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curriculum/Analog Communications
section 9 of 133 min read

9. Real-World Walkthroughs

9.1 An AM radio recovering the morning news

  1. Antenna. A loop of wire (or a ferrite rod with windings) picks up the entire AM band as an aggregate voltage of microvolts.
  2. RF preselector. A tunable LC tank, coupled to the ferrite rod, peaks at the desired station frequency. Out-of-band garbage is attenuated.
  3. Mixer. A diode or BJT acting as a switch driven by the LO multiplies the RF by cos⁡(2πfLOt)\cos(2\pi f_\text{LO} t). Sum and difference frequencies appear; the IF filter selects the difference.
  4. IF strip. Three or four cascaded transistor amplifiers tuned at 455 kHz, with bandpass coupling between them, provide 80 to 100 dB of gain. A diode-driven AGC line trims the gain so the signal stays in the linear region of the detector.
  5. Detector. A germanium diode (Ge for the lower forward drop, ~0.2 V instead of Si's 0.7 V; Ge envelope detectors hear weaker signals). RC time constant ~10 µs. Output: audio plus DC.
  6. AC coupling and audio amp. Block the DC, amplify the audio, drive the speaker.

A 1948 vacuum-tube radio and a 2024 transistor radio implement the same architecture. Total transistor (or tube) count: 5 to 8.

9.2 An FM stereo broadcast

  1. At the studio. Microphones capture left (L) and right (R) audio.
  2. Stereo encoder. Compute L+R (the mono baseband, audible to mono receivers) and L−R. Modulate L−R as DSB-SC on a 38 kHz subcarrier. Add a 19 kHz pilot tone (a phase-locked half of the subcarrier frequency) so that stereo receivers can lock to it. The composite signal occupies 0 to 53 kHz: L+R from 0 to 15 kHz, pilot at 19 kHz, L−R DSB-SC subcarrier from 23 to 53 kHz. RDS data on a 57 kHz subcarrier (= 3× pilot) extends the composite to ~58 kHz.
  3. FM modulator. Pre-emphasize the entire composite (75 µs in the US), then FM-modulate it onto the 88 to 108 MHz carrier with Δf=75\Delta f = 75 kHz peak deviation. By Carson's rule, the RF bandwidth is about 200 kHz, fitting the FCC channel.
  4. At the receiver. RF amp, mixer, IF at 10.7 MHz (band-defined), limiter, FM discriminator (PLL or older Foster-Seeley/ratio detector). Output: composite baseband.
  5. Stereo decoder. Phase-lock to the 19 kHz pilot to derive a clean 38 kHz local oscillator. Coherently demodulate the L−R subcarrier (multiply by 38 kHz local, low-pass) to recover L−R. Also bandpass-filter the 0 to 15 kHz band to recover L+R.
  6. Matrix. L=((L+R)+(L−R))/2L = ((L+R) + (L-R))/2, R=((L+R)−(L−R))/2R = ((L+R) - (L-R))/2. De-emphasize. Send to speakers.
  7. Mono fallback. A mono receiver simply ignores everything above 15 kHz (its audio bandwidth limit), playing only L+R. Fully backward-compatible.

This whole architecture was finalized in 1961 and remains the standard worldwide.

9.3 An aviation voice exchange

  1. Pilot keys microphone at 121.5 MHz (the international air emergency frequency).
  2. Aircraft transmitter modulates the AM carrier with the pilot's voice. Output power 5 to 25 W, AM, ~6 kHz channel.
  3. Multiple aircraft within radio horizon hear the transmission simultaneously, including the airline operations center via a remote receiver.
  4. If two pilots key simultaneously, both transmissions reach all receivers. AM mixes them; controllers and pilots hear "stepped-on" or "blocked" audio and request that one party repeat. FM would have silently dropped the weaker one, hiding the conflict. Aviation's choice of AM is deliberate.