I remember when I worked at Motorola some decades ago, there was a little section in the factory area in Schaumburg that made quartz crystals. It's all long since been sold off (I think all the equipment went to China) but I remember all the signs for cyanide alarms, presumably due to some step in the manufacturing process.
The quartz crystals themselves were grown with carefully controlled levels of specific impurities (like scandium) in order to reduce their temperature sensitivity.
The article is a nostalgic reminder of when I was first interested in electronics and got interested in quartz resonators for projects. Grinding down old war surplus FT243-style crystals and other types so as to resonate at frequencies I needed was a common practice with hobbyists back then.
I used many similar techniques to the same end of removing quartz which raised its frequency. Grinding materials included abrasives such as jeweler's rouge, cerium oxide, commercial polishes such as Brasso and Silvo and even HF solution. I'd place the quartz on a small section of plate glass and slide it through a slurry of the abrasive periodically testing its frequency until I'd reached my target.
There's an art to this that's too long to mention here except to say abrasives were used strategically, course grinding would get me near the desired frequency and I'd finish off with a fine abrasive. Then there was the job of re-aging the crystal after its recent abuse to increase its stability. Other techniques were involved such as not lowering its Q factor, etc. which I'll not cover here.
The most desired crystal cut was from the XT-plane (being the most stable) but it was generally difficult to get as it's only a small section of the quartz crystal (also each cut oscillates only over a limited range of frequencies). I used to have a book that explained these cuts in detail, their frequency ranges and electronic properties along with the basic crystallography which I lost years ago. A quick glance at the book would have shown that a great deal of science, engineering and skill is involved in the selection of quartz and its manufacture into useful resonators.
BTW, the mentioning of HF will likely horrify chem-phobic readers. We were well aware of its dangers and took special precautions never to come in contact with it.
> BTW, the mentioning of HF will likely horrify chem-phobic readers
I would not say I am chem-phobic, but yes indeed that stood out. HF is nasty stuff, and yes requires some care I suspect.
The other details are fascinating, though - the intersection of mechanical, crystallographic, and RF (?) properties of a crystal that you can adjust through abrasives and selection of the cut.
Working with HF was extremely difficult before WWII, but it has become much easier after the invention of Teflon.
Teflon is not affected by HF, so if you use only vessels of Teflon to hold the HF solution and tweezers made of Teflon for handling anything that you submerge in the solution of HF, it works fine.
Besides using Teflon for anything that is in contact with the HF solution, you must do all work under a hood that evacuates the vapors of HF emitted by the solution, otherwise handling a HF solution would be very dangerous. It is good that gaseous HF is lighter than air, so after being evacuated it will continue to rise in the air, while becoming more and more diluted.
I've just spent the weekend tuning brass reeds from an organ. It sounds like a very similar process, except you can grind both ends of the tongue to raise and lower frequency.
Adding solder has also been frequently used to correct the resonance frequency of quartz crystals that have been ground too much, and I mean during industrial mass-production, not only in a home-lab setting.
Oh that is so cool. I played the one in Liepaja, Latvia for a bit and it was absolutely amazing. It's love/hate for me (like the harpsichord), I love the instruments but I usually do not like the music that is played on them because of the grating effect. I have pretty bad tinnitus which really spoils a lot of music for me, extremely annoying.
Believe or not, when I explained to many non techies how quartz watches work and how any computers’ hardware clocks work in the same principle, they were all surprised how elegant and how efficient the mechanism is. I was also impressed when I first learned about it. True science and engineering beauty.
You mean could you get it down to a low enough frequency? Hmm
I guess you can get down into audio frequency but maybe the amplitude will be tiny, you probably want a piezo mechanism that will give you more of a rumble.
You can get quartz to resonate down to upper audio frequencies with certain crystal cuts and manufacturing techniques but it's difficult. Typically, the lowest frequency in common use is with its use in watches with a frequency of 32,768Hz (that's about the lower limit where manufacturing and frequency combine to make a useful product).
For electronic circuits such as frequency reference markers where frequency stability is important the lowest practical frequency is 100kHz with 1Mhz preferred, and where frequency tolerances are tight 5 and 10MHz are much preferred with operation in a temperature stabilized oven to minimize frequency drift.
The most frequency-stable crystal cuts are at those frequencies, as frequencies increase (say >10MHz to 100MHz), which at the highest frequencies require the crystal to operate in overtone mode, frequency stability again tends to decrease.
As mentioned in the article there are a lot of common watch crystals that oscillate at 32.768 kHz, but they are tuning fork crystals rather than bulk mode (or modern SAW), which have much higher frequencies. It would be challenging to lower the frequency of these, but perhaps you could evaporate Au or W onto the tips and reseal them to get into the audible frequencies. Much easier would be to get two crystals which beat against each other in the audio range. Even a couple of 6MHz crystals 100ppm off would be ~6kHz (temp sensitive), and you would need some driving circuits, but you might be able to hear the beat by driving an electret microphone or a really tiny tweeter coil speaker (probably need an amp though).
It's much easier to build/buy electrical rather than mechanical LC components to hit audio frequencies ~100Hz-10kHz.
The quartz crystals themselves were grown with carefully controlled levels of specific impurities (like scandium) in order to reduce their temperature sensitivity.
reply