Frequency sources épisode deux

A project log for Bipolar Discrete UART

Because making discrete clocks is not an end in itself.

yann-guidon-ygdesYann Guidon / YGDES 03/03/2020 at 21:310 Comments

So I received the quartz tubes and I'm characterizing them :-)

The 2-transistors circuit mentioned earlier works like a charm. The GS507 from @jaromir.sukuba  flies without effort.

There is one tube marked as 2400Hz on the box but with no marking on the tube itself and I just measured it a 2402,4 Hz. I don't see how to center it better to 2400 precisely, as temperature and supply voltage don't affect the frequency enough.

I started playing with the liaison capacitor and it indeed has an effect but not the one expected : too low and it oscillates on 5th harmonic and sometimes more (11th harmonic ?). However this confirms what @SHAOS said about its importance.

I'm still playing with the circuit and it's a lot of fun, you can hear something like tinnitus if you hold the crystal to your ear and the driving strength is high enough. The oscillator can work down to a very low voltage, which is very fun to do with germanium :-D

I'm not overly concerned by the 1/1000th of frequency mismatch because it's still well below 1% required for serial communications and the other tube is clocked at 2403.1 Hz.

The quartz has a large inertia and can stand some power that would damage a wristwatch crystal. The output of the oscillator is strong enough to drive a simple amplifier. It's so fun to see a crystal that is more than 60 years old become part of a digital circuit ! However the circuit saturates a lot. Maybe the crystal is designed for a Pierce oscillator ?

I also tried the 19200 Hz quartz tubes from Ukraine (another great find on sr71's eBay shop) and they are easier to "pull" due to a different cut, probably, as well as 30 years younger. I find a 5Hz drift that is easier to remove by adjusting the feedback capacitance, it's only 1/5000 to correct instead of the much larger relative drift of the slower crystals.

The OC139 drive it easily, even with swapped emitter & collector... It's fun that I can also pull the frequency by shining light on the black-tinted glass ;-) the paint is not perfectly opaque and now I understand why the metal cans have been a necessary evolution.

The GS507 seem to work well at 19K2Hz and the digital logic should work fast enough.