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.
Looks like Series, Generally CL can be ignored. It would be PCB related.
If you specify it, you can always ad it to the circuit.
The 10pf compensates for the propagation delay in the gates.
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.
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.
I learned a little more about this circuit.
It's a proprietary video card card from a piece of German offset printing
I used a sig. gen. in place of the crystal. The circuit generated a video
signal without any data (just sync pulses). The video card is separate from
the system processor, so being on the bench the card has no data to display.
Viewed on a scope, varying the 8.867 MHz frequency doesn't change the video
signal at all.
I'm guessing (with my limited understanding of how video works) that the
8.867 runs the dot clock, basically how fast the dots are shoved out of the
data bus into the video generator IC.
So, how critical *is* this frequency?
Dave (not an EE)
On a sunny day (Wed, 13 Feb 2013 19:33:37 -0800) it happened DaveC
Likely not, that 8.8 is 2x Fc and used for PAL color generation.
You need a much higher dot clock for video, I used 18MHz in my video card
for 15625 lines and 80 characters per line.
If it only does 40 characters / line then it COULD be 8.8 MHz.
Measure horizontal frequency from that card, it should be 15625 Hz.
If it changes when you change the 8.8MHz, then it is the dot clock.
Else it is only for color.
If it is for color it needs to be 2x443... you know.
What is the video controller chip?
Look up its datasheet!
it's hard to say. it depends on the monitor.
try a 9MHz crystal it's within 2% of the target , you might need to tweak the
vertical and/or horizontal hold adjustemnt on the monitor.
Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here.
All logos and trade names are the property of their respective owners.