[USRP-users] LO leakage on receiver when tuning the transmitter

Marcus D. Leech mleech at ripnet.com
Thu Dec 4 11:35:15 EST 2014

On 12/04/2014 11:23 AM, Stefano Speretta via USRP-users wrote:
> What surprises me a bit is that the glitch is exactly in DC (if we 
> look at the baseband signal) or exactly at the LO frequency (seeing it 
> as an RF signal) no matter which frequency you tune to on the tx side. 
> I would expect spurs of the transmitter frequency to appear in case of 
> a leakage from the transmitter to the receiver (and maybe more than 
> one) or, at least, to move when the transmitter frequency changes but 
> this never happens.
> I tried changing the size of the frequency jumps and I noticed that, 
> when the jump is more than 1 MHz, the RF front-end is retuned and in 
> that case I do not see the glitch. If the jump is smaller (10-100 kHz) 
> the tuning is done using the internal DSP and in that case I see the 
> glitch.
> Regards,
> Stefano
Hmm, that makes it sound like a different class of problem, so I wonder 
if Balint/Ben/Ian want to comment?

> Il 04/12/2014 16:26, Marcus D. Leech via USRP-users ha scritto:
>> On 12/04/2014 04:14 AM, Stefano Speretta via USRP-users wrote:
>>> and I clearly see  glitch appear every time I tune the transmitter. 
>>> The amplitude of the glitch, according to the FFT, is between -55 
>>> and -65. The same file was run with a USRP B100 and a USRP1 using a 
>>> WBX daughter-board but the glitch never appears. In the patch I also 
>>> added a message handler to stop displaying messages because 
>>> sometimes using a USRP B100 there were some messages which were 
>>> disrupting the FFT plot.
>>> Best regards,
>>> Stefano
>> What you're seeing is the VFO on the TX-side as it sweeps into "lock" 
>> when you change frequencies.  That action will create spectral 
>> components that
>>   will briefly appear (sometimes, not always) in your RX passband.  
>> There's no way, given that both LOs are inside the same TINY chip 
>> that you can get
>>   perfect isolation between TX and RX.  We live in a world where 
>> receivers are very, very, sensitive--like at or below 3dB, so even 
>> with pains taken
>>   to try to isolate things (as much as can be done when you have an 
>> LO in the same chip as the RX gain and mixer chain), you'll still see 
>> these sorts
>>   of things.
>> The only thing I can suggest is to logically mute your RX chain when 
>> you're tuning.  And also, how do you cope with "glitches" that 
>> originate from
>>   the outside world?  Because there will always be some--the outside 
>> world isn't generally kind in this regard, it's a useful exercise to 
>> consider how
>>   to deal with them.

Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium

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