How does a Temperature Compensated Crystal Oscillator work?

Foundations of Amateur Radio

You know when you walk down the street and you lift your foot and all of a sudden you realise that you stepped in something and now it's stuck to your shoe?

I had that feeling during the week.

Last week I mentioned that I had purchased a TCXO, a Temperature Controlled External Oscillator. Lowell NE4EB set me straight by pointing out that XO stood for crystal and that TCXO stood for Temperature Compensated Crystal Oscillator, which then lead me on a merry goose-chase trying to learn about all that.

I mention this because while the stickiness on my shoe kept me busy, it also highlighted that I'm still a babe in the woods on a steep learning curve to knowledge with some roadblocks, diversions and potholes along the way.

That reminds me, if you ever feel the urge to pull me up on something I've said, you can email me via my callsign at gmail.com.

So, how does this Temperature Compensated Crystal Oscillator actually work?

Without getting into the circuitry behind the scenes, as I mentioned previously, a crystal oscillates and the frequency is dependent on temperature. Turns out this is a predictable curve, which makes it possible to account for changes in temperature.

In addition to keeping the temperature stable, another way to keep the frequency of a crystal stable is to have an electrical circuit that changes depending on temperature and have that create something like an opposing curve, so you can add the two together and end up with a pretty stable frequency. Before you start asking how exactly, let me just remind you of the shoe with the stickiness on it.

In essence you have something like a resistor that changes resistance depending on temperature, it's a component called a thermistor, and that in turn affects a resonant circuit, also known as an Electronic Oscillator, or LC circuit, which in turn affects the circuit that is driving the crystal. These days most if not all of that is on a chip and you get a neat little package that you can plug into your radio to give it frequency stability and hopefully accuracy.

I did say I was going to talk about accuracy this week, but the doo-doo I stepped in put a swift halt to that. Besides, now I know that there is a thing called a thermistor, the second portmanteau I ever learned, together with Gerrymander, so there's that - oh, also, Tanzania, Eurasia and Oxbridge.

Back to Amateur Radio. The oven controlled crystal I mentioned last week, they exist in high-end measuring gear, not in the $26 TCXO I have installed in my radio. While I'm on the subject, you can also compensate for temperature with software, using either a purpose built micro-processor, or even the host processor that is using the crystal, but that gets into magic self-referencing voodoo pretty quickly.

And while I've been playing, Japan is finally being received here and I heard a station 18656km away during the week. Mind you, AA3GZ in Doylestown, Pensylvania, on the Atlantic Ocean side of the United States was putting out 100 Watts, so there's that.

I'll leave you with a thought that I hope to be able to answer next week.

If your radio has a crystal that determines what frequency it's tuned to, how do you use that to determine the accuracy of the frequency, more self-references, just because I can and besides, I'm a software developer and recursion is part of my make-up.

I'll give you a hint, it's not all to do with MHz.

I'm Onno VK6FLAB.