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

[Jonathan Friedman]

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:
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>>> Sent from the GnuRadio mailing list archive at Nabble.com.
>>>
>>>
>>>
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>>
>>
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>
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>
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