[USRP-users] Odd behavior with received power on SBX 120

Marcus Müller marcus.mueller at ettus.com
Tue Sep 16 06:55:19 EDT 2014


Hello Hrishikesh,

this looks like crosstalk; you should expect at least -60dB crosstalk on
any RF system that runs in the same box, basically. Given that the RX
side is quite sensitive, there's not much you can do to circumvent that;
it's just that physics doesn't allow circuit designers to have
attenuations higher than that without going great lengths and tolerating
high costs.

Greetings,
Marcus

On 16.09.2014 06:03, Hrishikesh Shelar via USRP-users wrote:
> Hi all,
>
> I am experiencing some odd behavior with loopback tests and received power
> with the SBX-120 daughter card. After tuning both the TX/RX and RX2 side to
> the same frequency I have observed that the input signal power on the RX
> chain doesn't depend on the external loopback cable plus attenuators used
> (its as if the two sides of the daughter card were connected internally). I
> have confirmed this by removing the cable all together and applying dummy
> loads to both sides.
>
> Based on my custom logic in the RX chain I have seen the received power
> swing from the 13th MSB to the 21st MSB (out of 24 bits) just by altering
> the tx-gain (0-31dB) and rx-gain (0-31 dB) parameters through UHD. So 8bits
> of swing is about 10*log(2^8) or ~55 dB ratio which make sense if the
> combined realized gain of the tx-gain and rx-gain parameters is 0-62 dB.
>
> With my custom logic I was able to loopback OQPSK at 10 mbps, *all with no
> external loopback cable!!!* Is there anything I am missing here? I can't
> make sense of these results. o.O
>
> Thanks,
> Hrishikesh
>
>
>
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