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

Stefano Speretta s.speretta at isispace.nl
Thu Dec 4 11:23:00 EST 2014


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

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.
>
>
>
>


-- 
Stefano Speretta
ISIS - Innovative Solutions In Space BV
Molengraaffsingel 12-14
2629 JD Delft
The Netherlands

Phone:   +31(0)15 256 9018
Fax:     +31(0)15 257 3969
E-mail:  s.speretta at isispace.nl
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