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Re: [Discuss-gnuradio] benchmark_xx.py Transmits Images at +/- 0.5MHz Wh

From: Josh Blum
Subject: Re: [Discuss-gnuradio] benchmark_xx.py Transmits Images at +/- 0.5MHz When Center Freq Is 2.4GHz [USRP N200 and XCVR2450]
Date: Fri, 04 May 2012 01:45:32 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:11.0) Gecko/20120410 Thunderbird/11.0.1

On 05/03/2012 01:08 PM, Jason Tran wrote:
> Hi All,
> Here are images of what I'm seeing in my setup.
> Pictures of Images/Spikes:
> center freq set to 2.4GHz. random image (oscillioscope)
> http://img585.imageshack.us/img585/5871/img20120503124151.jpg
> center freq set to 2.4GHz. images +/- 0.5MHz (uhd_fft.py)
> http://img801.imageshack.us/img801/3015/imagesonuhdfft.png

I believe that you are seeing the LO of the XCVR2450. The default
behaviour when tuning the transmitter on this board is to tune the LO
with an offset such that it stays out of your passband.

So I dont think is is harming the deframing and demoding of packets. You
might try messing around with some other parameters, gain, symbol rate,
modulation type.

> Pictures of Clean Signal:
> center freq set to 0; Tx "defaults" to 4.8GHz but is actually 2.4GHz 
> (uhd_fft.py):
> http://imageshack.us/photo/my-images/213/noimagesuhdfft.png/
> (oscilloscope version)
> http://img801.imageshack.us/img801/4141/img20120503124257.jpg

I think whatever screwy thing is happening to make the tune request clip
to 2.4GHz is affecting the logic that decides the LO offset.

I'm not sure at what level the coercion is happening. Pardon me, I am
away from my USRP :-) If you run one of the examples that comes with UHD
try tx_waveforms --freq=4.8e9 --rate=1e6 and see the warning it prints
about the tune frequency.


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