Improving A Cheap Frequency Counter With GPS

Frequency counters are useful tools for anyone that finds themselves regularly working with time-variant signals. There are a huge range available, from cheap eBay specials to expensive lab-grade hardware. [itakeyourphoto] had a counter on the lower end of the cost spectrum, and decided to make some improvements with the help of GPS (Youtube link, embedded below).

The fundamental weakness of a cheap frequency counter is usually the internal reference against which all other signals are measured. The more accurate this is, the more accurate the counter will be. [itakeyourphoto] determined that a great way to generate a reasonably good reference frequency was by using a uBlox GPS module. Once locked on to satellites, it can use a numerically controlled oscillator to output any frequency up to 15MHz with good accuracy.

The cheap frequency counter in question used a 13 MHz internal reference, so the uBlox module was programmed to match this. [itakeyourphoto] reports that it compares favorably to his higher-end GPS-disciplined oscillators, displaying very little drift or other aberrations.

We see plenty of clocks using GPS for its accurate time, but we’ve seen projects that attempt to go even further than that, too. Video after the break.

[Thanks to jafinch78 for the tip!]

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Fun With Negative Resistance II: Unobtanium Russian Tunnel Diodes

In the first part of this series, we took a look at a “toy” negative-differential-resistance circuit made from two ordinary transistors. Although this circuit allows experimentation with negative-resistance devices without the need to source rare parts, its performance is severely limited. This is not the case for actual tunnel diodes, which exploit quantum tunneling effects to create a negative differential resistance characteristic. While these two-terminal devices once ruled the fastest electronic designs, their use has fallen off dramatically with the rise of other technologies. As a result, the average electronics hacker probably has never encountered one. That ends today.

Due to the efficiencies of the modern on-line marketplace, these rare beasts of the diode world are not completely unobtainable. Although new-production diodes are difficult for individuals to get their hands on, a wide range of surplus tunnel diodes can still be found on eBay for as little as $1 each in lots of ten. While you’d be better off with any number of modern technologies for new designs, exploring the properties of these odd devices can be an interesting learning experience.

For this installment, I dug deep into my collection of semiconductor exotica for some Russian 3И306M gallium arsenide tunnel diodes that I purchased a few years ago. Let’s have a look at what you can do with just a diode — if it’s the right kind, that is.

[Note: the images are all small in the article; click them to get a full-sized version]

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Fun With Negative Resistance: Jellybean Transistors

The concept of negative resistance has always fascinated me. Of course, a true negative resistance is not possible, and what is meant is a negative differential resistance (NDR). But of course knowing the correct term doesn’t do anything to demystify the topic. Negative resistance sounds like an unusual effect, but it turns out to be relatively common, showing up in places like neon lamps and a number of semiconductor structures. Now’s as good a time as any to dig in and learn more about this common principle.

NDR means a portion of a device’s I/V curve where the current falls with increasing applied voltage. The best-known semiconductor device exhibiting negative resistance is the tunnel diode, also known as the Esaki diode after one of the Nobel-Prize-winning discoverers of the quantum tunneling effect responsible for its operation. These diodes can perform at tremendous speeds; the fastest oscilloscope designs relied on them for many years. As the transistor and other technologies improved, however, these diodes were sidelined for many applications, and new-production models aren’t widely available — a sad state for would-be NDR hackers. But, all hope is not lost.

Rummaging through some old notebooks, I rediscovered an NDR design I came up with in 2002 using two common NPN transistors and a handful of resistors; many readers will already have the components necessary to experiment with similar circuits. In this article, we’ll have a look at what you can do with junkbox-class parts, and in a future article we’ll explore the topic with some real tunnel diodes.

So, let’s see what you can do with a couple of jellybean transistors!

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Casio Watch Gets A MEMS Oscillator Upgrade

We’ve got to admit to being a bit of a Casio G-Shock watch geek. The big, chunky watches were every day carry items that survived everything we dished out, right up until the smartphone made wearing one seem redundant. But others continue to use and abuse G-Shocks, and some brave souls even hack them.

Replacing the standard quartz crystal with a temperature-compensated MEMS oscillator is one hack that [Alex] tried, and it appears to have worked out well. His project write-up doesn’t specify which MEMS oscillator was used, but we suspect it’s the SiT1552 TCXO. With its extremely small size, stability over a wide range of temperatures, and ultra-low power requirements, the chip is a natural choice to upgrade the stock 32.768-kHz quartz crystal of the watch. Trouble is, the tiny 1.5 mm x 0.8 mm chip-scale package (CSP) device presented some handling problems. After overcooking a few chips in the reflow oven, [Alex] was able to get one mounted to a tiny breakout board, which went into the space formerly occupied by the watch’s quartz crystal. He stole power for the TCXO from a decoupling capacitor, sealed the watch back up, and it’s back in service with better stability and longer battery life to boot. The video below shows the TCXO undergoing tests alongside the original quartz crystal and a comparatively huge DS3231 RTC module, just for fun.

[Alex]’s MEMS transplant seems a long way to go and a lot of fussy work for marginal gains, but who are we to judge? And it does make the watch susceptible to punking with a little helium, which might make things interesting.

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Crystal Oscillators Explained

We’ve read a lot about oscillators, but crystal oscillators seem to be a bit of a mystery. Hobby-level books tend to say, build a circuit like this and then mess with it until it oscillates. Engineering texts tend to go on about loop gains but aren’t very clear about practice. A [circuit digest] post that continues a series on oscillators has a good, practical treatment of the subject.

Crystals are made to have a natural resonant frequency and will oscillate at that frequency or a multiple thereof with the proper excitation. The trick, of course, is finding the proper excitation.

The post starts with a basic model of a crystal having a series capacitance and inductance along with a resistance. There’s also a shunt or parallel capacitor. When you order a crystal, you specify if you want the resonant frequency in series or parallel mode — that is, which of the capacitors in the model you want to resonate with the inductor — so the model has actual practical application.

By applying the usual formula for resonance on the model you’ll see there is a null and a peak which corresponds to the two resonance points. The dip is the series frequency and the peak is the parallel. You can actually see a trace for a real crystal in a recent post we did on the Analog Discovery 2. It matches the math pretty well, as you can see on the right.

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Energy Sipping Neodymium Sphere Keeps On Spinning

At this point we’re sure you are aware, but around these parts we don’t deduct points for projects which we can’t immediately see a practical application for. We don’t make it our business to say what is and isn’t worth your time as an individual hacker. If you got a kick out of it, great. Learned something? Even better. If you did both of those things and took the time to document it, well that’s precisely the business we’re in.

So when [Science Toolbar] sent in this project which documents the construction of an exceptionally energy efficient spinning neodymium sphere, we knew it was our kind of thing. In the documentation it’s referred to as a motor, though it doesn’t appear to have the torque to do any useful work. But still, if it can spin continuously off of the power provided by a calculator-style photovoltaic cell, it’s still a neat trick.

But how does it work? It starts by cracking open one of those little solar powered toys; the ones that wave or dance around as soon as any light hits the panel in their base. As [Science Toolbar] explains, inside these seemingly magical little gadgets is a capacitor and the classic black epoxy blob that contains an oscillator circuit. A charge is built up in the capacitor and dumped into a coil at roughly 1 Hz, which provides just enough of a push to get the mechanism going.

In the video after the break, [Science Toolbar] demonstrates how you can take those internals and pair it with a much larger coil. Rather than prompting a little sunflower or hula girl to do its thing, the coil in this version provides the motive force for getting the neodymium sphere spinning. To help things along, they’re even using a junk box zero friction magnetic bearing made up of a wood screw and a magnetized screwdriver tip.

It’s an interesting example of how a tiny charge can be built up over time, and with a nice enough enclosure this will make for a pretty cool desk toy. We’ve previously seen teardowns of similar toys, which revealed a surprising amount of complexity inside that little epoxy blob. No word on whether or not the version [Science Toolbar] cannibalized was quite so clever, however.

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[Ben Krasnow] Gasses MEMS Chips, For Science

Why in the world does helium kill iPhones and other members of the Apple ecosystem? Enquiring minds want to know, and [Ben Krasnow] has obliged with an investigation of the culprit: the MEMS oscillator. (YouTube, embedded below.)

When we first heard about this, courtesy in part via a Hackaday post on MRI-killed iPhones, we couldn’t imagine how poisoning a micro-electromechanical system (MEMS) part could kill a phone. We’d always associated MEMS with accelerometers and gyros, important sensors in the smartphone suite, but hardly essential. It turns out there’s another MEMS component in many Apple products: an SiT 1532 oscillator, a tiny replacement for quartz crystal oscillators.

[Ben] got a few from DigiKey and put them through some tests in a DIY gas chamber. He found that a partial pressure of helium as low as 2 kPa, or just 2% of atmospheric pressure, can kill the oscillator. To understand why, and because [Ben] has a scanning electron microscope, he lapped down some spare MEMS oscillators to expose their intricate innards. His SEM images are stunning but perplexing, raising questions about how such things could be made which he also addresses.

The bottom line: helium poisons MEMS oscillators in low enough concentrations that the original MRI story is plausible. As a bonus, we now understand MEMS devices a bit better, and have one more reason never to own an iPhone.

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