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

WarrenS warrensjmail-one at yahoo.com
Sat Dec 31 13:23:00 EST 2011


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 

A couple basic ICs and a few Rs and Cs and you're done. This makes a basic 
PI controller that will cause the 10MHz osc to track the GPS PPS.
The less you make the Osc's EFC tuning range the better this works, and Once 
it is tracking you can fine tune the freq adj pot every now and then to keep 
the Filtered FF voltage at near 2.5 volts if the Osc tends to drift outside 
of the control range.

Not very high tech and there are Lots of possible ways to add more parts to 
improve it further, depending on what your goals are and how much you want 
to learn about GPSDO and PID control loops.

If the definition of "simple"  is less parts and more programming you can 
replace all the active parts with a simple PIC and get better performance by 
controlling the Osc's EFC using a software PID and PWM with an external RC 
filter as the Dac.


Chris Albertson albertson.chris at gmail.com
Sat Dec 31 06:01:38 UTC 2011

On Fri, Dec 30, 2011 at 10:16 AM, Hal Murray <hmurray at megapathdsl.net> 
> Software.

As soon as you say "Software" the device is no longer simple.    Even
a microprocessor is a very complex device and so is its development
system.   The software inside the uP is not simple either if you count
the number of possible paths through the code (2 raided to the power
of the number of branches.)

I have nothing against software, that is what I do for a living, every
day.  But you can't count a uP with software indise as "simple".
And the point of this exercise is to find the simplest thing that can
still work.
Chris Albertson

Redondo Beach, California

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