Re: [Discuss-gnuradio] WBX CW transmit problem

[Matt Ettus]

On 06/30/2011 07:13 AM, Aleš Povalač wrote:
> Dear all,
> 
> I have been testing USRP2 with WBX board (#957). Using GNU Radio, I
> have created a simple CW TX system. The problem: output signal is
> oscillating when I set the TX frequencies to 900MHz band. Signal
> output (in time domain) at TX/RX connector from the FSL analyzer is in
> the attachment.
> 
> Settings:
> 868 MHz, 200 kSps, gain 15 dB
> 
> Same results with:
> ./tx_waveforms --rate 250000 --freq 868000000 --gain 15
> 
> The oscillation with ca. 1dB amplitude is present for almost all gain
> settings except the lowest values. I have observed the problem at
> several frequencies from 700M to 1G, sometimes with amplitude over
> 2dB.
> 
> Any ideas about the cause of this problem? Some PA instability or AGC
> loop issue?


There is DC offset in the signal.  Since you are using a 10 MHz RBW, you
are seeing both the DC offset and the desired signal beating with each
other.  If you take your spectrum analyzer out of zero-span mode and
zoom in on the signal you will see that the amplitude is constant.

Matt