Question E8D07:
What is a common cause of overmodulation of AFSK signals?  

Choices:  
A: Excessive numbers of retries  
B: Excessive frequency deviation  
C: Bit errors in the modem  
D: Excessive transmit audio levels    

Correct Answer is  D: Excessive transmit audio levels  

To understand AFSK overmodulation, remember that AFSK is just two audio tones driving a transmitter. If those tones are too loud, the transmitter’s audio chain or modulator is overdriven, causing distortion and excessive occupied bandwidth.

Why D (Excessive transmit audio levels) is correct:
- Overmodulation of AFSK is most commonly caused by audio drive that is too high into the transmitter.
- In FM, the peak frequency deviation is proportional to the modulating audio amplitude:
  $$\Delta f = k_f \, V_m$$
  So excessive audio level ($V_m$) produces excessive deviation, clip/limiter action, distortion, and splatter.
- In SSB/AM, too much audio drives stages into nonlinearity, creating distortion products.

Why the other choices are incorrect:
- A: Excessive numbers of retries
  - Retries occur at the data/protocol layer due to poor link quality; they don’t cause overmodulation. They might be a symptom, not a cause.
- B: Excessive frequency deviation
  - That is the result of too much audio drive, not the root cause. The cause is the high transmit audio level.
- C: Bit errors in the modem
  - Bit errors are a consequence of distortion, noise, or interference, not a cause of overmodulation.

Useful formulas and rules of thumb:
- FM deviation vs audio level:
  $$\Delta f = k_f \, V_m$$
  Control $V_m$ (audio level) to keep $\Delta f$ within the intended standard.
- Carson’s Rule for FM occupied bandwidth:
  $$B \approx 2(\Delta f + f_m)$$
  If $\Delta f$ is too high, your signal gets too wide.
- Practical tips:
  - Reduce the transmit audio (modem output or radio mic gain) until limiting/clipping indicators disappear.
  - Disable speech processing/compression for AFSK.
  - Aim for the recommended deviation (e.g., for 1200‑baud AFSK packet, adjust so tones are clean and do not hit the limiter; use a service monitor if available).