An “Infinite Impedance Detector” would possibly sound a bit like one thing that [Zaphod Beeblebrox] would use to zip across the galaxy. It’s not, in fact, however it’s an fascinating and helpful demodulator for AM radio alerts, as [Sebastian Westerhold] over at Baltic Labs explains within the temporary however well-done video under.

When you’ve ever browsed by schematics of previous vacuum tube radios, [Sebastian]’s JFET-based detector circuit would possibly look unusually acquainted. That’s as a result of this demodulator is about as near a direct translation between a vacuum tube circuit and a silicon circuit as doable. In reality, [Sebastian] even used literature from the triode model of this detector to determine the values for a few of the elements. The one energetic element is a BF256B JFET; the remaining are a small handful of resistors and caps. Development is within the ever-popular ugly fashion.

The check setup is straightforward — a perform generator set to 455 kHz and modulated with a 1,000 Hz sine wave. The detector demodulates the audio sign very cleanly, judging by the oscilloscope traces. Only for enjoyable, [Sebastian] additionally tried a ten.7 MHz provider with a 1,500 Hz audio modulation, and that labored nice too. He additionally tried a variation on the circuit with an IF transformer on the enter. That circuit works nearly the identical because the transformerless model, though it does present a bit acquire.

Earth-shattering stuff? In all probability not. But it surely does present the enjoyable you may have with a scrap of PCB and some elements, and looks as if it may simply be the type of mission that may take you down the RF rabbit gap. Due to [Sebastian] for sharing this one with us.