DC Nonexistent Peak at Transmitter

GNU Radio
#1

Hi all,

I am trying to create a complex cosine signal in 100kHz and transmit it
with USRP in 110MHz using WBX daughterboard. I have observed this signal
in an oscilloscope and a spectrum analyzer. It can be seen that there is
a nonexistent peak at transmit frequency of USRP. I also added the
related photos to be clear.

Any help would be kindly appreciated.

Sema

#2

Hi Sema,

You are most likely seeing the local oscillator at the centre due to the
WBX being a direct conversion daughterboard:

http://en.wikipedia.org/wiki/Direct-conversion_receiver

To remove this, you can put the following in the center frequency box of
the UHD source/sink block in GRC:

uhd.tune_request(uhd_center_freq,
rf_freq=(uhd_center_freq+uhd_lo_offset),rf_freq_policy=uhd.tune_request.POLICY_MANUAL)

The ‘uhd_center_freq’ and ‘uhd_lo_offset’ need to be specified. Set
‘uhd_lo_offset’ to be something like ((samp_rate/2)*1.1).

Regards,

Mike

–
Mike J. M0MIK BSc MIET
Ettus R. Technical Support
Email: [email protected]
Web: http://www.ettus.com http://www.ettus.com

#3

Sema -

Also, please be sure you run the calibrations provided by UHD. The TX DC
Offset calibration addresses exactly what you are talking about.

http://files.ettus.com/uhd_docs/manual/html/calibration.html

Cheers,
Ben