Re: [Discuss-gnuradio] Strange side-band when transmitting using WBX on USRP1

[Mark J. Blair]

On Oct 13, 2010, at 8:45 PM, Drew Read wrote:
> We're having some problems with transmitting using the WBX board on a
> USRP1 (not an old one).
> We have a flow graph with a NULL source going into a NBFM, then
> through a multiply_const and into the USRP.
> At low frequencies (100MHz) the signal looks ok, but when we get up
> passed 500MHz we can see/hear that what should be an unmodulated
> carrier also has a single side-band (and is audible as a tone when
> demodulated) . And by increasing the const in the multiply_const, we
> can see our wanted signal getting stronger while the side-band remains
> there at a constant level.


That sounds (and looks) like an issue that I encountered yesterday with my USRP 
+ WBX. I was using it to generate some test signals in the 1.5-1.7 GHz range, 
and I found that there was a constant spur about 1-2 kHz above the 
transmitter's center frequency. I also found that I could reduce its effects by 
playing with the signal level, transmitter gain and an external attenuator. For 
now, I think I can work around it by simply generating my test signals at an 
offset from the transmitter center frequency and adjusting the center frequency 
to put them back where I want them, thus moving the spur out of the middle of 
my test signal.


> We've had a play with manually adjusting the DC offset of the USRP
> sink, which seems to change the size of the side-band a bit, but we
> don't really know what to do with that.

I also found a bit of the transmitter center frequency bleeding through when I 
modulated it with no DC component. I haven't tried to mitigate this yet, but my 
workaround for the spur should also let me ignore this little bit of unwanted 
carrier for the time being. I presume that it's due to a small DC offset error 
in the DACs, based on the my observation of what appears to be a small 
(sub-LSB) DC error in the receiver ADCs. In my receive path, I found there to 
always be a received component at the tuned receiver frequency, and I found 
that I could null it out by adding a small (magnitude < 1.0) complex constant 
to the USRP source output before it passed to the rest of my receive chain. 
Over the course of a few hours and a power-cycle, I found that I had to 
readjust the constant once or twice. This little error  is pretty negligible 
when receiving larger signals, but it was annoying while I was trying to look 
at some pretty weak signals. I may be able to swamp it out with some external 
gain, but I didn't have an LNA handy yesterday to try that.

For the time being, I'm going to try to ignore these DC components in both the 
receiver and transmitter by simply offsetting the LO a bit from the signal that 
I want to transmit or receive. I have to do some more tests tomorrow using my 
USRP as a signal generator, so I hope this will work.

In the longer term, I'm interested in looking into whether there's a better 
approach to trimming out any DC offsets in the DACs and ADCs. I'm also 
interested in learning more about that unwanted spur in the transmit output 
(particularly since it's quite a bit larger than the component that appears to 
be caused by a small DC error).

> We've tried several USRPS and several WBX boards and seen the same
> problem on all of them.
> We don't see it on the USRP2.

Interesting.




-- 
Mark J. Blair, NF6X <address@hidden>
Web page: http://www.nf6x.net/
GnuPG public key available from my web page.