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

[WarrenS]

Here is another analog control example based on the quick and dirty example 
below.
It is a  simple and Very poor GPSDO Rb design as far as noise jitter goes 
because of the  nonlinear and high Phase detector gain, and high 1e-8 noise 
jitter on the PPS,
but still no problem to do with cheap basic parts.


Given:
1) LPRO Rb with a + - 1e-9 range analog tuning range plus an internal freq 
adj pot with the same range.

2) 1pps GPS control signal with 10 ns noise at 1 second

3) Desired 1e-11  GPSDO jitter at one second

4) Total Attenuation needed from phase detector to EFC input 1000 to 1. Can 
be a combination of resistor and cap.

5) Use a 74HC74  D Flip-Flop phase detector whose 1ns sensitivity is good 
enough because it is much less than the 10 ns control signal jitter.
5a) 1 sec control signal to FF clk in, 10MHz divided by 100 using a 74HC390 
to the FF D input  (100KHz will give a 5us  range before jumping a cycle)

6) limit range of fine EFC control input to + - 1e-10 freq change with 
resistor divider, and use the LPRO's course adj pot for course freq setting.

7) If Rb phase is before the control signal the 5V FF Q_not output will 
drive the Rb freq 1e10 lower in freq (1ns/10 seconds)
7a) if the Rb phase is later than the control signal, the 0 volts of the FF 
Q_not output will drive the Rb freq 1e-10 higher.

8) LPRO Rb EFC input Z is 50 Kohm with a 0 to 5V control, designed to be 
left open when not in use so resistor noise is not an issue (best to add a 
RF cap to gnd)

9) Output the FF thru a 10 sec low pass prefilter RC using 10K and 1000uf 
cap to give a 10 sec average of the 10ns phase noise

10) Feed the Rb EFC from this RC  thru two 250K ohm resistors in series to 
give a 10 to one attenuation (500K to 50K)

11) Add the main slow LP filter as desired to the center of the 250K res 
center tap, with small resistor in series with a big cap.

12) use >= 1000uf cap for 100 sec + TC,   >= 100 to 1 cap attenuation Plus 
10 to one resistor attenuation => over all 1000 to one jitter attenuation

Maybe no free lunch, but total controller cost can be under a dollar.
Note that all the crappy LP filter RCs that many seem to be most concerned 
about are all Inside a closed loop,
so their poor, less than ideal performance does not mater so long as the cap 
leakage is not so great that they never charge.

ws

*****************

Hi

For some designs (like a Rb) the time constant may be in the "days" range..

Even for something simple, you can easily get out to several thousand 
seconds.
You also need low noise past the cutoff time (for short times the cap may 
help you).
That's going to get you into resistor noise and / or op amp noise. All of 
this will push up the capacitance required.

Quick and dirty example:

1 pps comparison setup
10 ns jitter on the GPS ( 1x10^-8 at one second)
1x10^-11 as the desired GPSDO jitter at one second.

. you need > 1,000X attenuation of the jitter at one second.   With a 1 uf 
cap, that's gets you to pretty noisy resistors. Most TBolt's are quite happy 
doing the sort of thing in the example.

Bob