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Re: [Discuss-gnuradio] External clock source Info?

From: Mark J. Blair
Subject: Re: [Discuss-gnuradio] External clock source Info?
Date: Tue, 17 Aug 2010 15:14:07 -0700

On Aug 17, 2010, at 2:24 PM, Gregory Maxwell wrote:
> I'd also like to echo the 10MHz comment.  GPSDOs Clocks with excellent
> long term stability show up at fairly low prices on ebay all the time
> (excess from cell site deployments, I assume).  I have a couple of
> them.   What they don't usually have is the very low phase noise that
> I'd want for a clock which is eventually going to multiplied up to GHz
> levels.   So a USRP1 clock board which was primarily a 10->64MHz
> up-converter with very low phase noise would be exactly what I would
> want.

For my bench frequency reference, I selected a surplus Trimble Thunderbolt with 
the newer version of OCXO that's supposed to be nearly as good as the HP 
double-oven OCXOs. I'll power it from an HP bench supply, since the switchers 
that are often supplied with the surplus Thunderbolts are said to add a lot of 
phase noise to the oscillator output. If the statistics from the monitoring 
program that I used during my first test run of the oscillator are to be 
trusted, then it should be able to provide a reference accurate to within tens 
of parts per trillion. I haven't finished installation of my GPSDO yet; I still 
need to finish repairing the (cheap, broken) HP bench supply that I got from 
eBay and assemble a power cable. I may get this done tonight, as the parts I 
needed just arrived today. The outdoor antenna (a surplus Lucent +26dB antenna 
as used at cell sites) is already installed and cabled into my house.

My own idea for a USRP clocking replacement was to use a common and fairly 
inexpensive VCTCXO rated for temperature stability of around +/-2.5ppm 
(available at Digi-Key in frequencies commonly used in cell phones, GPS 
receivers, etc.), drive a PLL+VCO with that to generate 64 MHz (or 52 MHz, or 
any other frequency that the TCXO+PLL+VCO can generate), with an additional PLL 
to lock the VCTCXO to an external 10 MHz input. I'd also include a 
microcontroller which could measure the VCTCXO control voltage while locked to 
an external reference, and then drive that same voltage with a DAC when the 
external reference isn't present. Thus, while attached to the GPSDO the 
internal reference would be slaved to a very good reference, and then when that 
reference isn't available (such as when operating "in the field") the on-board 
VCTCXO would at least be trimmed to compensate for aging. The memorization of 
the nominal VCTCXO tuning voltage might be triggered automatically, or by a 
push button, or by a GPIO controlled by the USRP hardware.

The VCTCXO might be replaced with a voltage-controllable OCXO if desired for 
better holdover stability, but I figured that VCTCXO should be adequate for my 

Does that architecture sound reasonable? If so, would anybody like to save me 
the trouble of designing and building it myself? ;)

Mark J. Blair, NF6X <address@hidden>
Web page: http://www.nf6x.net/
GnuPG public key available from my web page.

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