On Mon, 19 Apr 2004 18:53:16 GMT, "W3JDR" wrote:

Using CMOS inverters to build oscillators works best when using "unbuffered"
inverters. These can be identified by the letter "U" in the part number, as
in "74HCU04".

If you use a buffered part or a part that has Schmitt trigger inputs, then
the input-output transfer function becomes very steep (too much gain) and
the oscillation becomes hard to control. When this happens, the circuit
often oscillates, but at some frequency much higher than the crystal
frequency. I've made oscillators out of XOR gates, but if it doesn't come up
on the right frequency it can be hard to debug without a scope.

To determine if it's oscillating at all, I suggest measuring the DC current
consumption with the feedback removed (crystal out of circuit), and then
with the feedback closed (crystal in circuit). When the circuit is
oscillating, the current will be markedly higher. Once you know it's
oscillating, then you have to determine where it's oscillating. This might
be challenging if all you have is a receiver.

Certainly would be. The other thing about using Schmidt type inputs is
the likelihood of causing longer-term damage to the crystal. They
'prefer' not to be driven to hard and the sharpness of the buffered
gates ain't good for 'em.