I have a Flat lapping machine with me and have been trying to lap some quartz blanks but after lapping when i measure the frequency I found that the frequency spread is too high all the time, Can someone give me some expert comments on how can I decrease the frequency spread. Also I will highly appreciate if someone can please provide me links to some helpful documents about flat lapping and some Do's and Don't to improve the quality of my workpieces.
I'm not sure that this is the correct newsgroup for this question -- though I'm not sure that one exists.
From your question, I'm presuming that you are attempting to make frequency controlling crystals, not optical flats or some of the other possible products which might be made by lapping quartz.
I've never done it, but I would expect that flatness (on both side) is not the only important criterion. I suspect that the two surfaces must be very parallel as well. Have you tried measuring the thickness at all four corners -- and in the center? And you may need a something with more resolution than a micrometer capable of reading down to 0.0001" (or even 0.001mm)
Also, it may depend on how the crystals are mounted. I know that really old ones in my collection are mounted between two plates of metal so designed that they contact the crystal only at the corners. I have seen others were the crystal was apparently vacuum plated with metal on both sides (and perhaps the edges were ground free, as these were round), with wires bonded to the center on each side.
I also believe that I remember that the orientation of the blank relative to the original quartz crystal changes the behavior -- one orientation may give a lower frequency, another may give a narrower response curve, and a third orientation may give a lower temperature coefficient relative to the frequency.
But -- this is really faded memory of something read over forty years ago.
Just some thoughts, and a lack of hard information.
Please define what you mean by "frequency spread". I'm interpreting it as you're saying that the Q isn't as high as you'd like it to be, but perhaps that's not what you mean to tell us.
The last time I did crystal grinding (which is what you are
*really* doing:), I used a very flat piece of 1/2" plate glass for the surface. The agent (grinding compound) was jeweler's rouge and a bit of distilled water as a vehicle.
We held the crystal with our fingers (in Platex gloves) and took a few very measured circular passes. We counted the revs. A wonderful ham we knew (Mr. Irvine), designed and built a very neat tester/oscillator to judge the results before sticking the futtering FT-243 crystal holder all back together.
It consisted of wide respsonse oscillator using a computer switching transistor (2N706 or thereabouts), a *very* thin copper wire soldered to a small brass flathead woodscrew. The top of the screw was polished down and could be lifted and placed anywhere on the surface of the rock ... errh ... crystal.
The crystal sat on a polished brass flat (an old brass gear). The gear and the wood screw served as the two contacts for the rock.
I suspect that your "Flat lapping machine" may be taking too much off the blank. Go easy with it. We also fooled around with HcFl
- hydroflouric acid - VERY bad stuff to play with - grinding your rock is much safer.
By frequency spread I mean that the when heaps of crystals are lapped at one time in the lapping machine, the difference in frequency from one crystal to the another is quite big which is the frequency spread. How Can I minimise this spread.
Are you doing this for a few amatuer crystals, or is it for production? What kind of reliability do you want?
My understanding of short-run crystal manufacture is you calibrate the machine for how much it'll change the frequency per pass (or whatever), you measure the frequency of a part-way-done blank, then you take off a controlled amount from the blank.
AFAIK crystals need to be etched a bit if you want them to be resistant to frequency changes as they age. Time-controlled etching is also a good way to take off a controlled amount of material. But HF acid is nasty stuff, so I wouldn't play with it unless you're in it for money.
When I did it I was told to make those passes in a "Figure eight" movement.
We counted the revs.
I had pretty good luck raising the fundamental frequencies of some WWII "war surplus" FT-243 crystals up into the 40 meter ham band circa 1950.
If I overshot a and raised the frequency a bit too much, I used to lower it a little by rubbing soft solder (cold) onto the center of the crystal blank, adding a little mass back onto it. Those crystals were the types held between two metal plates formed to grab them at their four corners.
I've still got about twenty of those FT-243 crystals in a box somewhere. I guess they'll stay where they are until my survivors toss them out, along with a couple of stacks of QSL cards from those halcion days.
The frequency that I am lapping the crystals upto is 20 MHz and the average spread observed is around 200 KHz, This frequency spread is the difference in the frequency between the highest frequency crystal and the lowest frequency crystal obtained out of one batch of crystals i.e. when few hundred crystals were lapped together the highest frequency crystal observed was of 20,150,000 Hz =
20.15 MHz while the lowest frequency crystal was around 19,950,000 Hz =
19.95 MHz thereby giving a spread of 200 KHz. I hope its very clear now. My target is to bring this spread below 50 KHz.......I want some help Please....
ref given earlier has a formula, about 40% into the page: Thickness = 66.2 / Frequency (Thickness in inches, frequency in KHZ) for AT cut crystals. That would give: 66.2 /20000 - 66.2 /20050 ~ .0000083 inches difference, ie about 8 millionths for 50 KHz, and about 33 millionths for 200 KHz. Near the end the page says, "a film of water one molecule thick will lower the frequency of a 10 MHz crystal about 10 parts per million."
Note, 50 KHz at 20 MHz is 2500 parts per million. About the cheapest crystals you can buy are 100 ppm; for a few cents more,
20 ppm is common in cheap crystals. So y2kvickyindia (Pankaj Trivedi) must have some serious material, measuring, or process control problems, being out of the commercial ballpark by a factor of a hundred.
As someone else recommended, Trivedi started a thread in sci.electronics.design, "Re: Flat Lapping of Quartz Crystal." vs. "Flat Lapping" here, and has more details all in one post,
That's the question. If he's measuring thicknesses that are inconsistent with the spread in his frequencies, there's some other uncontrolled parameter in his process.
If he would list up the extremes of thickness that correspond to the extremes in frequency, then it would be obvious if this was a routine exercise in process control (goal to get thickness controlled) or if it is a snipe hunt, where the goal is to find the mystery parameter that's pulling the frequencies, in *spite* of good statistical process control.
Lets start with what you are lapping. Is it a sawn disk ? I suspect so. From a large xtal. This then would be one of the issues.
From Semiconductor background, the wafers (a xtal themselves) are lapped but across the surface - up to 300mm now - the flat isn't flat. There are hills and valleys all across the wafer. There are machines that measure this and qualify a wafer to use. Sometimes it is lapped again. Often the wafers are very think and after testing the die are back lapped for packaging.
SO I suspect the source and the lap is not to the spec level it needs to be. After lapping - measure the flatness of the material. Light helps, but there are machines. Research mirror lapping and measuring. Good stuff there!
I suspect you are fighting some of this - microscopic hills and valleys.
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