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I noticed a tank could make a peaky little bandpass filter, and wanted to use it. Remembering that (some) infrared remote controls use a 40kHz AM encoding, I decided to try that.
As with all my circuits, I haven't built this in hardware. I'm saying that here because my description below makes me sound like I know what I'm talking about. :)
The left side including the FETs is a test rig. It encodes a digital signal in AM, which would be done in the remote control, and amplifies it to feed the filter which would be part of the IR receiver in the TV.
Next is the tank filter. The frequency is controlled by the capacitor and the inductor together. Its peakiness and ringing are both controlled by the input impedance, here primarily the 5Ω resistor. The FETs have been adjusted to minimize their resistance. Controlling its ringing takes work; here it draws up to 400mA, so I guess you'd want something else entirely for mobile use. :)
Finally, the signal is buffered and amplified a little, rectified, weakly smoothed, and fed to a Schmitt trigger.
Rejection is very good! The signal passes with a 40kHz carrier, but is rejected with 42kHz or 37.7kHz. If that's too good, reducing the 5Ω resistor should do the trick.
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