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

From: Josh Blum
Subject: Re: [Discuss-gnuradio] Synchronization issue with multiple USRPs
Date: Tue, 26 Apr 2011 13:12:08 -0700
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20110223 Lightning/1.0b2 Thunderbird/3.1.8

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.


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