[USRP-users] Phase compensate issue

Marcus Müller marcus.mueller at ettus.com
Sun Nov 12 12:19:38 EST 2017


Dear Hojoon,

the B200 can't go that far down in frequency, so 3 MHz is impossible.

But let's say you want to do 300 MHz.

You say "phase is 60°", but phase is always *relative* to something. So,
your question only makes sense in conjuction with another wave or
theoretical source of time.

> But, I heard that there is a consistent phase offset after tuning.


That isn't true for the B200. Here, the phase after tuning the LOs is
random.


> Thus, when we transmit the cosine signal as it is, it will have phase offset. I dont
care the amplitude, but only care the exact phase.


What does "the same phase" even mean for two tones of different
frequencies? This makes no sense to me!


> In order to compensate the phase offset, we are going to receive the
> signal which is transmitted from the Tx/Rx channel since we don't know
> the phase offset before it transmits. If we receive the signal, then
> we can know the phase offset 'theta'. since we know the phase offset
> theta, to compensate the phase offset, We multiply that signal by -theta.
>

You will need to tune your receiver too, since receive and send
bandwidths are the same, so without more work (you could implement
something like a staggered alternating TX and RX tuning, but that
doesn't sound like a feasible solution to any problem I can think of),
this is not a solution to a problem you could encounter.

>
> In summary,
>
>
> (a) USRP B210 transmit the signal through the Tx/Rx channel, which
> will have phase offset, say X.
>
> (b) the usrp receive the (a) signal to know the phase offset X through
> RX2 channel(i.e. Aloopback using a single USRP.)
>
If you add another USRP to the mix, you add, again, phase uncertainty!
>
> (c) multiply the signal samples by -X  to compensate the phase and
> transmit!
>
You mean exp(-jX), right?

> 1. Will it be possible??

Yes, I think so, but as explained above, I'm not sure what "compensated
phase" even is if you have two different frequencies.
And, your receiver would need to be able to phase-coherently sample both
your old and your new frequency – but since you had to tune TX, you
would also have to tune RX, so, no, I don't think this approach works.

> 2. Marcus said that The SBX has a special architecture based on a LO
> synthesizer that allows to have a deterministic phase after "tuning"
>
> What is the exact meaning of tuning? it means frequency lock?
>
Oh, ok. Here, tuning is the act of changing the mixing frequency of a
receiver or transmitter.

Best regards,
Marcus


On 12.11.2017 17:44, Hojoon Yang via USRP-users wrote:
>
> Hi guys!
>
>  
>
> please let me know whether following operation is possible!
>
>  
>
> we have USRP B200.
>
>  
>
> All I want to do is to transmit a specific signal without distorting
> phase.
>
>  
>
> Say, I want to transmit the cosine wave, which have center frequency
> fc = 3Mhz and phase is 60 degree(i.e. y = cos(2*pi*3e6*t + pi/3) )
> through the Tx/Rx channel in USRP B200.
>
>  
>
> But, I heard that there is a consistent phase offset after tuning.
> Thus, when we transmit the cosine signal as it is, it will have phase
> offset. I dont care the amplitude, but only care the exact phase.
>
>
> In order to compensate the phase offset, we are going to receive the
> signal which is transmitted from the Tx/Rx channel since we don't know
> the phase offset before it transmits. If we receive the signal, then
> we can know the phase offset 'theta'. since we know the phase offset
> theta, to compensate the phase offset, We multiply that signal by -theta.
>
>
> In summary,
>
>
> (a) USRP B210 transmit the signal through the Tx/Rx channel, which
> will have phase offset, say X.
>
> (b) the usrp receive the (a) signal to know the phase offset X through
> RX2 channel(i.e. Aloopback using a single USRP.)
>
> (c) multiply the signal samples by -X  to compensate the phase and
> transmit!
>
>
> 1. Will it be possible??
>
>  
>
> 2. Marcus said that The SBX has a special architecture based on a LO
> synthesizer that allows to have a deterministic phase after "tuning"
>
> What is the exact meaning of tuning? it means frequency lock?
>
>
>
> _______________________________________________
> USRP-users mailing list
> USRP-users at lists.ettus.com
> http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com

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