[USRP-users] Removing the whole LO signal when transmitting

mleech at ripnet.com mleech at ripnet.com
Fri Jul 14 11:56:46 EDT 2017

LO suppression in mixers is never perfect.  Consider that the LO signal
is entering the mixer at a level of perhaps +7dBm, there's 62dB of LO
suppression, which is pretty good. 

On the transmit side, using LO offset doesn't *suppress* the LO, it just
moves it to a place that is convenient for analog filtering. 

Nearly every type of superhet or direct-conversion TX chain has this
issue, and it's just a question of where the LO lives with respect to
your desired carrier and your analog filtering capability. 

On 2017-07-14 11:42, Brais Ares via USRP-users wrote:

> Hello everyone 
> I'm trying to implement a RX-TX RF LOOPBACK but I may be having problems with local oscillators.
> The signal to capture is 100 KHz, so I configure rx-bw and tx-bw to its minimum values (286 KHz and 782 KHz), and Fs to 286 KSPS.
> As far as I understood reading this thread [1], I can avoid capturing the LO in RX by adding an LO_OFF VALUE TO "TUNE_REQUEST", this value being larger than Bsample/2. So I configure the following lo_off:
> RX (bw=286 KHz), lo_off = 200e3 
> TX (bw=782 KHz), lo_off = 500e3 
> ​If I capture the transmitted signal with a spectrum analyzer I can see my ​signal centered where expected, but also, out-of-band I can find the LO signal with high power and a replica.
> Is this the expected behaviour or is there any way to actually remove them? 
> ​Regards, 
> Brais.​ 
> ​​ 
> _______________________________________________
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> USRP-users at lists.ettus.com
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