Re: [Discuss-gnuradio] unknown phase-relation between rx and tx

[wolfgang buesser]

Sorry,

I may be completely off track, but I dont' see any clock-source on the
schematic of my 
LF-RX/TX (not basic RX/TX).
As far as I understand they only process the signals as baseband without any
frequency
conversion.
Yes, I have to use 2 daughter-boards (one LF-RX and one LF-TX) to complete
the path,
but if they both transfer the signals to the same AD9862 without any
conversion I expect
the phase-correlation to be an issue of the interpolator/decimator inside
the AD9862 or
potentially the FPGA-code.

Thank's
Wolfgang


Jonathan Friedman-2 wrote:
> 
> That's true, but their are two frequency synthesizers (ADF4360-0 on my
> flex2400) on each flex daughter card (one for TX and one for RX -- or
> more generically one for each sub-subdevice). On the basic daughter
> cards (which only have one sub-subdevice) there is only one
> synthesizer, but you need a second daughter card to complete the path
> (one for TX, one for RX).
> 
> Ultimately, this is not a software (firmware fix) or sampling problem
> (use the same ADC or supply sync signal). This is a physics problem
> that results from using two independent oscaillators that are not
> phase locked to each other. Resultingly, you need a hardware fix to
> achieve coherence. That said, this is a problem that will manifest
> itself in your real-world application and is not easy to solve except
> by phase locking the oscillators through a cable (which doesn't work
> well for anything, except really short separation distances).
> 
> What is your ultimate application?
> 
> Cheers,
> -Jonathan
> 
> On Sun, Jun 22, 2008 at 3:58 AM, wolfgang buesser
> <address@hidden> wrote:
>>
>> Just to make sure:
>>
>> I am using LF RX/TX-boards.
>> TX and RX are on the subdev (thus using the same AD9862)
>>
>> Thanks
>> Wolfgang
>>
>>
>>
>> Jonathan Friedman-2 wrote:
>>>
>>> Unless I misunderstand what you are doing, what you are describing is
>>> the coherence error of the system. The long-answer made short is "no".
>>> You have to calibrate it out or otherwise live with it. The TX and RX
>>> paths of the USRP are derived from separate hardware local oscillators
>>> so there will always be an unknown phase error between them. What's
>>> worse is that it will vary slowly and dynamically as temperature and
>>> voltage change during system operation.
>>>
>>> Cheers,
>>> -Jonathan
>>>
>>> On Sun, Jun 22, 2008 at 2:42 AM, wolfgang buesser
>>> <address@hidden> wrote:
>>>>
>>>> Hi,
>>>>
>>>> Iam using
>>>>   gr.sig_source_s (self.usb_freq (), gr.GR_CONST_WAVE, 0, 0, 0)
>>>> to generate a carrier and
>>>>   usrp.source_c(nchan=2,decim_rate=self.interp) to resample it at the
>>>> same
>>>> frequency.
>>>>
>>>> I get what I expect: 2 complex numbers which are pretty constant. But
>>>> with
>>>> every restart
>>>> of the program the phase changes (apparently by the same amount for
>>>> both
>>>> channels). The
>>>> amplitude does not vary much from run to run.
>>>>
>>>> It seems to me, that the phase relation between the interpolator in the
>>>> TX
>>>> and the decimator in
>>>> the RX changes with every restart.
>>>>
>>>> Is there a way to "reset" the  interpolator/decimator so that I get the
>>>> same
>>>> reading for every start
>>>> (as long as I don't change frequencies) ?
>>>>
>>>> Thank's
>>>>   Wolfgang
>>>> --
>>>> View this message in context:
>>>> http://www.nabble.com/unknown-phase-relation-between-rx-and-tx-tp18052839p18052839.html
>>>> Sent from the GnuRadio mailing list archive at Nabble.com.
>>>>
>>>>
>>>>
>>>> _______________________________________________
>>>> Discuss-gnuradio mailing list
>>>> address@hidden
>>>> http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
>>>>
>>>>
>>>
>>>
>>> _______________________________________________
>>> Discuss-gnuradio mailing list
>>> address@hidden
>>> http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
>>>
>>>
>>
>> --
>> View this message in context:
>> http://www.nabble.com/unknown-phase-relation-between-rx-and-tx-tp18052839p18053437.html
>> Sent from the GnuRadio mailing list archive at Nabble.com.
>>
>>
>>
>> _______________________________________________
>> Discuss-gnuradio mailing list
>> address@hidden
>> http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
>>
>>
> 
> 
> _______________________________________________
> Discuss-gnuradio mailing list
> address@hidden
> http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
> 
> 

-- 
View this message in context: 
http://www.nabble.com/unknown-phase-relation-between-rx-and-tx-tp18052839p18054066.html
Sent from the GnuRadio mailing list archive at Nabble.com.