From: "Bruce Lane" kyrrin@bluefeathertech.com
Subject: [time-nuts] 10MHz to 32MHz?
Date: Sat, 21 Apr 2007 11:26:23 -0700
Message-ID: 200704211126230290.4117897A@192.168.42.129

The most straight-forward fashion is naturally to lock it through a PLL.

Highest common comparator frequency is 2 MHz, so divide 10 MHz by 5 and 32 by
16, compare with SR-gate, use either a passive lag or better yeat, and active
loop. Three digital ICs and an op-amp. If the 32 MHz reference is a crystal
then hooking into the caps for a varicap is easy enought for most hams.

Cheers,
Magnus

You might look at using a Reflock II
(http://www.tapr.org/kits_reflock_ii) to discipline a 32MHz VCXO from
your 10MHz.  Unfortunately, the Clock-Block phase noise is nowhere near
good enough to use for an RF application, so that's not an option. :-(

73,
John

Bruce Lane said the following on 04/21/2007 02:26 PM:

Bruce,

Buy an HP 3586 on eBay (I paid $90 for my HP 3586A with OCXO), lock its
reference to the GPSDO, set it to 32 MHz and use the LO output. All you
need is a sturdy shelf (!!!) and a couple of cables. No soldering iron
required, no PIC programming or anything else :-)

Maybe not the most elegant, but you could find all sorts of things to do
with an HP 3586...

That will work with all sorts of non-standard Xtal oscillators in the
400 Hz to 32 MHz range, with 0.1 Hz resolution and pretty darn good
phase noise.

PS: I have a number of synthesized ham transceivers here, and they all
have funky reference oscillators...

Didier

Bruce Lane wrote:

Hi John:

Is there any data on how the Reflock II does as the basis of a GPSDO compared
to other ones?

Have Fun,

Brooke Clarke
w/Java http://www.PRC68.com
w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml
http://www.precisionclock.com

John Ackermann N8UR wrote:

There is a firmware image for the Reflock II that will lock a 10 MHz (or
other relatively-low-frequency) oscillator to PPS.  The last time I
looked at it, it worked OK but not up to a "real" GPSDO -- which makes
sense since it's a pretty basic PLL.  Luis Cupido has updated the PPS
code since then, though, and for all I know it might be much better now;
I haven't had a chance to test the new code.

John

Brooke Clarke said the following on 04/21/2007 06:29 PM:

John Ackermann N8UR wrote:

John

Unless the hardware has been radically redesigned, changing the software
doesn't get around the fundamental limitation that, since the VCXO being
disciplined is used to measure the phase error, the phase error
resolution is limited by the OCXO period. For a 10MHz VCXO the phase
resolution will at best be 100ns (it may be even worse depending on
actual implementation details).
There is also the likelihood (in the absence of insufficient noise) that
averaging the phase values so obtained will not improve the precision
over the one shot values since the VCXO is being disciplined to be a
multiple of the PPS frequency.

Thus while the REFLOCK II is better than nothing its performance will
fall far below a well designed real GPSDO in this application.
It will work much better for locking to higher frequencies where an XOR
or phase detector with similar resolution is employed.

The obvious solution of multiplying the 10MHz VCXO frequency up to say
100MHz or so to improve the phase error resolution to a multiple of
10ns, is unlikely to be particularly satisfactory as the designer hasn't
used a synchroniser in the design.

Bruce

One solution: double the 10 MHz twice to get 40 MHz, divide by 5 with any
handy 74HC or 74F counter to get 8 MHz, and mix that with the 40 MHz signal
to get 32 MHz and 48 MHz.  Basic LC filtering should be adequate in the
10->40 MHz multiplier and the 32 MHz output sections.

Another solution: drive an AD9852 DDS with the 10 MHz, and pick a
clock-multiplier setting that gives the best SFDR near 32 MHz.  That only
requires two chips, but one of them would have to be a microcontroller.

-- john, KE5FX

Dr Bruce Griffiths said the following on 04/21/2007 06:59 PM:
multiple of the PPS frequency.

Absolutely agreed.  The Reflock II was really designed to lock high(er)
frequency oscillators to low(er) frequency references.  The PPS input
capability was a sort of "well, yeah, we can do that" and for some
applications it's adequate -- but it's in no way intended to replace a
real GPSDO.

John

John Miles wrote:

John

Another solution:

1. Divide the 10MHz by 5 with a synchronous counter configured for a 2:3
   (or 3:2) mark spacer ratio for the 2 MHz signal.
2. Isolate the 16th harmonic with a bandpass filter

With a 2:3 mark space ratio the 5th, 10th, 15th harmonics of 2MHz will
have zero amplitude.
This leaves only the fundamental plus the 2nd, 3rd, 4th... 14th, 17th,
... harmonics to filter out.
However the amplitude of the 16th harmonic will only be about 1/16 of
that of the fundamental.

Bruce