On Fri, 30 Jun 2006 15:48:03 -0500, Ben Jackson wrote:

I've got a 74HC86 (quad XOR) and I'm using 2 of the gates (with one input
tied high) to make inverter oscillators. One crystal at 14.85MHz in a
SMT package was relatively easy to start up and stabilize. The 25MHz
crystal (in a short can package) was very sensitive to Rs (series resistor
between the inverter output and one leg of the crystal) and low supply
voltage. To get it to start reliably I ended up with Rs=110 @ 5V. The
whole thing is drawing about 25mA.

The problem I have is that the 74HC86 and 25MHz crystal seem to warm
up quickly and cause the 25MHz to drop by about 200Hz. This oscillator
isn't intended to be on for long periods of time, so I'd like to avoid
that. I've isolated those two parts by a combination of heating and
cooling components to see which contribute the most (the temp swing is
too small for me to feel, I'm just warming with a finger and cooling
with a puff of compressed air).

So if I'm putting 15mA+ into that crystal, is that unusually high?
What would be a typical amount for such an oscillator? Is there
anything I can do about the effects of chip heating (I'm assuming
the gate capacitance is going up as it heats)?

From a stability factor point of view those "inverter" based
oscillators are amoung the worst. I doubt it's all crystal heating
and I bet there are parameter shifts in the "chip" as it warms up
operating at high frequency like that. Also If you drive the crystal
hard like they do (15ma!!!) bad things happen. Those circuits are OK
for micros and other less critical apps.

I suggest a discrete Bipolar or FET where you can control the
operating point of the device better.

Allison