[time-nuts] Thunderbolt? (re simple gpsdo.)

WarrenS warrensjmail-one at yahoo.com
Sat Dec 31 17:07:56 EST 2011

Chris posted:

>What level of performance did you get?

Correct it depends on what parts you use and how nutty you want to get.
So much also depends on how you define performance, and where you want to
Compared to WWV and WWVB it was much better, Compared to a correctly set up
TBolt much worse.
In a "NUT-Shell",  It is good of enough for most any REAL non-Nut
Ball park numbers: Freq error of 1e-8 is simple, 1e-9 is easy, 1e-11 gets
hard without some good parts and lots of care to details.
Besides the GPS engine and the Osc, The time period the freq is averaged
over is an important factor, because of the jitter.
If you do not loose sync, like all GPSDO, over a long enough time period of
many days or weeks, it is good enough to check and calibrate ANY Osc,
because it can be set up so that there is NO long term accumulative drift,
just short term freq jitter.

> just one flip-flop, divider and a capacitor.
AND some resistors
>I'm looking for a controller that is much more like the bimetallic spring

For a Bang bang type two state controller like your bimetallic example, you
don't even need the cap which is added to filter out freq jitter.
Take out the filter cap, scale and adjust things right and what it does is
if the freq is less than the GPS,
when the FF toggles it will raise the freq above the GPS and then when the
phase matches,
it will toggle back and lower the freq below the GPS.
This will continue forever keeping the AVERAGE Osc Freq dead nuts on
bouncing back and forth between
a couple frequencies in what then becomes a PWM like function of the OSC
bouncing between two frequencies, one higher and one lower than 10.000 MHz.
The freq step size and jitter is a function of the resistor divider used and
the EFC sensitivity.
The PWM cycle rate depends on freq step size, the speed of the PPS signal,
the osc divider used and GPS PPS phase noise.

Lots of other uses for this type of  D FF as a basic ns hi-low Phase
detector for low freq signals.
Remove the EFC feedback, Reduce the 100 to 1 divider to two or so and you
can use this to measure and/or  manually set a Rb Osc to be on frequency if
you have an accurate 1PPS signal.


[time-nuts] Thunderbolt? (re simple gpsdo.)
Chris Albertson albertson.chris at gmail.com
Sat Dec 31 20:34:14 UTC 2011

I think this is the simplest design that can still work, just one flip
flop, divider and a capacitor.

What level of performance did you get?    I think it depends on how big the
integrating capacitor is and how stable the VCXO is.   I guess if you
switched to using the t-bolt the performance was not as good as a t-bolt.

On Sat, Dec 31, 2011 at 10:23 AM, WarrenS <warrensjmail-one at

> Chris
> Here is a GPSDO I built that better fits Your definition of "Simple". I
> used this as my freq standard before getting a TBolt.
> 1) Feed the PPS output of an oncore GPS timing engine which has 1 Hz or
> better yet 100 Hz output to the clk of a D FlipFlop (74HC74)
> 2) Feed the FF's D from a 10 MHz osc which has been divided down to 100KHz
> or less using 74HC390.
> The FF output shows if the Phase of the Osc is greater or less than the
> GPS signal and the FF will toggle back and forth when the phases are near
> equal due to the typical 40 ns jitter on the GPS pulse signal.
> 3) Add a RC filter to the FF output using a big cap, so the voltage out of
> the RC filter is 0 to 5 volts depending on the duty cycle of the FF.
> (A small R in series with the cap will help stabilize it if a real Big cap
> is used).
> 4) Feed the filtered analog FF output voltage (No buffering necessary) to
> the EFC of an 10 MHz osc that has its EFC input desensitized with a couple
> of Rs and has been set to be real near 10 MHz at the nominal analog FF's
> 2.5 volts output using the Osc's mechanical tuning and/or add a fine freq
> adj pot.......

Chris Albertson
Redondo Beach, California

Home heating thermostats can be simple of complex.
Some use LCD displays and a computer.
Other have a simple bimetallic spring inside.

But for now I'm looking for a controller that is much more like the
bimetallic spring thermostat.

Chris Albertson
Redondo Beach, California 

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