Re: [Discuss-gnuradio] Multi-N210 synchronization issue

[Marcus D. Leech]

On 06/11/11 06:11 AM, Khalid Jamil wrote:
> Hi,
>
> I am facing a strange problem in synchronizing and recording data from
> multiple (4 or 8)  N210s. 
>
> The problem: 
> After each data writing to the files on disk, it seems that each N210
> re-tunes/ re-locks to the frequency references. It changes the fixed
> phase offset from channel to channel. It kills the purpose of
> providing a calibrating signal to ascertain the fixed channel-channel
> phase differences. Why?
>
> Setup: 
> Eight N210+WBX devices are synchronized with external 10MHz and 1PPS
> signal. The data is routed from all channels through a gigabit switch
> to computer where GRC gnuradio-companion is being used on Ubuntu. Here
> a multi-usrp block is created, each channel tuned to the same
> frequency (900 MHz) with same sampling rate (1 MS/s).  USRP source is
> connected to either a scope or file sink. The type of sink is chosen
> in runtime using a switch block in GRC. 
>
> The purpose is to apply and record a calibrating signal which is
> essentially a tone at the carrier frequency to find out the fixed
> phase offsets from channel to channel. This is used to apply the
> correction phase errors later to the actual signals of interest.
>
> The problem occurs when I apply a calibrating signal and push the
> button to switch from scope sink to file sink for a couple of seconds.
> After I switch it back to scope, the channels have now a different
> phase offsets than before. It seems that channels have re-tuned and
> re-locked. So, this way, I am unable to know the phase offsets from
> channel-channel and hence the system cannot be calibrated. 
>
> The attached file shows a four channel setup. I have a similar eight
> channel one.
>
> Any suggestions on why it is happening (re-tuning after file write)
> and how to calibrate the multi-channel system.
>
> Thanks,
>
> Khalid.
>
>
>
>
My recollection of the way the "Select" block works is that it causes
flow-graph hierarchy reconfiguration.
  Which requires that the graph be *stopped* and *restarted* every time
you switch.  This would cause
  phase-hits in the recorded data, and I think it may also cause the UHD
sources to shutdown and
  re-instantiated.

Consider a different approach.  I'm assuming that you are using a
"select" here so that you can watch
  for correct phasing on the 'scope display before you start recording. 
Perhaps instead, record
  all the time, and insert zeros into the data stream while you're in
the "mucking about" state, and
  when you're happy, you record the actual data.  Just insert a
multiplier into the stream in front
  of the file-sinks, and have the multiplier be either '1' or '0'
depending on what state you're in.

There are probably thinks with stream tags you could do as well, but I'm
not that familiar with them.


-- 
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org