Hello again.

Go back to fundamentals. An oscillation occurs when there is positive
feedback with a circuit gain of 1. That is, the signal is not inverted at
the desired oscillation frequency. Another way to say this is that the
phase of the signal at the input and output are the same. If the gain is
less than one, the oscillation will die out---inother words it will not
oscillate at all. If the gain is greater than one the circuit will 'lock
up' at either 0 volts or the power supply voltage.

Most oscilaltors have some type of phase shift device between the input and
output to ensure that the phase of the input and oputpuit signal are the
same.

Note that in some circuits the feedback and phase shift may not be obvious.
A vacuum tube oscillator, for example. Here the feedback is sometimes
provided by the capacitance inside the tube.

Everything must be just right for the oscillation to occur and be stable.

With an oscillator built from inverting logic gates, you need two gates so
there is no overall phase inversion through the gates. If you are using XOR
gates, I would recommend tying one of the input lines on each gate high,
That way it will act as an logic inverter.

Jim
N8EE

"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