Goal
Learn to control level and phase and take into account how fast the feedback reacts.
Prerequisites: Parts 06–07, 11; digital communication examples from Part 12.
Future lessons
- AGC — automatic gain control: level detector, target level, attack, release and hang times.
- PLL — phase-locked loop: phase detector → loop filter → oscillator; acquisition, bandwidth and stability.
- The Costas loop for carrier recovery; a return to Part 12.
- ALC — automatic level control of the transmitter: overshoots during transients and the effect on modulation.
Lab plan
- Apply a level step and an impulse interference to the AGC; plot the input and output envelopes and the gain.
- Apply frequency and phase steps to the PLL; compare the acquisition speed and the response to noise.
- Compare ALC with hard clipping by envelope and by spectrum.
We do the experiments in Python with NumPy, SciPy and Matplotlib. We keep the parameters, units and plots. The base models use float64 and complex128; we add bit-depth limits in separate experiments.
Completion criteria
- I can explain the trade-off between a fast response and distortion of the wanted modulation.
- I understand why a digital AGC does not correct an overload that already occurred in the ADC.
Result for the main project
AGC, PLL and ALC with a saved state and plots of the transients.