Re: [Discuss-gnuradio] Synchronization issue with multiple USRPs

[David Evans]

This has never been clear to me! 
What about the phase ambiguity due to the fractional-N VCO's, as Marcus has 
mentioned?

This has to be calibrated out, yes/no?, but how?, unless after a retune you 
then 

TX a calibration sequence that you can correlate against and correct. But after 
this, then, do you really need to accurately wait for a PPS? It's just stuff in 
the buffer, with the computational overhead of alignment.
Basically, how do you compensate for the ambiguity? (maybe it doesn't matter 
and 

I'm probably talking rubbish :-/ )

Cheers,

David


----- Original Message ----
From: Josh Blum <address@hidden>
To: Khalid Jamil <address@hidden>
Cc: address@hidden
Sent: Tue, 26 April, 2011 21:12:08
Subject: Re: [Discuss-gnuradio] Synchronization issue with multiple USRPs



On 04/26/2011 11:22 AM, Khalid Jamil wrote:
> Please correct me if I am wrong. I think that this random channel-channel
> phase offset at each acquisition depends on when each usrp 100MHz LO locks
> to 10MHz external frequency reference by PLL. How PPS will help in that
> case? Unless PPS is locked somehow to 10MHz clock?
> 

Yes, but I strongly recommend that you get time-aligned samples working
before trying to calibrate for the LO phase offsets.

In your GRC flowgraph, each USRP source block will provide samples that
start at a different random time that could be offset by many
milliseconds. I don't think you want that?

In order to get time-aligned samples from N devices, you must

1) synchronize the the time registers (seconds/ticks) across all USRP
devices. You should use the PPS to do this

2) tell the devices to stream at the same time.

> Is there a good example to follow, may to start with 2 or 4 channels?
> 

There is an example that comes w/ uhd called rx_multi_samples. if you
want to do this in GRC, follow the instructions in the previous email.

-Josh

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