Tom
I ran across TvB on one of my ‘time
forays’ off the FlexRadio Forum last year. This guy is amazing! I
probably spent 8 hours reading about his ‘shack’ and what he has
done to get many stabilized time sources, and wandering around these websites.
Talk about dedication to one hobby and becoming a master! WOW.
All of this is worth the read folks! If
just to marvel at the work and dedication.
Eric
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED]]
On Behalf Of Tom Clark, W3IWI
Sent: Thursday, November 24, 2005
1:30 AM
To: Jim Lux
Cc: [EMAIL PROTECTED];
FlexRadio@flex-radio.biz
Subject: Re: [Flexradio] Frequency
stability and calibration
Jim Lux wrote:
There ARE actually sources with better close in phase noise than a quartz
crystal, just in case you see one at a hamfest or surplus place (or, you're
wealthy enough). A hydrogen maser, for instance (that's what we use at
work, JPL, when we're concerned about such things.. but then we have an
infrastructure to distribute the maser signal around, and a budget for the
support staff).
Actually, all H-Maser I know rely on a really high
quality xtal for their short-term stability (and hence intrinsic phase noise);
by high quality, I mean BVA xtal units costing in the $5k range. The transition
from the BVA xtal to the maser is typically done at times ~30-100 seconds or so
(see the AVARs in my tutorials I mention later, or http://www.leapsecond.com/museum/manyadev.gif
to see that the BVA performance is better than the Maser up to ~30 seconds. The
goal is to "hand off " from one oscillator to the next when their
Allen deviation is equal).
BTW -- we actually have a couple of "amateurs" that have both passive
& active H-Masers in their basements. One is Tom vanBaak (no call) whose
efforts can be viewed at http://leapsecond.com/
and another is Jim Jaeger (K8RQ) (see http://www.clockvault.com/
if you can stand the music!). TvB offered a review paper on amateur timekeeping
at the 2003 PTTI meeting, which can be fetched at http://tycho.usno.navy.mil/ptti/ptti2003/paper35.pdf.
Also be sure to note TvB's "Most Accurate WristWatch" when you log
onto leapsecond.com.
I've said it before, and I'll repeat it now -- you are better off thinking in
the frequency (and phase noise) domain when you are considering oscillators on
time scales shorter than tens of seconds, and in the time domain for minutes
and longer. If you are interested in these topics, you might want to fetch one
of my "Timing for VLBI" tutorials at http://gpstime.com/
. In my past incarnation I ran NASA's Geodetic VLBI program and was responsible
for H-masers as time and frequency standards.
While I am on here making comments on this thread, I note that Alberto, I2PHD
is using a circuit similar to the one I developed for locking an xtal to the 10
kHz output from the Connexant/Navman Jupiter-T receiver. A couple of notes on
what I found:
- My initial effort also used 74HC390 dividers as a
ripple counter to get from 10 MHz -> 10 kHz. But I found that the
propagation delay thru these dividers varied strongly with temperature,
amounting to a couple of hundred nsec in a day. I fixed this problem by
using a simple, but elegant circuit developed by Tom van Baak (see http://www.leapsecond.com/tools/ppsdiv.zip)
which uses a PIC with its clock input driven by the 10 MHz signal and a
finite state machine that executes a fixed number of instructions to
generate lower frequencies. Not only is it a very stable synchronous
divider, but also it need only a couple of $$ worth of parts.
- I did a lot of work to optimize loop time
constants to try to achieve performance at the couple of nsec levels. Most
of the time, the Jupiter-T steered the oscillator very well, but about
once per hour, the 10 kpps (and 1pps) output sawtooth goes thru a
zero-beat, with a fixed bias error spanning intervals of 10s of seconds.
You can see some of these sawtooth "hanging bridges" that really
screw up the locking in my tutorials on gpstime.com. And you can see the
fix that Rick (W2GPS) is using in his latest CNS clock using the M12+ in
the latest of the gpstime.com tutorials.
73, Tom
