"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

Yes, you can get each XOR of the 74HC86 to work as an inverting stage by
tying one input high. Then you can get it to act like a linear inverting
amplifier by putting a 1 Meg resistor from output to the other input. Now,
if you have two of these linear inverting amplifiers in cascade, a feedback
path from the second output through a crystal to the first input should
produce oscillation. You may even find that just one of the linear
inverting stages will be sufficient, since there will be a phase shift if
you put a 30pf capacitor in series with the crystal. You can control the
feedback with the capacitors to ground that you mentioned.

To run a doubler, you need a non-symmetrical square wave or pulse waveform,
since a symmetrical square wave has no second harmonic content (only odd
harmonics). With vacuum tube amplifiers, this was called Class C operation,
since conduction occurred for considerable less than 1/2 the cycle and
doubling was easy. Doubling requires about 90 degree conduction (1/4 cycle).

You should get yourself an oscilloscope, if you are a sighted person.

Chuck