AM & the Envelope Detector
How music rides on a megahertz wave — and how a diode and an RC, your oldest friends, get it back off.
Builds on: 9.2 Tuning: Picking a Station3.1 Diodes & LEDs2.3 Capacitors & the RC Time Constant5.2 Rectifiers: AC → DC
Putting sound on a wave
Audio itself (20 Hz–20 kHz) won’t radiate from any reasonable antenna — the waves are kilometres long. So we let the audio ride on a high-frequency carrier that radiates beautifully. The oldest scheme is amplitude modulation: wiggle the carrier’s strength in step with the sound.
Squint at an AM waveform and you see it: a fast carrier filling a slow-moving outline. The outline is the audio. Recover the outline and you have sound.
The envelope detector: three old parts
Here is the loveliest circuit in this course, because you already know every piece:
- A diode (3.1) throws away the bottom half of the wave — a half-wave rectifier (5.2), just at radio frequency.
- A capacitor charges to each carrier peak…
- …and a resistor lets it sag just fast enough to follow the outline down. Charging fast, discharging slow — your RC smoothing (5.2) with a musical purpose.
The RC choice is a Goldilocks problem: too small and carrier ripple leaks through (buzz); too large and the output can’t follow the audio downhill (a distortion called diagonal clipping). The lab lets you find the sweet spot: 1/fc ≪ RC ≪ 1/fa.
The crystal radio
Chain the last three lessons — antenna, tuned LC tank, diode + RC detector, and a sensitive earphone — and you have a crystal radio: a complete receiver with no battery, powered entirely by the energy the station transmits. Generations of engineers began exactly there. (Fewer AM stations remain on air in some regions these days — which is why your hands-on capstone ahead is a different build — but the circuit remains the finest teaching machine electronics ever produced.)
FM hides the audio in the carrier’s frequency instead of its strength (better noise immunity); digital radio hides bits in phase and amplitude at once. All of them are still: a carrier, a modulation, a detector.
⚡ Lab — Riding the Carrier
An AM signal and a diode + RC detector chasing its envelope.
- Set modulation to 100% and watch the envelope pinch to zero between beats.
- Make RC tiny: the detector output buzzes with carrier ripple.
- Make RC huge: it sails over the audio valleys — diagonal clipping. Find Goldilocks.