RF Field Detector

RF Field Detector — your ADC is secretly a radio

An ADC can't sample 2.4 GHz. Nyquist forbids it: to digitize a wave you must sample at least twice its frequency, and no microcontroller pin runs at 5 billion samples a second. So how does a one-dollar circuit let an ordinary ADC pin feel the Wi-Fi in the room?

It doesn't sample the RF. It measures its envelope — how much RF energy is present — and hands that to the ADC as a slow DC level. This is the oldest trick in radio: the crystal-radio envelope detector.

How it works

1. The antenna is just a wire. Hold it near any transmitter — a phone, a router, a walkie-talkie — and a few millivolts of RF couple onto it. It isn't tuned; it picks up whatever's loudest nearby.
2. The Schottky diode rectifies that RF. A Schottky's low forward drop and fast recovery let it follow gigahertz signals where an ordinary silicon diode simply can't keep up — it chops the wave so only the positive half gets through.
3. The RC (C1 + R1) smooths those rectified pulses into a steady DC level. The capacitor holds each peak; the resistor bleeds it so the level falls when the field weakens. What comes out tracks the strength of the RF, not its content.
4. The ADC pin reads that DC level. Phone transmits → the number jumps. Walk away → it falls. You built a field-strength meter out of four passives.

The honest part

This measures how much RF is here, not what's being said. There's no tuning, no station selection, no demodulation of data — just power. That's the honest scope of an envelope detector: a cheap, wideband "is something transmitting near me?" sensor, the same core that sat inside foxhole radios and every AM crystal set. To turn field strength into a clean reading, bias the diode just below conduction with a high-value resistor so even tiny signals push it into rectifying, and pick a true detector Schottky (a zero-bias part like an HSMS-285x) when you want to feel weak fields.

The lesson generalizes: an ADC that can't possibly sample a signal can still measure a property of it, if you do the hard part in hardware first. The diode is doing the down-conversion the silicon never could.