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Re: [Discuss-gnuradio] USRP1 Clock drift

From: Nick Foster
Subject: Re: [Discuss-gnuradio] USRP1 Clock drift
Date: Thu, 06 Jan 2011 15:42:31 -0800

On Fri, 2011-01-07 at 10:01 +1100, Matt Robert wrote:
> Hi Guys,
> I ran Kalibrate against my pair of USRP1's using GSM900 towers (around
> 940MHz) and found my first unit to me 5kHz off and the second was 13kHz off.
> Is this within limits for the built in oscillator? I have been using
> them to transmit P25 frames around 435Mhz and found that my commercial
> P25 radios will not decode anything coming from the second USRP. The
> first USRP works OK. I have swapped around all daughterboards between
> both USRPs and the only common factor is the second USRP motherboard so
> I am sure its due to the clock in that unit.

This is within spec. It doesn't tend to drift that much so if you
characterize how far it's off it'll stay reasonably constant (to a few
ppm) unless temperature changes drastically.

> Perhaps clocktamer or a secondhand GPSDO is needed for my Tx applications.

You will need an oscillator with a ~64MHz output to run the USRP1. There
is no 10MHz reference input on USRP1. I used a GPSDO and a homebrew PLL
based around an Si570 VCXO to keep mine synchronized, but it wasn't a
quick hack. Unfortunately there don't seem to be many 64MHz TCXOs in a
reasonable form factor out there.


> Cheers,
> Matt
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