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Re: [Discuss-gnuradio] changing the vctcxo

From: Matt Ettus
Subject: Re: [Discuss-gnuradio] changing the vctcxo
Date: Mon, 26 Mar 2007 16:21:52 -0700
User-agent: Thunderbird (X11/20070302)

Roshan Baliga wrote:
Matt Ettus wrote:
> The rev 4 and newer USRPs have a clock generator chip that is spec'ed to
take a signal as low as 150 mV p-p. However in my testing I found I needed a voltage as high as 8 or 9dBm. 8dBm is about .8V, so that should still work.

Sorry if this isn't relevant to everybody on the list, but it is *very* important to those of us who are using external clocks with the Rev 4 USRPs.

I tried out varying clock inputs, and verified Matt's claim that 800 mV pp was required for consistent USRP operation. This didn't seem correct to me, because Analog (who makes the clock distribution chip) normally is very good about their specifications.

After further testing (and datasheet reading) we discovered two things:

(1) If you're using an external clock, in addition to the changes specified under "Rev 4 USRPs - For the Slave Boards" at:


You should probably also remove C924, the 0.01 uF cap going to X2 pin 3.

Thanks for figuring this out! I've updated the version of the page on the new wiki:



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