Understanding The Quartz Crystal Resonator

Accurate timing is one of the most basic requirements for so much of the technology we take for granted, yet how many of us pause to consider the component that enables us to have it? The quartz crystal is our go-to standard when we need an affordable, known, and stable clock frequency for our microprocessors and other digital circuits. Perhaps it’s time we took a closer look at it.

The first electronic oscillators at radio frequencies relied on the electrical properties of tuned circuits featuring inductors and capacitors to keep them on-frequency. Tuned circuits are cheap and easy to produce, however their frequency stability is extremely affected by external factors such as temperature and vibration. Thus an RF oscillator using a tuned circuit can drift by many kHz over the period of its operation, and its timing can not be relied upon. Long before accurate timing was needed for computers, the radio transmitters of the 1920s and 1930s needed to stay on frequency, and considerable effort had to be maintained to keep a tuned-circuit transmitter on-target. The quartz crystal was waiting to swoop in and save us this effort.

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Fail More: The Story of [CNLohr]’s Clear Keytar

[CNLohr] is kinda famous round these parts; due to some very impressive and successful hacks. However, for his 20k subscriber video, he had a bit to say about failure.

Of course glass circuit boards are cool. Linux Minecraft things are also cool. Hacks on the ESP8266 that are impressive enough people thought they were an April Fool’s joke are, admittedly, very cool. (Though, we have to confess, posting on April 1 may have added to the confusion.)  For a guy who puts out so many successes you’d think he’d talk about the next ones planned; hyping up his growing subscriber base in order to reel in those sweet sweet Internet dollars.

Instead he shows us a spectacular failure. We do mean spectacular. It’s got beautiful intricate copper on glass key pads. He came up with clever ways to do the lighting. The circuit is nicely soldered and the acrylic case looks like a glowing crystal. It just never went anywhere and never worked. He got lots of people involved and completely failed to deliver.

However, in the end it was the failure that taught him what he needed to know. He’s since perfected the techniques and skills he lacked when he started this project a time ago. We’ve all had experiences like this, and enjoyed hearing about his. What failure taught you the most?

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The (Copper) Crystal Method

One of the staples of kitchen chemistry for kids is making sugar crystals or rock candy. Why not? It is educational and it tastes good, too. [Science with Screens] has a different kind of crystal in mind: copper crystals. You can see the result in the video below.

To grow pure metal crystals, he used copper wire and copper sulfate. He also used a special regulated power supply to create a low voltage to control the current used to form the crystal. The current needed to be no more than 10mA, and an LM317 holds the voltage constant. However, that regulator only goes as low as 1.25V, so diodes cut a volt off the output.

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Atmel Removes Full-Swing Crystal Oscillator

It is one of our favorite chips, and the brains behind the Arduino UNO and its clones, and it’s getting a tweak (PDF). The ATmega328 and other megaX8-series chips have undergone a subtle design change that probably won’t affect you, but will cause hours of debugging headaches if it does. So here’s your heads-up. The full-swing oscillator driver circuitry is being removed. As always, there’s good news and bad news.

The older ATmega chips had two different crystal drivers, a low-power one that worked for lower speeds, and higher-current version that would make even recalcitrant crystals with fat loading capacitors sing. This “full-swing” crystal driver was good for 16 MHz and up.

The good news about the change is that the low-power crystal driver has been improved to the point that it’ll drive 16 MHz crystals, so you probably don’t need the full-swing driver anymore unless you’re running the chip at 20 MHz (or higher, you naughty little overclocker).

This is tremendously important for Arduinos, for instance, which run a 16 MHz crystal. Can you imagine the public-relations disaster if future Arduinos just stopped working randomly? Unclear is if this is going to ruin building up a perfboard Arduino as shown in the banner image. The full-swing oscillator was so robust that people were getting away with a lot of hacky designs and sub-optimal loading capacitor choices. Will those continue to work? Time will tell.

The bad news is that if you were using the full-swing oscillator to overcome electrical noise in your environment, you’re going to need to resort to an external oscillator instead of a simple crystal. This will increase parts cost, but might be the right thing to do anyway.

Whenever anyone changes your favorite chip, there’s a predictable kerfuffle on the forums. An Atmel representative said they can get you chips with the full-swing driver with a special order code. We’re thinking that they’re not going to let us special order ten chips, though, so we’re going to have to learn to live with the change.

The ATmega328 has already gotten a makeover, and the new version has improved peripheral devices which are certainly welcome. They don’t have the full-swing oscillator onboard, so you can pick some up now and verify if this change is going to be a problem for you or not. We don’t have any of the new chips to test out just yet.

Thanks to [Ido Gendel] for tipping us off to the change in our comment section! If you have any first-hand experience with the new chips, let us know in the comments and send in a tip anytime you trip over something awesome during your Internet travels.

Improving the RTL-SDR

The RTL-SDR dongle is a real workhorse for radio hacking. However, the 28.8 MHz oscillator onboard isn’t as stable as you might wish. It is fine for a lot of applications and, considering the price, you shouldn’t complain. However, there are some cases where you need a more stable reference frequency.

[Craig] wanted a stable solution and immediately thought of a TCXO (Temperature Compensated “Xtal” Oscillator). The problem is, finding these at 28.8 MHz is difficult and, if you can find them, they are relatively expensive. He decided to make an alternate oscillator using an easier-to-find 19.2 MHz crystal.

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Oscillator Design by Simulation

[Craig] wanted to build a 19.2 MHz crystal oscillator. He knew he wanted a Pierce oscillator, but he also knew that getting a good design is often a matter of trial and error. He used a 30-day trial of a professional simulation package, Genesys from Keysight, to look at the oscillator’s performance without having to build anything. He not only did a nice write up about his experience, but he also did a great video walkthrough (see below).

The tool generates a sample schematic, although [Craig] deleted it and put his own design into the simulator. By running simulations, he was able to look at the oscillator’s performance. His first cut showed that the circuit didn’t meet the Barkhausen criteria and shouldn’t oscillate. Unfortunately, his prototype did, in fact, oscillate.

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Strobe Light Slows Down Time

Until the 1960s, watches and clocks of all kinds kept track of time with mechanical devices. Springs, pendulums, gears, oils, and a whole host of other components had to work together to keep accurate time. The invention of the crystal oscillator changed all of that, making watches and clocks not only cheaper, but (in general) far more accurate. It’s not quite as easy to see them in action, however, unless you’re [noq2] and you have a set of strobe lights.

[noq2] used a Rigol DG4062 function generator and a Cree power LED as a high-frequency strobe light to “slow down” the crystal oscillators from two watches. The first one he filmed was an Accutron “tuning fork” movement and the second one is a generic 32,768 Hz quartz resonator which is used in a large amount of watches. After removing the casings and powering the resonators up, [noq2] tuned in his strobe light setup to be able to film the vibrations of the oscillators.

It’s pretty interesting to see this in action. Usually a timekeeping element like this, whether in a watch or a RTC, is a “black box” of sorts that is easily taken for granted. Especially since these devices revolutionized the watchmaking industry (and a few other industries as well), it’s well worthwhile to take a look inside and see how they work. They’re used in more than just watches, too. Want to go down the rabbit hole on this topic? Check out the History of Oscillators. Continue reading “Strobe Light Slows Down Time”