RF DesignLine Blog | Bluetooth, Wi-Fi, Cellular, Microwave, Options/Analysis

January 30, 2010
Are "RF" and "wireless" defined by their frequency or their media?
By Bill Schweber

Like most professions, we use the same words for different things, and different words for the same things. Think, for moment, of the many meanings of the word "buffer," or the many words and phrases we use for the analog/digital converter function.

Generally, this is not is not a problem, since the context of the discussion and expertise of the people we are talking to eliminates, or at least minimizes, ambiguity and misunderstanding. (I did, however, attend one conference where there was a heated argument about some aspect of "IP"–it turned out that one of the arguers meant intellectual property, while the other was referring to "Internet protocol"! And I'm sure they weren't talking about "intermodulation product.").

But what do we mean when we say RF? Do we mean radio frequency? OK, but what's a radio frequency? In the early days of radio–and I am talking Marconi-era here– radio operated down in the tens of kilohertz at its highest, which is almost DC from our present vantage point. But it was radio, in every sense of the word.

And when we have a signal on a wire or cable in the hundreds of MHz or above, we now routinely use the term "RF" to describe its attributes, even though the signal is confined to a physical conductor, rather than air or vacuum medium. Yes, it's RF, but is it radio?

What about infrared (IR)? Those ubiquitous remote controls are definitely wireless, but they operate well beyond the conventional RF bands, and they observe the same Maxwell's equations which conventional radio links must follow. Yet, when engineers speak of RF or wireless, they usually don't mean IR–but I suppose they could.

The lesson here is not so much to "watch your language" and try to be more precise. That's not going to happen, nor is it necessary in most cases. But notice that I said "in most cases": there will be times you'll want to make your definitions clear up front, to avoid misunderstandings and even embarrassment among participants! ♦
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January 03, 2010
Why I am sometimes not so enamored of short RF links
By Bill Schweber

Earlier in 2009, I wrote about how the wireless bike speedometer I got to replace an older wired one was a case where an RF link can beneficially take the placed of even a short wire link, see "Wireless or not? When are short-distance, point-to-point RF links worthwhile?". At the time, I was thrilled.

Well, times change and thrills fade. It turns out that the wireless set-up is just not as reliable as a wired one was. Whether it is this particular unit, or this model, or wireless speedometers as a class, I can't say. The handlebar-mounted readout shows the right speed much of the time, but often drops down to about half or a third of the actual speed and then recovers, even when I am going at a fairly constant rate.

I know the sensor pickup is OK, since I can hear the clicks of the internal reed switch as the spoke magnet goes by. But after that, I don't know where the unreliability is. It could be that I am at the border of the claimed 70-cm wireless range, or that the frame geometry is somehow in the way, or maybe the fork-mounted sender and/or the handlebar-mounted receiver/display don't like colder weather. Whatever the cause, it's frustrating. (And yes, I have put fresh batteries in both units.)

But the underlying issue and subsequent lesson goes beyond this non-critical application. To save power, reduce size, and keep costs down, the wireless bike speedometer is a simplex (unidirectional) design. There is no reverse channel and acknowledgement from the handlebar unit to the sender. And that's the weak link, so to speak. There is no way for the fork-mounted unit to know that its clicks were received (it's a fairly slow repetition rate, 0.2 clicks/sec per mile/hour speed).

Unlike a wireless mouse or TV remote control, for example, where the user can see a problem, and thus (in effect) implement the ACK/NACK protocol, and repeat the action if the signal was not received (frustrating, for sure, but at least doable), I don't have that luxury on the bike. Either the switch pulses are received and totaled, or they are not.

While low-cost, unidirectional wireless links may seem attractive—and they are in many cases, since they eliminate connectors and cables—be sure to take time to understand what the impact of a lost message will be. Regardless of the cause, whether it is distance interference, weak batteries, or other, the consequences can span minor to major. And if they are more than minor, you may want to embed a most costly, complex half- or full-duplex link, with associated protocols. Or, you may decide a wired link, with its greater reliability and immunity to outside factors, is a better choice after all. ♦
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December 02, 2009
Is contactless payment approaching an inflection point?
By Bill Schweber

I recently saw a summary of a market-research report predicting a somewhat optimistic future for contactless payment, a form of RFID where the credit card has an embedded chip which communicates with the reader over a short distance. (I commented about this several months ago, and you can also see two recent IC press releases for this market, in the links below.)

The press release for "The World Market for EFT-POS Terminals and Contactless Readers" from IMS Research (Wellingborough, UK) indicates that despite the retail and general economic slowdown, more banks and other credit card issuers are shipping contactless cards, and reader manufacturers are also seeing modest growth in demand. (In case you did not know: EFT is electronic funds transfer, while POS means point of sale.) The press release also includes the customary crystal-ball graph showing the annual market in dollars, not units, for both contactless and conventional readers out to 2013; frankly, I have absolutely no faith in such long-term projections, so I will ignore them completely.

Where do you think these contactless cards are going? I still have my doubts, for three main reasons.:

First, the card itself is more expensive than a conventional magnetic stripe card, and that adds up over the millions of cards shipped.
Second, more establishments now have the card owner personally "swipe" the card through the reader, so the card never leaves your possession, which is a nice feature (I do bristle at how the word "swipe", which used to mean "to steal", now means pulling a card though a reader!).
And finally, I just don't see the benefits of contactless card reading versus a regular card with its magnetic strip.

I know that our industry is always searching for "the next big thing", a market or niche that will bring growth of 10-20% per year, and maybe this is it. Then again, "contactless" may be sexier, and it may be a handy when you are using it as in ID card to access an area, but for payment at the store's checkout, I just don't get it, sorry. ♦
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November 21, 2009
Higher antenna vs. transmission-line loss: an RF (and engineering) tradeoff example
By Bill Schweber

Engineering is about tradeoffs, some obvious, some vague, some quantifiable, some qualitative, some based on judgment and sense of the situation. Here's one I just did some thinking about, though I haven't decided what to do–since the problem is not at all critical.

The over-the-air reception for my digital converter box ahead of my analog TV is decent, not great. (Yes, some of us don't have cable or dish or even a digital TV.) Although I am nearly to line-of-sight to various TV towers about 20 miles away (about 30 km) with just some trees in the way, I get image breakup and freeze on some of the lesser channels, especially when there is rain or snow.

Since my antenna is indoors in the attic (much easier to deal with than outside, for sure) I have been thinking about moving it to another part of the attic, or maybe moving it outside and up, to see if there is improvement. Right now, I have a run of about 100 feet (30 meters) of coax from the antenna to the converter connector, and I'd have to add another 50 feet (15 meters) if I moved it around inside, or perhaps another 100 feet if put it outside.

So the question becomes: is the improved signal strength I may get worth the additional loss in the transmission line? This is a simple, yet realistic, example of the kinds of tradeoffs engineers must make in their system and circuit design over and over.

I did some quick research and found that the typical loss for various coax cable types is roughly between 0.1 and 1 dB per 100 feet (about 0.15 and 1.5 dB per 50 meters) at 1 MHz, and goes up to between 5 and 15 dB at 1 GHz (about 7.5 to 22 dB per 50 meters); it varies depending on the type and quality of cable. I'll assume I am at the higher-loss side, it's actually hard to find better-quality cable locally (I have looked for it) and I'm not ready to buy it online, yet. The TV signal is in the 1-GHz range, of course, so we are at the higher end of the losses.

I haven't decided what to do, and it is very likely I will do nothing, since the solution may be more of a nuisance and cost in money and headache than the problem is worth to solve. And no, I won't looking to add a preamplifier at the antenna, since that involves more cost, plus a power supply, and assumes the problem is signal strength itself rather than SNR (a likely but not assured assumption).

My TV reception is a very modest problem, of course. But it reminds us that in engineering, as in life, it's often about tradeoffs in performance, effort, and cost. That's message engineers should both keep in mind, and make clear to others. ♦

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