[USRP-users] Amplitude oscillation on B210

Javier Coronel (Expo Impex, S.L.) jcoronel at expoimpex.com
Wed Nov 15 11:56:45 EST 2017


Hi Marcus,

Sorry for the error in the plots (I forgot doing an axis square;).

I am talking about why I get a normal I vsQ plot using a pure frequency and
why I get a romboidal shape when I transmit a varying frequency.

This phenomenon does not depend on the carrier frequency, the sampling rate
or the gain.

The patterns I send in these examples are sinusoidal waves of constant 2Hz
frequency, sweep from 2Hz to 10 Hz and sweep from 2Hz to 50Hz. All these
waveforms are 1 buffer length and I is 1/4 wave out of phase from Q.

Is you see an amplitude plot of the I+Q signal, there is an AM envelope on
the sent waveform (The waveform is correct, but with this AM envelope).

This Friday I will go to the lab to do some precise measurements with an
Agilent oscilloscope, so I will send you my conclusions about this. I guess
this could be due to impedance differences (without the Tx antenna, the AM
envelope is softer), digital filtering (antialias, low-pass, ...) and
amplification.

Have a nice day!

Javier
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