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.

Joe
W3JDR


"PaoloC" wrote in message
...
[Slightly off-topic request]

Hi.
As of the other post of mine, another part of my weekend was spent
trying to get a XOR gate to oscillate with a XTAL.

I have a working 74HC14 oscillator:
"One gate of a 74HC14. 470ohm resistor from gate output to the parallel
of 1Mohm//XTAL. 10pF and 33pF (from the junkbox) capacitors to ground on
each side of the XTAL."

Since my project calls for a frequency doubler, which I want to
implement with a XOR gate, I wanted to use one XOR gate of a 74HC86 as
oscillator. Remaining gates would work as buffer, delay line, digital
mixer.

(I have Googled newsgroups and found an interesting discussion about XOR
frequency doublers, so I am aware of its limitations.)

So I moved the oscillator circuitry (R; C; R, XTAL; C) from the 74HC14
to the 74HC86, pulling the other input of the XOR gate to "1", so that I
would produce an inverter.

Nothing happens. I used a 10.0 MHz XTAL.

I understand that the 74HC14 has Schmitt trigger inputs and that a
74HC86 might have higher propagation delays. Still, is it possible to
use a XOR gate as an inverter and oscillator? Do I overlook something?

In the end I recovered the 74HC14 oscillator, but this adds one
component to the final circuit I have in mind that wastes energy, space
and is underutilized.

Looking forward to your always helpful replies!
Paolo IK1ZYW