Modern radio receivers have a distinct advantage over the common early designs which I covered in my previous article. Most of the receivers you will have worked with over the past couple decades are designs by Edwin Armstrong; regenerative, superregenerative, or most commonly superheterodyne. These are distinguished by a few fascinating key traits that bring both benefits and drawbacks.
Today let’s dive into Mr. Armstrong’s receivers. I’ll also talk about DC receivers which, despite the name, are not made to listen to batteries. These are receivers you are much more likely to encounter in modern equipment.
Regenerative and Superregenerative
The regenerative receiver is all about doing more with less. You still see some of these in simple applications like RF remote controls. The idea derives from how an oscillator works. In a simple way of thinking, an oscillator is an amplifier with enough positive feedback that any tiny signal at the right frequency will amplify and then, through feedback, continue to output over and over. If everything were perfect, then, an oscillator would have infinite gain at a given frequency.
Of course, things aren’t perfect, but they are close enough. You have to set the feedback network up just right to get the frequency you want. Also, things in nature tend to be linear, so it isn’t like the amplifier has no gain at the given frequency and then suddenly has infinite gain. The gain increases until it meets the Barkhausen criteria and achieves stable oscillation.
In fact, sometimes we want to build an amplifier and find that it oscilla ..
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