24 Hour Digital Clock Gets a Better Timebase

The last post on this clock built out of 7400 series TTL chips gave a bit of back story on it and the initial troubleshooting of it’s timebase drift and noise issues that caused extraneous counts to occur. Now that the noise is figured out and solved, it is time to do something better than a plain crystal with a trimmer 7400 ( NAND Gate ) oscillator for a timebase.

Driving the clock with an OCXO

The goal was to allow the clock to be driven by an oven controlled crystal oscillator (OCXO), which would surpass the performance of the oscillator on the board. The on board oscillator can be easily pulled off it’s frequency and slaved to an external source. By using a 7414 Schmitt Trigger Hex Inverter, it is possible to take the low level sine wave output from the OCXO and convert it into a digital signal capable of entraining the on-board oscillator. By using an external master, I can pull the master OCXO off if I need it to calibrate anything else, meanwhile the clock keeps running off of it’s board oscillator. Most of the time the OCXO is not being used and can just remain with the clock.

7414-Schmitt-Trigger-on-breadboard
7414 Schmitt Trigger Hex Inverter on Breadboard

The hookup is pretty simple, I breadboarded it and found out the biasing one of the inputs of the 7414 mid range using a pair of 33K Ohm resistors in series from +5V power to ground with the input to an inverter stage on the 7414 in the middle at 2.5V, feeding in the OCXO signal, capacitively coupled  and then feeding the output of that stage into another inverter on the 7414 for a buffer, I was able to get a clean square wave at 10MHz. This I coupled to the board oscillator using a 370 Ohm resistor. The resistor is more or less a protection in case I touched it to something that I shouldn’t and I don’t want too much current to be able to flow in either direction. I also temporarily drove an LED via a 370 Ohm resistor off of the same output as a check that I was actually getting output, the LED is lit at half the brightness when it is excited by a square wave.

Initial testing with the breadboard shows that the clock now tracks time very well as compared to another clock I have that is synced to the WWV 60KHz signal.

OCXO to 7414 to TTL Clock
OCXO to 7414 to TTL Clock: Signal path is via yellow clip leads.

 

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