Sunday, June 20, 2021

A Ham Radio Series 26 – The World Runs on Quartz

In today’s world of computers everywhere, and trillion-transistor processors, it’s easy to think that the modern world is running on silicon.  It’s true, but there’s an aspect of running on silicon that escapes most people.  Virtually all of those computers, smartphones, and every other gadget we think of runs on silicon dioxide: quartz.  All of those things, and far more than I can summarize, have a clock circuit (so-called because the clock’s timing pulses run everything) with their timing set by a slice of a quartz crystal.  

In 1880, two brothers working at the Laboratory of Mineralogy at the Sorbonne, Pierre and Paul-Jacques Curie, discovered that a crystal would generate a small voltage if it was pressed.  They called this the piezoelectric effect, piezo- derived from the Greek piezen, “to press”.  A year later, they showed that if voltage was applied to the crystal, it would change its shape in response.  Through 1915, the piezoelectric effect was a laboratory curiosity studied largely in France and Germany.  Other types of crystals were studied and the effect found in some other materials. 

The first use of a crystal for setting the frequency of an oscillator circuit is credited to Dr. A.M. Nicolson working at the Bell Telephone Laboratories in 1918.  Nicolson was using a crystal called Rochelle Salt, because it had a stronger piezoelectric effect than quartz.  W.G. Cady switched to a quartz crystal slice by the following year, and was the first to use quartz.  In 1920, Cady submitted a patent application for a piezoelectric resonator describing possible uses as not just a resonator but also as a filter and a coupler.  

With the start of amateur radio in the US, quartz crystal oscillators became a subject of wide experimentation.  The first commercial market for quartz crystals was selling shaped slices of the best mined quartz to hams, who then did the “finish work;” lapping the quartz on grinding plates to set their frequencies, most often in an oscillator in their transmitters.  

The history of quartz production and the industry is fascinating.   An area of concentration was central Pennsylvania, near the city of Carlisle (pdf warning), which happens to be the location of the first crystal vendor I was ever sent to investigate and interview.  While the history of the industry and oscillators is interesting, I don’t want to dwell on histories; I prefer to talk about the technical side. A good summary of the history is by Virgil Bottom, here.  Other papers I have on my computer are behind IEEE paywalls and I can't link to them.

Today, natural, mined quartz crystals aren’t used out of the ground.  The crystals are refined and regrown in a hydrothermal process (yes, “hot water;” very hot water).  Quartz crystals have complex geometries, and the ways to cut a crystal to get better properties out of the resulting crystal have been refined over the years.  This figure shows just some of the crystal cuts that are commonly used.  

Now compare that short, kinda squat crystal to a lab-grown quartz crystal.  The world market for these is in the billions.

Synthetic quartz crystals are grown in bar form; the position of the crystal before cutting is determined by x-ray diffraction and the crystal is then bonded to a substrate to be cut on.  The cutting is done with a grinding process: parallel bands of steel like a bread slicer that carry abrasives along their edges and the quartz is ground through, resulting in many slices.  The slices are ground into the desired shape (usually round) and then lapped on a plate to frequency.  In the place where I saw this being done, the frequency of the batch of blanks was monitored on a shortwave radio.  As the blanks get thinner, the frequency they’re heard on goes up.  As the final frequency is approached, the abrasives get switched to finer and finer grit sizes.  The sound turns from a growl in the speaker to finer note until it’s becomes like one audio tone.  If the blanks are inadvertently made too thin and too high in frequency, it’s possible to sputter some metal onto the crystal slice, lowering the frequency.  In the 1920s and 30s, hams lapping a crystal on a glass plate with very fine abrasives would rub some graphite from a pencil lead on the crystal to lower its frequency (and they still do it today).

Much as Cady predicted in 1920, quartz slices can be used in oscillators and in filters, and the vast majority of ham rigs out in the world have some quartz in them.  Further, the older and “lower tech” ham stations are, the more quartz they’ll have.  There are literally millions of low power (QRP) ham transmitters that are made from a quartz crystal oscillator and a single transistor amplifier.  Until just before I got my first ham license in 1976, Novice class licensees were required to use quartz crystal frequency control.  The majority of VHF/UHF “handie-talkies” used a pair of crystals for every channel the user wanted.  Quartz crystal filters for adjacent channel suppression are still used in many receivers.  Only the most advanced of modern receivers can substitute software defined filters that exceed the performance of good crystal filters.

Crystal oscillator design is a wide and deep topic.  Any topology of oscillator can be built with a crystal resonator in it.  This chart (from a presentation by John Vig that runs over 300 pages), shows where a crystal (in blue) would be in the various circuit types. The red triangle is active device, with the B and C pins corresponding to a bipolar transistor's base and collector.



12 comments:

  1. Oh, yeah! I remember grinding/lapping some surplus crystals I was given so I could get them into the Novice band. Valve grinding compound of increasingly finer grits, followed by a toothpaste polish.

    And a small variable cap in series or parallel (depending on circuit config) with the crystal could "rubber it" a few tens of Hertz one way or the other.

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  2. Thank you very much. I haven't seen this great a review since I studied for my Advanced (back around '65 or so).

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  3. You wrote:

    In the place where I saw this being done, the frequency of the batch of blanks was monitored on a shortwave radio. As the blanks get thinner, the frequency they’re heard on goes up. As the final frequency is approached, the abrasives get switched to finer and finer grit sizes. The sound turns from a growl in the speaker to finer note until it’s becomes like one audio tone.

    and at the bottom of one of your links was this:

    Also in the '49 Hints and Kinks book is a method (page 88) of monitoring the frequency of a quartz crystal DURING GRINDING.

    Take a flat piece of aluminum or copper about 6" square, and connect it to your receiver antenna post using a short lead. Place the plate glass on which the grinding is being done on this sheet. You can tune in the crystal frequency on the receiver by the scratches you hear as the crystal is being ground.

    This is the first time I've ever heard of something like this, but I assume the laws of physics and piezoelectricity allow this to work.


    !!!

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    1. Yeah, it's pretty cool.

      I have a pdf of an article from QST dated November 1925, "Crystal Control for Amateur Transmitters" Someplace else was saying transmitters were the first to be crystal controlled because the current surges during transmitting would move the transmitter.

      It assumes you have a quartz crystal out of the ground and goes through cutting the blank with the back of a hacksaw blade dribbled with carborundum and water, then grinding it to thickness and putting it on frequency using 301 emery and kerosene.

      If anyone wants that, I don't have it on Dropbox or something like that, but I can email it. SiGraybeard at gmail.

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  4. Some piece of gear I used to test would occasionally have a crystal that would fix on 4/3 of its rated frequency. First clue was gibberish serial comm.

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    1. Interesting. Up until fairly recently, like 10-15 years ago, no crystals operated above somewhere around 20 MHz. It's possible that circuit had a third overtone (harmonic) crystal, which was the most common type of OT crystal and would operate on the 4th harmonic of the actual crystal frequency instead of its third.

      Crystals will run on spurious frequencies, too, but a good circuit design minimizes that chance. It could be that running on the 4th overtone instead of the third was a design deficiency, too.

      Far too many unknowns in there for me to have a real idea of why, just things I'd think about troubleshooting.

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  6. I'm bummed that I didn't get anything at all about this in my GMDSS (Element 7&9) class. I got 80 hours of the instructor teaching the test and reliving his days touring the brothels of Thailand as a ship's radio operator. Had I known that there was some mad science like hand-lapping crystals to tune them, I'd have been in my element.

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    1. Anybody can tour the women in brothels, but how many men can put a radio on frequency with fine emery, some light oil, and a plate of glass?

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    2. Only the few of us who have actually done it!

      It's a lot harder now that the crystals are all soldered into little metal cans, but in the Olde Dayse, we could have the crystal apart, lapped, on-frequency, and reassembled in short order.

      After you ruined a few at first......

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    3. It's probably bad to say the FT-243 holder was an abomination, but the hermetically sealed crystals do perform better.

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    4. No doubt about it. They age far better, too. I had WWII surplus crystals marked as "dead" that just needed disassembly and cleaning to make them work again.

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