Precision series resonancy crystal oscillator - The SJOSTROM KLOK

[peranders]

Hi!

We talked about crystal oscillators to CD players. The best oscillators are those which uses series resonancy crystals. I have got requests about those. Here comes some design ideas.

LM317 as power supply for kwak-clock

Tips: Gain = 1.5 -10, lower is better but maybe the oscillator get hard to start.

Supply voltage 5-10 volts, not too much because of higher drive level of the crystal.

Transistors, BC5xx etc for <5 MHz, HF types above

Currents, a few mA's

Well stabilized power! Very important in order to get high stablity. Important also not to load the oscillator. An extra buffer in essential.

Careful: You must choose a crystal for the purpose!!!! You can't tune a parallel resonancy crystal all the way to series mode. You loose lots (almost everyting) of Q-factor and stability!!! Very important!!!!

[peranders]

Sorry, forgot to upload the file.

BTW: I called my project KLOK without a C! Just to create a personal touch.

[peranders]

Possible enhancement is to have variable gain and stabilized amplitude in order to get lower distortion and jitter.

You can also put the whole oscillator in an owen! Not very hard to fix. Search Linear Technology. They have very good examples of that.

[Jocko Homo]

Try this one out.

It also uses a series resonsant crystal, has low distortion, and comes with a low-noise regulator. Puts out 1.6 V p-p.

This is the first version, so more revisions are probably in the works.

It is easy to measure if a crystal is series or parallel resonant. If you need to order one, and the supplier doesn't know which one it is, find a different supplier.

Jocko

[HarryHaller]

Are all the BJTs 2240s? What kind of LED and what is the voltage out of the voltage regulator? I like it.....

H.H.

[Jocko Homo]

1.) Yes.

2.) Cheap. Green.

3.) 12 V.

Are you sure? It uses BJTs, and not FETs.

Jocko

[CG]

Might it be better to:

a) run the LED string directly from the current source (that's the way it already is - just reviewing)

b) connect a 100 Ohm resistor (or maybe even some higher for a lower -3 dB point) from the top of the string to the base of the emitter follower

c) connect the 120 uF and other filtering capacitor to the base of the emitter follower?

The idea is that the LED string is a more or less low impedance (hence the reason for using it as a voltage reference). Bypassing this low impedance directly isn't really as effective as using an RC lowpass filter to get rid of noise. The voltage regulation under dynamic conditions shouldn't be affected.

Good? Bad?

[Jocko Homo]

For a zener: yes. The noise with LEDs is low enough that RC filtering is not needed. It is there only for low impedance on the base, as the LED string is around 400 ohms.

Jocko

[CG]

Gotcha. For a 400 Ohm source, the resistor would be waste.

Have you ever measured the noise voltage density across an LED string? I'm curious how low it is.

Do you find blacking the LEDs out with goop of some kind, or otherwise shielding them from light, lowers the noise? (LEDs - and semiconductor lasers - also exhibit photoelectric effects, although they aren't real efficient.)

[Jocko Homo]

I have only measured it on a RTA, not a wide-band spectrum analyzer, but is low. Very low. Also, the 'lytic cap is there to reduce any ripple that might be on the base, as that will modulate the oscillator. Which is why the whole thing is fed from a current source.

Jocko