Amateur radio, and how to avoid it

Why can’t you really use a mobile phone in a petrol station forecourt? Why do you have to turn off digital cameras and MP3 players during take-off? The reason is that every piece of electronic equipment you will ever own has a secret life as a radio transmitter.

I’ll start explaining what happens using sound waves instead of electromagnetic ones, as they’re slower, less mysterious, and you can hear them. An organ pipe has a natural tendency to resonate. If you place a source of turbulent air at one end, it will travel to the other end. Some of it will leave the tube, but some will be reflected back: this back-propagation is a kind of shock-front caused by the travelling wave suddenly emerging into the open air. That reflection is reflected again when it reaches the first end of the pipe, and so on. A pressure wave can travel up and down a pipe for some considerable time without dissipating. The resulting periodic pressure also influences the source of air, to the extent that a really good standing wave is set up. As long as the air flow keeps it energised, it is quite stable.

So, organ pipes need three things: a source of energy, a long, thin shape to guide the sound wave along, and an end that’s open to the world to allow the sound to radiate and the shock-front to propagate back down the pipe. Electromagnetic waves in wires share many similarities to acoustic waves in the way that they propagate, and it turns out that these similarities include the propensity for resonance.

So, what does an FM radio antenna look like? It’s generally a piece of wire of a carefully-selected length, with a high-frequency signal being driven into one end, and the other end left unconnected. A signal travels up the piece of wire, and some of it travels back down the wire as a reflection. The fact that the wire is tuned by choosing its length to match the carrier frequency helps the system to resonate, and electromagnetic radiation propagates into the air.

ImageThe Hannington transmitter in Hampshire: if you squint, it’s an organ pipe.

Yes, this is a simplification: many antennae look like this; those that don’t are designed so that that they focus their radiated energy, or their coverage, in a particular direction. If we’re going to call Hannington an organ pipe, we can call the microwave mushrooms that formerly decorated the BT Tower flugelhorns. The principle is the same.

Anyway, it turns out that a resonant system is created on a printed circuit board whenever we connect two chips that exchange information: one of them produces a high-frequency signal with nice sharp transitions. It travels down a copper track with all its harmonics stretching gloriously to infinity, and into the input pin of the device that’s reading it. Usually, the input pin is designed with efficiency in mind, so that it doesn’t use much current to read the signal (a certain class of engineers will now be uncontrollably murmuring ‘high input impedance’ at the screen). The far end of the track therefore appears to be very similar to a piece of unconnected wire. The signal and its harmonics travel back down the wire and, even if the length of the track isn’t quite matched, we are suddenly transmitting a little bit of radio. This process is reciprocal, so a little current will flow in the wire in response to strong radio waves. In fact, it’s more complicated than that, because the power connection that supplies both chips behaves in a similar manner.

Why does any of this matter? Well, ordinarily it wouldn’t, until you happen across a piece of equipment that radiates a bit of energy at a certain frequency, and try and make it work at the same time as another piece of equipment that is susceptible to interference at that frequency. Suddenly you have a fault that stops them playing nicely together for mysterious, magical reasons. Neither manufacturer accepts culpability because it’s not entirely their fault and, if you’re really unlucky, the fault kills people.

Of course, we now live in a world full of radio energy, and we rely increasingly on electronic devices for our safety and security, so we are rapidly accumulating anecdotes of near-lethal situations in which electronics fail. A friend of mine managed to crash his heart pacemaker during a compliance test, and it was broken for weeks. His hospital found out during a routine check, discovering that its system clock had stopped at the time he was wandering about inside an EMC test chamber. If it doesn’t kill you, it might just cost you money. That’s why customers are required to turn off mobile phones at the petrol pumps, just in case they mess up the payment system.

So, why does any of this matter to a manufacturer of audio toys? The simple answer is legislation.

That’s the sound of the police

Since the mid-1990s, various laws have ensured that designers and manufacturers need to take electromagnetic radiation seriously. We must certify our own equipment as complying with these regulations in order to sell it, and have to produce documentation on demand to show that we have made the effort. If our products don’t comply, we will be fined. Sometimes we do the compliance testing ourselves; sometimes, when we expect compliance issues to be straightforward, our manufacturer does it for us.

CE/FCC
As well as electromagnetic tests, there are tests to see if equipment can withstand static shocks without crashing or being destroyed, to check that mains-powered equipment can withstand power surges and drop-outs, and to ensure that harmful radiation isn’t propagated down interconnecting cables into neighbouring equipment. This is all very reassuring for the consumer but, interestingly, we often find good, defensive radiofrequency design practice is at odds with the most optimal audio circuit design: particularly when so much of the audio world is still using those horrible phono cables to transmit its most sensitive signals. This leads us to invent a host of novel circuits that serve both masters, and to a ceaseless and obsessive evaluation of our best practice.

The details of what we must do are not the most exciting subject for a non-specialist, but they have shaped the world around us in a few ways. One thing you’ll have noticed is that more and more pieces of equipment tend to come with separate power supplies, rather than accepting an input directly from the mains as they used to. This allows the designers to buy in a power supply from a third party that has already been tested, and prevents them having to test their own for compliance, in accordance with the volume of extra legislation that covers mains-powered equipment.

The second thing is those ubiquitous lumps on computer power supply cables and USB leads.
USB with ferrite This is a ferrite. It’s just a doughnut of material containing mostly iron powder and resin, which magnetises slightly in response to current flowing through the cable and, in magnetising, generates its own field in opposition. That’s its only function. In doing this, it suppresses nasty electromagnetic problems below about 80MHz surprisingly well. Nobody likes them, but we put them on the cable because the CE marking criteria are very strict, and most equipment will fail by a hair’s breadth without them.

Radio silence

To give a quick indication of what we need to do, here is a picture of a bit of one of our products, with parts marked that we use only to make it pass electromagnetic legislation. If we designed the product without them, everything would work in the same way, but the unit wouldn’t get certification.

Product X

  1. A multi-layer circuit board. Most circuit boards these days have internal layers, and this is often just for electromagnetic compatibility. Multi-layer boards are built up from sandwiches of thin boards glued together, and this makes them more expensive than standard one- or two-layer boards. The internal layers sit only a fraction of a millimetre from the outer ones, and the proximity helps to dampen any resonance: a bit like lining the organ pipe with foam.
  2. Series resistor packs also quell standing waves, by absorbing a little reflected energy.
  3. Gigahertz capacitors and chip ferrites selectively admit certain high-frequency signals, while increasingly absorbing their troublesome harmonics.
  4. Common mode inductors sort out particular classes of problems on power and data lines from the USB and power socket by dispersing unwanted high-frequency energy.

This is an atypically complicated product, which is why I chose it. There’s a DSP on there, some fast RAM for audio, a microcontroller, USB, and quite a few wireforms that join up to other circuit boards using long runs of cabling. That is quite a lot of systems interchanging information quickly, and some signal paths are fairly long. Even with care, it is hard to execute such a system correctly because if something resonates, a signal somewhere will find it, couple into it, and use it as a transmitter.

The first time we took measurements, we found a couple of problems – we expected them. However, after two days and two revisions of circuit board, we could spin the device on a turntable, point a steerable antenna at it, and watch it sail under the pass line provided by EN55022 class B with a couple of decibels to spare.

Product X emissions test

We’d normally expect a far more comfortable pass than this, but a pass is a pass, and a tricky product is a tricky product. Whatever we design, electromagnetic performance has to be considered from the start. We need to know more than we used to about where our electrons are going, generally take a mature approach to designing products and reviewing best practice, or we risk spending a fortune on making things that we’re not allowed to sell.

ZX Electromagnetic Spectrum

Now, we get to the fun bit. This year is the 30th anniversary of one of my favourite inventions of all time, the Sinclair ZX Spectrum. A few weeks ago, I finally bought one: a non-working one on eBay that I nursed back to health. Fortunately there was very little wrong with it. Unfortunately it’s a 16K model, and a fairly early one at that, which won’t run much software in its native state. This probably accounts for its unusually pristine condition.

We took half an hour in the chamber to perform an approximate series of EN55022 measurements, to check its radiated emissions against today’s standard. The question is, what have we learned as an industry since 1982? Does a 30-year-old computer, that embodies Sinclair’s mastery of cost-engineering and elegant design like nothing else, pass modern legislation that would render it saleable?

We gave it a fighting chance. One of the things I did was to disable its TV modulator which, as well as stopping it from generating a UHF carrier signal, renders it compatible with modern televisions. Machines of this vintage are notorious for their flaky video circuitry, and it needed all the help it could get just to render a yellowish picture.

speccyThe ZX Spectrum, interpreting cyan with modest success.

I replaced all the electrolytic capacitors, trimmed the colour circuitry as best I could, and ensured via some old documents online that its modification state is up to date: not because this would have helped, but because it’s good practice for any piece of old electronics. The only other modification from stock was a 1 megaohm resistor across the 14MHz system crystal, just to allow the aged thing to work properly. I’m trying to help here.

speccyinside

The inside of my refurbished Issue 2 ZX Spectrum. The empty chip sockets would normally contain an extra 32K of RAM.

Remember when all technology was this exciting? Here it is with the radiation detection antenna pointed at it. Normally we would test such a device in a more representative setup: ideally, it would be attached to a cassette recorder and a TV, but we gave it a fighting chance and left most of the cabling in the box. I’ll talk about what happened next week.

speccy emc

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