Crystal load?



When I vary the sig. gen. I used to simulate the crystal in this circuit, the output video signal doesn't vary at all (sync pulse timing stays the same).
Therefore I presume that video timing is determined by the other crystal (6 MHz) that is running the 8085 uP on this antique board.
So I also presume that this 8.867 MHz crystal drives the dot clock (shifting bus data into the 8275 CRT controller IC).
Is the 8.867 frequency critical at all? Since it doesn't control the video sync pulses, etc., what effect will it have on the (ASCII text only) video display?
Thanks, Dave
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Or rather, loading 8-bit bus data into a parallel-to-serial shift register and then into the 8275 CRT controller.
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usually the CRTC manges the raster size and does its own data fetches... I'm not familiar with the 8275, but all the CRTCs i've encountered had internal counters to determine the sync rates and sync pulse widths. from the pixel clock that crystal may be doing something else, colour-burst perhaps.
Is this circuit used with some sort of genlock - text over video ? if so then the crystal frequency may be critical.
perhps feed an FM signal into where the crystal goes and probe around to see where it's going.
--
?? 100% natural

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On 2/14/2013 3:56 AM, DaveC wrote:

It is hard to say just how critical that value is. How did you arrive at this value anyway? A 1 ppm value would be hard to measure.
Anyway, much of it depends on the device that will be displaying the data. Is this a TV or a computer monitor? A monitor will likely tolerate a good range of values relative to the number you have specified. The electronics in the display generator may or may not be pushed to its timing limits. It would be hard to imagine that upping the frequency 1000 ppm would cause a problem, but certainly lowering it is not likely to disturb a good design. Even a 10,000 ppm deviation shouldn't cause any real problems.
Here are links for parts available from Mouser and Digikey in stock. Low cost and you can get them within the week.
http://www.mouser.com/ProductDetail/ECS/ECS-85-S-4X/?qs=sGAEpiMZZMsBj6bBr9Q9aQVbz5FLOGxjmYeJ9iHcTC8%3d
http://www.digikey.com/product-detail/en/9C-8.912MBBK-T/887-1830-1-ND/3585929
The Mouser unit is 8.5 MHz and should work unless there is something "particular" about the monitor. The Digikey device is 8.912, closer to the requested frequency, but is high, so there is a small chance it will over clock some part of the design. It is also a surface mount unit that might be harder to solder in.
No promises, but this is certainly an easy thing to try.
BTW, the one reason to get a part that is within 100 ppm is because in the "old" days monitors would have some noticeable affects from power line noise. If the display is closely synchronized to the power line frequency this is not such a problem. Today's monitors don't have this problem.
--

Rick

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On 2/14/2013 4:33 AM, DaveC wrote:

You can still buy it in germany. Standardquarz, Grundton, 8,867238 MHz 0.18 euro (0.24 us dollar)
https://secure.reichelt.de/Crystals-Quartz/8-8672-HC18/3/index.html ?;ACTION=3;LA=2;ARTICLE520;GROUPID173;artnr=8%2C8672-HC18;SIDUNXklX8AAAIAAA-vdIc96bf9a2d5cb50299c3e3c87380e0d4b7
Minimum order 10 euro. Only drawback is the transport cost :((
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On 2/13/2013 9:18 PM, Michael A. Terrell wrote:

Did you miss the fact that he has specified a common crystal to nearly 1 ppm? They don't make crystals that will hold a frequency to 1 ppm. You can get units specified to some 10's of ppm along with aging of similar ranges. To do better you need to temperature compensate and perform other "magic" in the oscillator.
Why would you think the monitor gives a durn? It may not take the signal from an 8 MHz crystal, but 1000 ppm off should be no trouble.
--

Rick

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rickman wrote:

Have you ever worked with video? It doesn't sound like it.
Here is a photo of a mono monitor being driven by a CGA card, which is less than 16 hz off at the H sweep:
<http://i.ebayimg.com/t/Mono-Video-Card-with-Artefacts-/00/s/MTIwMFgxNjAw /$T2eC16RHJF0E9nmFSIbDBQ-CLvDguw~~60_12.JPG>
It is outside the capture range of mono monitors. If it wasn't critical, they would have used a low tolerance ceramic resonator instead of a crystal. Being off 10 Hz at 3.579,545 MHz is out of spec. With the typical 14.318,180 MHz crystal used for broadcast video gear, that is a max of +/- 40 Hz. or less than 2.8 PPM. I've seen more than one mono monitor die because it was used on a CGA board, and some went up in flames. The flyback is designed for a specific frequency, and quickly loses efficiency as the Q drops.
3,579,545 Hz = color 3,583,185 Hz = mono --------- 3,640 Hz = difference in dot clocks.
I have repaired monitors for over 40 years. I was a broadcast engineer at three TV stations. You keep spouting vauge crap. Should. Maybe...
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On 2/14/2013 5:34 PM, Michael A. Terrell wrote:

<http://i.ebayimg.com/t/Mono-Video-Card-with-Artefacts-/00/s/MTIwMFgxNjAw /$T2eC16RHJF0E9nmFSIbDBQ-CLvDguw~~60_12.JPG>

I notice the machine the monitor is sitting on looks like it has a tape drive backup in it. What is this a 486 some 20 years old? Yes, the earlier monitors were difficult beasts that were very sensitive and could even be damaged by running them off frequency. But they haven't made that sort of thing for a long, long time. Far too long to still be in use. If the OP is actually driving one of those monitors, I would be surprised. In that case, the crystal might need to be 1000 ppm accurate. But for any modern monitor it will adapt to whatever you send it nearly without limit. Have you worked with a monitor that isn't 40 years old?
My point is that for $5 the guy can get some crystals that he can try. Otherwise he seems to be having a ton of difficulty finding anything he can use. I believe he hooked the board up to a monitor to test running it from a signal generator. Did the monitor burn up then? Why don't you try getting a little more real rather than busting my chops when you don't know what type of monitor he is driving?
I can't believe you have worked for so long and actually believe the crystal driving a CGA board was 3 ppm accurate. That's an order of magnitude more expensive oscillator than what they actually used, perhaps two!
--

Rick

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rickman wrote:

Have you used a 8275 CRT controller in the last 30 years? (Think! That was used with the 8085 8-bit processor)
'Modern' monitors are not NTSC. They are digital, and the current types are HDMI. Therefore, all timing is generated by the onboard display controller. I still repair monitors, including LCD types. Not many, and not often but I do know how they work.

It's in an old Geramn made printer he's trying to reserect. Why would it have a modern monitor. You need your 'chops' busted, in real life. He is working on an old, retired system. You keep tossing out crap that has nothing to do with his problem. He has been pointed to multiple sources of the proper crystal.

Damn, you're stupid. CGA video used the 14.318180 system clock in the PC. It was divided by 4 to provide the required burst. Early PCs had a trimmer capacitor to set the exact freqency.
Have you ever seen a CGA only video card with a crystal? Once again: COLOR BURST. SEVEN CYCLES. PHASE LOCKED TO BURST ONCE PER SCAN LINE. Colorburst crystals were made by the millions, and for decades. They only had to be able to be pulled to the burst, and remain stable for one scan line. The two only have to be close enough to be pulled into phase. I have looked at a lot of the burst crystals in free running mode, and they were within 5 Hz. Is that good enough for you? Really, you need to stop making a fool of yourself and look at the real circuits.
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On 2/14/2013 7:49 PM, Michael A. Terrell wrote:

Yes, an old piece of equipment that is in tatters and may or may not use the original monitor. That is my point. Until the OP responds, you simply don't know what monitor is being used. If he wants to test the equipment with a $1 crystal rather than a $50 crystal that will take two months to get, that is his choice.

I'm not going to argue with you about this. The crystals used in PCs and nearly all consumer electronics are only accurate to a few 10's of ppm and typically are only stable over temperature to roughly the same range. You can tune the crystal to an exact frequency with a trimmer and it will be off by 10's of ppm by the end of the day or before the equipment has warmed up (or conversely after). Did PCs warm up the oscillator before turning on the monitor?
The point is that talking about depending on crystals being 3 ppm accurate is not valid unless it is a more expensive piece of equipment that can justify the cost of a temperature compensated oscillator. I don't care about how many crystals you have looked at, holding a 3 ppm spec on an oscillator is not something you do with a $0.50 crystal oscillator. CGA never had to rely on the timing being anywhere near that accurate, period; or any other display I have ever worked with. Heck, that is outside the spec of many frequency counters from HP and elsewhere! The model HP5383A timebase is specified for 3 ppm/month aging, 2.5 ppm between 0C and 40C and 0.5 ppm due to power 10% line variation. This is a piece of equipment with an accuracy limited by the crystal oscillator, so I think they would put a lot more effort into that feature than PC makers would in their reference clocks.
Your claims that CGA monitors (or any others) need to be driven to a timing accuracy of 3 ppm is not supported by the facts.
Go be rude with someone else!
--

Rick

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rickman wrote:

Sigh. the monitor has to be able to lock to the video source. PERIOD.

So, you don't have a GPS or WWVB derived in house standard?

Yawn. You continue to display boundless ignnorance. I've explained it several times, but you just ignore the facts. You must have voted for Obama. TWICE.

Yawn. It'es rude to hit you with facts.
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On 14/02/2013 22:34, Michael A. Terrell wrote:

<http://i.ebayimg.com/t/Mono-Video-Card-with-Artefacts-/00/s/MTIwMFgxNjAw /$T2eC16RHJF0E9nmFSIbDBQ-CLvDguw~~60_12.JPG>

I'm quite shocked at what you're saying. Like me you must have worked with monitors but I doubt much with alternative video sources.
Old CRT monitors are very forgiving on line frequency. You may have seen the picture size breath with brightness and perhaps a little with frequency. While the flyback time is fixed by self resonance you mention, the scanning period isn't, and the rate of change of the scanning magnetic field is largely determined by the power supply voltage across inductive coils. You may also remember all the corrective inductors, some designed to saturate earlier than other, and E-W modulation circuits to reduce pin-cushion and other related effects.
In short a monochrome monitor didn't give a jot about the precise frequency, after all they were built to a price and with component tolerances to suit.
In PAL and NTSC systems things became very different, where accuracy is now required to ensure colour lock in the monitor. As a result, because of the relationship between colour subcarrier and line rate, a video source would aim to be few ppm for broadcast and 100 ppm or so for most other applications.
I keep an open mind on whether a CGA card can make a mono monitor die. My instinct suggests that the monitor would have died at nominally the same time regardless of video source quality, especially if all the linearisation and E-W correction was still functioning as normal.
--
Mike Perkins
Video Solutions Ltd
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Mike Perkins wrote:

Pincushion on a mono monitor? How bad were the CRTS in Europe?

That depends n the monitor. The first monitors shipped with the IBM PC would literally burn up without horizontal drive. They were made in Japan, to European specs. That is why we saw that useless switched outlet on the back of the power supplies for so many years.

I personally saw more than one mono monitor smoke after a short while on a Mono drive. Hell, you could monitor the DC current into the monitor and see a 20% or so increase in current on some when you changed the drive. If the monitor was composite input, (Basically a mono TV less the tuner & IF) they could handle the change, but TTL input died almost on schedule.

Linearization? They had a coil in series with the H yoke to shape the flyback pulse. Vertical linearity was a pot and a few other passives.
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On 13/02/2013 05:24, DaveC wrote:

Recently you mentioned a source in Ireland that had crystals of this frequency. There are also some further source in the UK though a little more expensive.
I'm therefore surprised you're going for some $50 custom built ones.
Your oscillator circuit is a series resonant kind, where the oscillation frequency is a few kHz lower than parallel resonance.
I would have thought they would have been sufficiently accurate for your application as colour systems are generally quite forgiving. If you want to reduce the frequency of operation then a series inductor might assist, rather than the series capacitor in your current circuit.
For further info: http://www.mdpi.com/1424-8220/6/7/746/pdf
--
Mike Perkins
Video Solutions Ltd
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It's a monochrome system (even more forgiving?).
So, it's series resonance I want?
This is the one in Ireland:
<http://www.donberg.co.uk/catalogue/passive_components/quartz_crystals/8.86723 8mhz.html>
Should this one work in my circuit?
The only reason I was going with the manufacturer is that they can provide some expertise re. the application, whereas the Irish are only sellers.
I'm no engineer so cannot determine for myself the appropriateness of this crystal for my application (hence my posting here).
Thanks, Dave
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On 14/02/2013 03:47, DaveC wrote:

For the sake of lead time and cost, I would get the one in Ireland. I assume this is a one-off going by your other posts. There might be other suppliers with a reduced minimum order value.
Crystals oscillators tend to be quite forgiving in that its rare that they fail to oscillate. The main variable is the precise frequency, nevertheless they can still be pulled by a few kHz.
In your case I would drop a new one in circuit, see if it works. I will expect it to oscillate. If the frequency is outside of your systems tolerance, then there are methods to increase it towards the crystal's parallel resonant frequency.
While a manufacturer may offer some assistance, series resonance is less common than parallel resonance, and I doubt they will be able to assist in detail. Most crystal manufacturers are just that, or agents for Far East manufacturers and generally have little in-house electronic expertise.
--
Mike Perkins
Video Solutions Ltd
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little

oscillation

your

.86723

provide

this

It is my opinion that you can easily just use the crystal from the Irish or German vendor. For some reason that particular frequency is very standard in Europe and is mass produced by the millions. Very likely to be just fine. The high volume production lowers the price. I doubt that the actual frequency is any better than 50 ppm though, and more than 1000 ppm of frequency error may or may not cause problems.
?-)
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On 16/02/2013 18:09, josephkk wrote:

8mhz.html>

I generally agree, except that the 8.86723 MHz is no longer mass produced and rarely seen in any current catalogue.
It was a very common frequency though only the 4.433619 and 17.73446 MHz crystals are still available in UK RS and Farnell.
The latter was more commonly used in later colour decoders where the +/-90degs subcarrier references are conveniently created.
--
Mike Perkins
Video Solutions Ltd
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Isn't load capacity for the crystal a straightforward calculation?
http://www.tinyuploads.com/images/X4cOrL.jpeg
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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.
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