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Re: [Discuss-gnuradio] changing the vctcxo

From: Roshan Baliga
Subject: Re: [Discuss-gnuradio] changing the vctcxo
Date: Fri, 23 Mar 2007 15:04:46 -0800
User-agent: Thunderbird (X11/20060719)

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.

(2) Because the holes for J2001 come with solder in them, when soldering/pushing an SMA connector on, it is very easy to break or mangle the signal trace from J2001 to C927 (and therefore, to U702, the AD9513 clock distribution IC).

I believe that (2) is why Matt and I noticed a higher input signal requirement. On my board, with J2001 physically disconnected from C927, the leakage was enough to send a dirty/noisy clock into U702, which lead to flakey USRP performance.

However, once we connected the signal pin on J2001 correctly to C927, and removed C924, the USRP worked flawlessly, even down to 200 mV pp input.


NB For those EE types, you may wonder why I recommend removing C924. It's true that if pin 3 on X2 is DC, leaving C924 in shouldn't make a difference. However, removing C924 makes me feel better, because I know that U702 is completely disconnected from X2, and isn't affected if pin 3 on X2 changes at all.

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