An “Infinite Impedance Detector” could possibly seem a tiny like a thing that [Zaphod Beeblebrox] would use to zip all around the galaxy. It’s not, of study course, but it is an fascinating and valuable demodulator for AM radio indicators, as [Sebastian Westerhold] over at Baltic Labs describes in the quick but well-completed online video underneath.
If you’ve ever browsed by means of schematics of aged vacuum tube radios, [Sebastian]’s JFET-centered detector circuit may possibly search strangely common. Which is for the reason that this demodulator is about as near to a direct translation in between a vacuum tube circuit and a silicon circuit as feasible. In reality, [Sebastian] even made use of literature from the triode version of this detector to determine out the values for some of the factors. The only lively part is a BF256B JFET the rest are a modest handful of resistors and caps. Design is in the at any time-well-liked hideous type.
The exam setup is simple — a function generator established to 455 kHz and modulated with a 1,000 Hz sine wave. The detector demodulates the audio signal very cleanly, judging by the oscilloscope traces. Just for enjoyable, [Sebastian] also attempted a 10.7 MHz carrier with a 1,500 Hz audio modulation, and that labored great way too. He also tried using a variation on the circuit with an IF transformer on the input. That circuit functions just about the very same as the transformerless model, whilst it does supply a little obtain.
Earth-shattering things? Likely not. But it does present the exciting you can have with a scrap of PCB and a few factors, and looks like it could effortlessly be the variety of undertaking that would choose you down the RF rabbit gap. Thanks to [Sebastian] for sharing this just one with us.