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Lab 2 — AM/FM → Spectrum → Bandwidth

Lab 2 extends the tone experiment into classical analog modulation. The goal is not only to generate AM/FM signals, but to compare model expectations with measured spectrum and bandwidth.

Goal

baseband signal → AM/FM model → RF transmission → spectrum capture → bandwidth decision

Experiment flow

flowchart TB
    MSG["1. Message signal<br/>tone, audio fragment or sweep used as the modulation input"]
    MODEL["2. AM/FM model<br/>MATLAB / Simulink reference waveform and expected spectrum"]
    TX["3. RF transmission<br/>Zynq + AD9363 carrier, gain and analog bandwidth settings"]
    RX["4. External observation<br/>RTL-SDR / HDSDR spectrum, waterfall and level check"]
    IQ["5. IQ recording<br/>captured samples with documented Fs, Fc and gain metadata"]
    FFT["6. Offline spectral analysis<br/>FFT, sidebands, bandwidth and leakage checks"]
    DEC["7. Engineering decision<br/>modulation index, deviation, gain and sample-rate tuning"]

    MSG --> MODEL --> TX --> RX --> IQ --> FFT --> DEC
    DEC -. tune modulation index .-> MODEL
    DEC -. tune RF gain .-> TX

What to compare

Item AM FM
Main observable carrier and sidebands occupied bandwidth
Main tuning parameter modulation depth frequency deviation
Typical problem overmodulation too large deviation
Metric sideband level estimated occupied bandwidth

Common failure modes

Symptom Likely reason Action
AM carrier dominates everything modulation depth too small increase modulation depth carefully
AM envelope distortion overmodulation or clipping reduce modulation depth or gain
FM spectrum too wide excessive frequency deviation reduce deviation
asymmetric spectrum IQ imbalance, DC offset, LO leakage check receiver and RF settings

Demo figure

AM vs FM spectrum

Minimum report

  1. Modulation type and parameters.
  2. TX/RX sample rates and RF frequency.
  3. Captured spectrum plot.
  4. Bandwidth estimate.
  5. Explanation of mismatch between model and measurement.

Engineering takeaway

Lab 2 teaches that modulation is not only a formula. In real SDR work, modulation parameters are engineering trade-offs between bandwidth, power, distortion and receiver limits.