[sdiy] Help with oscillator

[Tim Stinchcombe]

Hi Tim,
 
> I have been humbled by the most simple of circuits: an 
> oscillator.

Don't feel too bad about it: in my view, crystal oscillators are amongst the
hardest oscillator circuits to get a real good grip on. I have most of the
better-known books on the subject, and they are either very, very involved,
or tend to oversimplify to the point whereby you think you understand what
is going on, but when you try to apply what they say, you can't figure
anything out!

> Its one of those crystal oscillators used 
> everywhere. The circuit is here:
> 
> http://circuitabbey.com/files/EqualTog_sch.pdf
> 
> Its the circuit in the upper left corner: U6. This same 
> circuit worked fine on the first board. I have since swapped 
> the crystal and 18p caps from the working board to the bad 
> one, but no joy. The output at pin 6 has about 4 KHz on it, 
> and pin 1 has no signal at all, just a DC bias. I tried 
> cleaning the board with flux remover. Now even the 4KHz 
> signal is gone. 

Does this circuit, a Pierce oscillator, really work with Schmitt triggers?
Schmitt triggers are good for relaxation-type oscillators, like
multivibrators and astables, with a cap on the input to ground and a
feedback resistor, but for a crystal oscillator you want it to work like a
simple inverting amplifier, so I'd have thought a simple inverter, biased in
its linear region by the large feedback resistor is what is needed. My guess
is that it was pure luck that the first one you tried just happened to work,
because that gate was OK with it, but now you have one that isn't. Is this a
tried and tested circuit? I'm fairly sure I've never seen a Pierce
oscillator with Schmitt triggers in it...

Tim
__________________________________________________________
Tim Stinchcombe 

Cheltenham, Glos, UK
email: tim102 at tstinchcombe.freeserve.co.uk
www.timstinchcombe.co.uk