[time-nuts] advice: frequency calibration to 1 ppm possible without GPSDO?

[Chris Albertson]

On Fri, Feb 18, 2011 at 11:51 AM, beale <beale at bealecorner.com> wrote:
> Hi Kasper,
>
> Thank you, your nft program looks very interesting! It turns out the local ISP that provides my DSL, sonic.net runs a GPS-driven NTP server and I have a pretty low and stable latency, so this may work well.  To measure a relative frequency offset, I presume that what limits accuracy is the total length of time of the test. So with a stable latency, I should not need to poll the server very often.

You don't have to worry about how often to poll.  the NPT software
will figure thatout based on the characteristics of your servers.  Yes
an "S" on servers.  not matter how good never depend on one NTP server
the minimum usful number is three.  use "pool" servrs for the others
and they will be randomly asigned via round robin DNS.     The way NTP
works is to adjust the RATE of your local clock, not to sync it by
jumping.  So given enough time your PC will be a pretty good clock.
I'd gues it will be at 10E-3 in one second and 10e-6 in 1000 seconds.
Even  if I'm wrong by a factor of 10 or more you can get to 10e-6
(your goal) in a reasonable number of hours.
Your GPS will be about 1000 times better then NTP.   It is not hard to
connect the GPS to a computer running NTP software all you really need
is that PPS signal connected to DCD (pin 1) of a serial port.
Technically a PC running NTP and connected to a GPS'  PPS is a very
crude GPSDXO.


>
> Meanwhile I have a M12+ timing GPS and antenna on order, and am assembling some glue logic, case, 3.0V power supply etc. I may not ever manage a long-term GPS installation, since my apartment is surrounded by tall obstructions, and my workplace has metallized-film window panes which block all GPS signals, and no roof access.  It has a big open parking lot, though... I imagine using a car-mount GPS setup and sending a 1 Hz or 100 Hz LED or laser pulse up through the window to a telescope / photodetector at my workbench on the 2nd floor.... or maybe sending a frequency-locked 10 kHz tone through a FM wireless mic system would be adequate.

If you get an optical system to work be sure and tell us all the
details.  LEDS have recently become more popular than lasers because
aiming is easier and also you don't have atmospheric scintillation
(the effect that makes stars "twinkel") problems as much.  You can
modulate LEDs like radio, with FM or just use baseband and let if
flash at 1Hz


>
> -John Beale
>
>> I did a piece of software to provide calibration to equipment-deficient microcontroller-hobbyist frequency counter builders (think 100ppm crystal from the bin) over ntp:
>> http://n1.taur.dk/nft/nft.pdf
>> /Kasper Pedersen
>
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-- 
=====
Chris Albertson
Redondo Beach, California