Thanks for the suggestion.
I have tried this and it turns out, the noise is mostly phase noise. Simply by
subtracting the phase of the second channel from the first, I can reduce the
noise already by about 20 dB.
May be, more reduction is possible by some sophisticated mathematics.
Best regards,
Erik
________________________________
Von: Sylvain Munaut <246...@gmail.com>
Gesendet: Mittwoch, 11. Dezember 2019 10:52:51
An: Erik Heinz
Cc: USRP-users
Betreff: Re: [USRP-users] B210: LO and noise issues
Hi,
> I wonder if there is an operation mode where TX and RX use the SAME LO, or
> some trick to achieve this. Probably not?
The AD9361 itself allows for external LO injection ( RX_EXT_LO_IN ,
TX_EXT_LO_IN ), but that's not broken out in anyway on a B2xx.
You'd need serious rework skill to get to them. They are on the edge
of the BGA, so it's doable ...
> The difference between using one LO for TX and RX and using two separate
> synthesizers, even if they are phase-synchronous and set to the same
> frequency, is that two synthesizers have independent 1/f noise introduced by
> the PLL. If using one LO, and feeding TX back to RX, this noise is canceled
> by the mixing and does not show up in the base band signal.
How about using a direct cross connect from the second TX channel
(transmitting a DC tone) to the second RX channel. Wouldn't that allow
you to cancel out the noise mathematically ?
Cheers,
Sylvain
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