On 05/08/2018 02:17 PM, Reinhold Frederick William Hollender via
USRP-users wrote:
The additional cable length acts as a time delay, so I would calculate
the expected phase difference based on the frequency of the baseband
signal, not the RF signal. I have a couple different cable lengths,
so I can try running an experiment if I get some time later today.
Thanks again,
Steve
P.S. How do you respond within the mailing list? Each time I send
a new mail with similar subject and copying the previous replies
manually...
The list is copied on each email, so simply reply to all instead of
replying to just the sender.
Cheers,
William
The phase offset at baseband should be the same as at RF. If it wasn't,
then things like PSK modulators that do their modulation at baseband
would not
work correctly at RF. That clearly isn't the case.
I do phase-sensitive interferometry for radio astronomy. My fringe
rates are exactly what theory predicts from the *RF* frequency and antenna
spacing. If there was "phase scaling", that would not be the case.
Steve , I got the feeling from your original message, though you didn’t say
it outright, that you might be using the same USRP its self as the signal
generator with loopback cables.
If so, beware “self receive” via paths other than your calibrated length
cables confusing the results, a sensitive full duplex radio like this hears its
self via various leakage paths especially with RX and TX tuned to the same freq.
(I’m assuming in the original email the cable you added was 50.8cm or
0.508m, right?)
-Ian
>/ On May 8, 2018, at 9:34 AM, ROBIN TORTORA via USRP-users <usrp-users at
lists.ettus.com
<http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com>> wrote:
/>/
/>/ some small things:
/>/
/>/ I am pretty sure you cables are not phase matched (costs about 5K for a 26” matched pair), so you will get some difference there. Not sure how to quantify.
/>/ Splitters have a phase mismatch, I think its called phase unbalance,
proportional to cost :), but can be multiple degrees.
/>/ even between 2 channels on the same device, there will be some phase
noise
/>/ Still seems far away from 18 degrees, so cant help more than above...
/>/
/
>>/On May 8, 2018 at 11:17 AM "shachar J. brown via USRP-users"
<usrp-users at lists.ettus.com
<http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com>>
wrote: />>//>>/Hi Jeff, />>/Thanks for your response, but you
understood me completely wrong. />>/Of course I have set the RF
freq in the source block. Furthermore, I have fully analized the
signal from both chanels, and it appears crystal clear in all the
sinks throughout the flow graph (e.g., a pure max at the correct
bin after the FFT in the vector sink, and a beautiful phase gain
in the time sink after the phase extraction). />>/My problem isn't
receiving the signal or analizing it. My problem is that the phase
difference between the two channels does not match the theory. The
wire to one channel is longer than the other by at least 1/6 of a
wavelength, whilst the phase diff was only 1/10 of 2*pi. />>/Am I
understood? />>/Does anyone have a clue what's going wrong?
/>>/Thanks again, />>/Steve />>//>>//>>/On 05/07/2018 11:11 AM,
shachar J. brown via USRP-users wrote: />>/> Hi All, />>/> />>/> I
am trying to measure the phase difference of a received signal
between />>/> the two RX ports of the B210. (In the grc I simply
ran each of the two />>/> received signals through an FFT, took
the bin with highest amplitude and />>/> extracted it's phase, and
finaly - subtracted the two). />>/> />>/> I experimented with a
single signal source generator split in two. I />>/> first
connected both RX ports with matching wires and received zero
/>>/> phase difference as expected. />>/> />>/> Though when I
added a wire of some length to one of the ports, the />>/>
received phase difference was NOT as expected by theory. />>/>
/>>/> (In short, I sent 100[Mhz] pure sine wave, thus wavelength
of 3[m] or />>/> shorter, the extra wire was 0.508[cm] long, thus
I would expect a phase />>/> diff of about 60 deg or more. Frankly
I received a phase diff of about />>/> 18 deg). />>/> />>/> What
am I doing wrong? />>/> />>/> I thought maybe the phase
calculation of gnuradio is done on the />>/> baseband frequency
and not on the RF, and therefore the phase diff would />>/> be
different. Is this my problem? (e.g. if the baseband is only
30[Mhz], />>/> then expected phase diff would be 18 deg). If that
is the case - how can />>/> I know which baseband frequency the
AD9361 has chosen? />>//>>/GNU Radio works with signals at
baseband. It sounds like you might not />>/have set a RF frequency
in the USRP source block. The default is 0, and />>/I'm not sure
what the B210 would tune to in that case. />>//>>/I don't know
whether this experiment actually works, but to do it you />>/would
tune to 99M, set the sample rate to 2M, and see what happens. The
/>>/peak should be right in the middle. />>//>>/Also, make sure
your signal generator is sending out a very low signal - />>/try
something like -40dBm. Max is higher, but there's no need.
/>>//>>/> />>/> Thank you all for your time, />>/> Steve />>/>
/>>/> />>//>>/_______________________________________________/
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