Originally posted Aug 4, 2014
A rich, entertaining, and enlightening history
As RF engineers we spend most of our time working on the technologies of the present and the future. It can be a constant footrace, so sometimes it’s refreshing to take a look back and see how far we and our predecessors have come. With a history dating back to the very beginnings of RF, detectors or demodulators are an excellent historical example of innovations and progress.
In my post on signal analyzer detectors I mentioned cat’s whiskers and coherers. Though I had never used one, I was familiar with the cat’s whisker detector as a central element in a crystal radio set. The cat’s whisker is a simple rectifier, made by touching a thin metal wire (the “whisker”) to a semiconductor, typically a raw chunk of natural galena.
A tuned circuit is used in the crystal receiver to select a radio station, and the rectifying function of the cat’s whisker serves to extract the audio signal from the RF carrier. In other words, the cat’s whisker is a detector, acting as a demodulator. It’s essentially the same as the IF or video detector found in a signal analyzer, as shown at left in this block diagram.
This partial, simplified block diagram of a spectrum analyzer shows the two detectors and their locations in the signal path. The first detector demodulates the IF signal, converting it into a magnitude value in the same way a cat’s whisker would perform demodulation in a crystal radio.
Note that in this diagram the envelope detector is symbolized by a diode. Detection is accomplished with the assistance of other components such as a capacitor and resistor, but the critical and most challenging component is the rectifier.
From the standpoint of technical progress it’s interesting to note that a solid-state component—the cat’s whisker—came before vacuum tube solutions. The tube solutions were then superseded by newer solid-state diodes.
History has another curve to throw at us as we move further back in time: the coherer. When I first heard the term I had been already been working on HF and RF applications for more than a dozen years and was surprised I hadn’t encountered it before. The coherer is another—even older—solid-state device that performed a kind of RF detection.
Though complicated in theory, the coherer is simple in practice: it’s just a pair of electrodes in an insulating capsule, separated by metal filings. RF energy causes the metal particles to cohere and the resistance between the electrodes drops dramatically. The coherer detects RF energy in a way that’s useful for electrical circuits such as an RF telegraph.
A coherer, an early RF detector. Two metal electrodes are separated by metal particles and the resistance between them drops in the presence of RF energy. (Image from Wikimedia Commons)
The earliest radios were RF telegraphs, using spark-gap transmitters in an on/off configuration. They were entirely broadband, in a way that is shocking to modern RF sensibilities. When I screened video of a large spark gap transmitter to groups of RF application engineers in the mid-1990s it always produced an audible gasp as they intuitively grasped the nature of the emissions.
If you’re interested in the topic, this bit of RF history gives me a chance to recommend one of my favorite TV series on technology, the low-budget, refreshingly British, amazingly enlightening Secret Life of Machines by Tim Hunkin. These programs from the late 1980s do an incredible job of explaining commonplace technology of the home and office such as refrigerators, vacuum cleaners, TVs, VCRs, word processors, fax machines and radios. I think the episodes on radios and fax machines are some of the very best, and are worth watching for their quirky perspective and humor, in addition to their brilliant explanations.
This all reminds me of my adventures many years ago with spark gaps driving large Tesla coils. Perhaps that’s a topic for a future post—but perhaps some RF interference sins should go unconfessed!