AM demod looks simple on the surface, but under the hood it’s doing a couple of neat little tricks.

If the built-in AM demod block vanished overnight, you’d rebuild it from first principles like this:

🧠 1. Start With the RF Signal (Complex Baseband)

From your SDR you’ve got complex IQ samples centered on your AM carrier.

So conceptually:

s(t) = [1 + m(t)] * cos(2πf_ct)

After the SDR tunes and mixes to baseband, that carrier is sitting at 0 Hz.

🔧 2. Envelope Detection (The Core Trick)

Classic AM demod is just envelope detection.

In GNU Radio terms:

✔ Take the magnitude

Use:

• Complex to Mag block

This computes:

|I + jQ| = sqrt(I² + Q²)

That gives you the envelope.

At this point, you’ve essentially done what a diode detector does in an old radio.

🎚 3. Remove the DC Component

AM signals include a carrier offset (that “+1” in the equation).
So your magnitude output will have a big DC level.

You remove it using:

• DC Blocker or
• High-pass filter (very low cutoff, e.g. 20–50 Hz)

Now you’re left with just the modulation (audio).

🎛 4. Low-Pass Filter (Audio Bandwidth)

AM broadcast audio bandwidth is ~5 kHz (sometimes 10 kHz).

So you’d add:

• Low Pass Filter
  • Cutoff: ~5 kHz (or whatever your signal uses)
  • Transition width: reasonable
  • Decimate if needed

This cleans up high-frequency junk from the magnitude operation.

🔊 5. Audio Gain + Resample

Then:

• Multiply Const (adjust volume)
• Rational Resampler if needed to get to 48 kHz or 44.1 kHz
• Send to Audio Sink

So the Full Replacement Chain Would Be:

SDR Source
   ↓
Frequency translating filter (if needed)
   ↓
Complex to Mag
   ↓
DC Blocker
   ↓
Low Pass Filter
   ↓
Gain
   ↓
Audio Sink

Boom. Homemade AM demod.

🔬 If You Want To Be Fancy (Synchronous Detection)

Envelope detection works, but it:

• distorts under fading
• struggles with weak signals
• doesn’t like overmodulation

A higher-quality approach:

Multiply by a local carrier (product detector)

1. Multiply signal by cos(2πf_ct)
2. Low-pass filter result

In baseband SDR world this becomes:

• Ensure carrier is centered
• Use Complex to Real (or multiply by 1∠0)
• Low-pass filter

Even better:

• Add a PLL to lock onto carrier (for DSB or SSB cases)

That’s basically what “AM Sync” receivers do.

🛰 For Your 1420 MHz Work

If you’re using AM in some diagnostic context (be honest… are you sneaking AM into an interferometer chain? 👀), envelope detection is fine.

But for serious signal integrity:

• Coherent detection is cleaner
• Especially if carrier suppression is present

TL;DR

To replace GNU Radio’s AM Demod block:

Magnitude → Remove DC → Low Pass → Gain

That’s it.