[time-nuts] wwvb 60 khz tuning fork crystals Some insights

[paul swed]

Working on the wwvb remodulator and have learned a fair amount about tuning
fork crystals.
there is a fair amount of information out there. It seems generally the
same thing and as it turned out perhaps not all that useful.
This is what I have done a schematic in words.
Using a single 74hc14 inverter feeding a buffer inverter pins 3 and 4
22 M feedback resistor pin 1 to 2
Xtal feed back path
Pin 2 to 50K variable pot to a 68K and 100K resistor in series to 230 pf to
ground.
This same junction has the 60 Khz xtal feeding then back to pin 1.
Power supply is a CS5206 3.3V LDO 22uf and .047uf to ground.

The 230pf cap grossly gets the system close to frequency. About 5 Hz.
Adjusting the variable pot puts the xtal on frequency.
Granted this is simply changing the crystal drive. But it works well.
The 50K pot gives about 8 Hz adjustment range.
I did indeed use small variable plastic caps. They were a pain to use
actually.
Granted a high quality air variable would be good and you can tell how its
been adjusted.
Not so with the plastic caps. You are a bit blind.

I then tested 10 xtals out of 50 a random sampling.
They ranged from 59.996 to 60.005 some came close to the original mouse
xtal I built the system with below 1 Hz.

Conclusion
The 60 Khz xtals will need a way to adjust. They simply are never close.
The feedback resistor has to be very large I was surprised by how large. 22
M and I could go higher.
Lowering the drive raises the frequency to a point. Anything above 270-330K
cause startup issues.
The circuit as describes starts very quickly sub 200 ms.
I will update the wwvb remodulator schematic with this information and most
likely make a real schematic and also build up my own remodulator for use
until I add the remodulator to the d-psk-r to create a single package.
Regards
Paul
WB8TSL