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

Marcus D. Leech mleech at ripnet.com
Thu Dec 4 10:26:37 EST 2014

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 
   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 
   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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