[USRP-users] TX and RX paths sharing a coherent LO

Matt Ettus matt at ettus.com
Thu Oct 9 14:30:56 EDT 2014


Every LO in the system, whether for TX or for RX is generated independently
by locking to the same master reference, and so every LO will exhibit
mutual phase noise to the others.  There isn't a mechanism to generate a
single LO and share it between boards.

To be clear, the frequencies will all be exactly the same, and the phase
will be constant between them.  The only thing you lose is that they have
different uncorrelated phase noise between them.

Matt


On Wed, Oct 8, 2014 at 2:44 PM, Rickard Radio via USRP-users <
usrp-users at lists.ettus.com> wrote:

> OK, interesting.
>
> Is it the exact same situation with two (or more) different transmitters
> like TX1-TX2 as you explained here with TX-RX, namely that each path always
> have physically different LO's but they are generated from the same 10MHz
> reference?
> Or can TX1-TX2 (or RX1-RX2) indeed use physically the same LO (i.e. with
> same phase noise)?  Or is that impossible too?
>
> That is, is there any difference with synched TX-RX compared to synched
> TX1-TX2 (or RX1-RX2)?
>
> The thing is that in my experiment I preferably would like to use
> physically the same LO for all RX's and TX's (i.e., with exactly the same
> frequency and jitter etc.) but perhaps I need to resort to synched, but
> still ever so slightly different, LO's for each path.
>
> Regards,
> Rickard
>
> On Wed, Oct 8, 2014 at 7:19 PM, Marcus Müller <usrp-users at lists.ettus.com>
> wrote:
>
>>  No, the LOs used for mixing are individually generated from the same
>> 10MHz reference; they thus don't drift away from each other, but they don't
>> share the same jitter etc. However, the Ettus daughterboards usually use
>> rather decent synths, and thus the LO quality should be dominated by the
>> quality of the reference clock -- which again, is quite ok, for most
>> applications, and can be further optimized by GPS-disciplining the
>> reference.
>>
>> Usually, that's how close you can get to shared LO if you want
>> independent TX and RX mixers.
>>
>> Greetings,
>> Marcus
>>
>>
>> On 08.10.2014 19:04, Rickard Radio via USRP-users wrote:
>>
>> Sounds good!
>>
>> Does this mean it really shares the same LO, i.e., also with the same and
>> uniquely occurring phase noise shared to all paths?! (I want that!)
>>
>> Regards,
>> Rickard
>>
>>
>>
>> On Wed, Oct 8, 2014 at 6:54 PM, Marcus Müller <usrp-users at lists.ettus.com> <usrp-users at lists.ettus.com>
>> wrote:
>>
>>
>>   Hi Rickard,
>>
>> all the LOs are synthesized from the same motherboard reference, so Q1 is
>> answered with "you get that for free".
>> Q2 is "yes".
>> Q3 gets the answer "if you want to use definite times when sampling, use
>> the timed commands feature of UHD".
>> :)
>>
>> Greetings,
>> Marcus
>>
>> On 08.10.2014 18:48, Rickard Radio via USRP-users wrote:
>>
>> Dear usrp-users,
>>
>> I am interested in coherently LO-mixing different TX and RX paths for some
>> experiments.
>>
>> Q1: Which options exist to use the same(?) coherent(!) LO for different TX
>> and RX paths on the USRPs (N210 and/or B210)?
>> Q2: Related, with the N210s and the MIMO cable, can I obtain and use a
>> coherently shared LO for different TX and RX paths?
>> Q3: Follow up  - do I need something else or in addition ?
>>
>> Regards,
>> Rickard
>>
>>
>>
>>
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