[time-nuts] TCVCXO Adjustment

Wayne Holder wayne.holder at gmail.com
Fri Apr 13 19:13:21 EDT 2018

Again, thanks for all the great feedback and suggestions.

> Are you familiar with these devices which I just found this week?
> https://tentaclesync.com/products

Yes, that's one of the lower cost commercial units available.  Another is
the NanoLockiIt by Ambient
which is company that's been making timecode products for many years.
Compared to more traditional prices for timecode generators, these are
relatively inexpensive at about $300.  However you need at least two, or
more generators to be useful, so that adds up pretty fast for an amateur
videographer, or starving film school student.  In contrast,  BOM for the
design I'm working on is less than $30 (the TCVCXO being, by far, the
most expensive part.)

My plan is to also write a desktop application, probably in Java to make it
portable, that the person building the devices could use to perform the
initial calibration and also setup various options.  So, the NTP-based
solution is attractive in that it doesn't require any additional hardware.
I'm a Mac user so, after a bit of reading the NTP implementation on the
Mac, I tried a few experiments.  Typing "ntpq -p" in the terminal
app produced this response:

     remote           refid      st t when poll reach   delay   offset


*usdal2-ntp-001. .GPSs.           1 u  428 1024  377   51.131    1.944

and typing  "ntpq -c rl" printed out:

associd=0 status=0615 leap_none, sync_ntp, 1 event, clock_sync,

version="ntpd 4.2.8p6 at 1.3265 Fri Feb  5 17:38:17 UTC 2016 (124.60.2~39)",

processor="x86_64", system="Darwin/16.7.0", leap=00, stratum=2,

precision=-20, rootdelay=51.131, rootdisp=34.160, refid=,

reftime=de7ba9c1.937e5f86  Fri, Apr 13 2018 15:12:17.576,

clock=de7badf7.39f8d36a  Fri, Apr 13 2018 15:30:15.226, peer=7077, tc=10,

mintc=3, offset=1.944153, frequency=25.163, sys_jitter=0.000000,

clk_jitter=0.745, clk_wander=0.001

I believe that the "precision" of -20 value on the 4th line is supposed to
be interpreted as 2^-20 seconds which, if my math is correct, works out to
be a precision of about 1 PPM. Is that correct?  If so, it would seem like
I should be able to use my system's internal clock to perform a "tweak" in
around 10,000 seconds, or a little less than 3 hours.  Does this seem
correct, or have I missed something?

Alternately, if I included a GPS receiver in the design, the whole process
could be done within the device, which would probably be the easiest
approach to calibration for the person building one.  This would increase
the cost and make the device larger, but users could then maintain
calibration by periodically keeping them plugged in for a few hours.  Or,
perhaps I could just design a 2nd board for a GPS "calibrator" module that
could be plugged into the timecode generators to calibrate them.  Hmm...
lots to think about.


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