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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
needs.
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.