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

[Burt I. Weiner]

Paul,

The stuffs I ordered from DigiKey came yesterday.  Those are really 
small tuning fork crystals!

In your notes below... you said, " The feedback resistor has to be 
very large I was surprised by how large. 22 M and I could go higher."

hen you said, "Anything above 270-330K cause startup issues."

Did you mean anything below 270-330K causes startup issues?"

Burt

At 08:02 PM 6/26/2013, time-nuts-request at febo.com wrote
>Subject: [time-nuts] wwvb 60 khz tuning fork crystals Some insights
>
>
>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

Burt I. Weiner Associates
Broadcast Technical Services
Glendale, California  U.S.A.
biwa at att.net
www.biwa.cc
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