I'm pretty sure that the circuit does not function
as a crystal oscillator. Is it an own invention?
For a crystal, you should decide if the circuit
needs the series or parallel resonance. There are
both, pretty near each other. For details, get
e.g. the ARRL Handbook and read about crystal
I double-checked the board layout; it is as I describe.
The circuit is part of an existing video driver board that outputs ASCII text
to a monochrome CRT monitor. I have substituted a sine wave generator in
place of the crystal and the board works.
The original crystal is gone (missing when I obtained the equipment) else I
would have looked up the numbers on the original.
I'm not asking for evaluation of the circuit's function -- that is confirmed
-- only to help calculate the load on the crystal.
I see you're having difficulties in determining the load..
If you are looking for an exact figure, I think you need to use
your signal generator via 100 Ohm R for example and a scope
to measure the drop in the circuit.
First, test the scope probe by measuring the drop after the
R only, to make sure you know the exact cap value in your probe.
Feed the circuit with this signal via the 100 ohm R, measure the
drop. Calculate the load and remove the scope probe load from the
The net results should give you a load that is going to be close
enough.. You may want to operate the circuit for a bit before taking
final values. The logic chips are going to shift a little.
I'm guessing you'll end up with an approximate value that equals 8 pf.
In the capacitor manufacture world, the common practice was to zero
beat a tuned circuit with a fixed frequency. You attach a test subject
to the post which were part of this tuned circuit. You then moved the
calibrated dial which was nothing more than a capacitor, to make it
zero beat again. A scope was used with the X,Y inputs for that nice
lissajous circle or spiral curves.
In any case, this dial would give you the exact capacitance load.
On a sunny day (Wed, 13 Feb 2013 12:15:49 -0800) it happened DaveC
Fir * sake use a trimmer.
Those 8.8.. 2xFc crystals are not MENT to be exactly on frequency.
They are normally used in a PLL, possibly with varicap
or some other reactance, to lock to incoming color burst.
If you do not do such a genlock, and want it free running,
then you need a trimmer, zero tc components (caps),
and for sure not a 2 inverter LSTTL oscillator, but a real one,
and maybe even an oven.
JFET makes nice oscillator.
By load they mean the total capacitance in parallel with the crystal.
Some of this will be the obvious circuit, and the rest will be the
inherent capacitance of wiring to the crystal.
Just take a reasonable stab by quoting say 20pf, and if you need
precision, then use a trimmer capacitor across the crystal to bring it
to the required frequency.
Load capacitance applies when the crystal is operated at its parallel
resonance. Your circuit operates at the series resonant frequency. Tell
the crystal manufacturer you want it to be series-resonant at 8.86723 MHz.
I haven't looked at standard frequencies, but you might have better luck
finding that frequency if you drop a significant digit or two. It is
unlikely the original part was specified to 1 ppm, 100 ppm would likely
do the job just fine. Didn't you say this was for a display? It will
probably work fine with just five digits or even four digits of
frequency specified. Is either 8.867 MHz or 8.868 MHz a common value
perhaps? Really anything near 8.87 MHz should do the job.