren't particularly clear in what they're showing, neither is
your problem description. Is your list of "frequencies" in terms of the
baseband? In Hz? Khz? MHz?
ANY radio receiver (or transmitter) will have a passband response that
is dependent on the analog passband response of the various components
in the system. With
a DSP radio the passband response is conceptually the convolution of
the analog and digital responses. The passband for any SDR will
exhibit roll-off at the edges
of your passband--that roll-off CANNOT be infinitely steep without
the filters also being infinitely long.
I don't have an X410, so I don't know what the passband *should* look
like, but I imagine that there are both edge roll-off effects and the
passband is not
perfectly "flat".
If you inject a laboratory noise source into the receiver, and just look
at the spectrum integrated over a few seconds, you will see the
pass-band response of the
radio at the desired frequency, at the desired sample-rate.
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