Question E8D08:
What parameter evaluates distortion of an AFSK signal caused by excessive input audio levels?  

Choices:  
A: Signal-to-noise ratio  
B: Baud error rate  
C: Repeat Request Rate (RRR)  
D: Intermodulation Distortion (IMD)    

Correct Answer is  D: Intermodulation Distortion (IMD)  

Correct answer: D — Intermodulation Distortion (IMD)

Why D is correct:
- AFSK uses two audio tones fed into an SSB transmitter. If the audio is driven too hard, stages in the audio/IF/RF chain go nonlinear, creating intermodulation products. IMD is the standard parameter used to quantify this distortion (usually specified as IMD3 in dBc).

Key ideas and formulas:
- For two tones at $f_1$ and $f_2$, third-order IMD products appear at:
  $$2f_1 - f_2,\quad 2f_2 - f_1,\quad 2f_1 + f_2,\quad 2f_2 + f_1$$
  Example for packet AFSK (Bell 202): $f_1=1200$ Hz, $f_2=2200$ Hz gives IMD at $200$ Hz and $3200$ Hz (plus higher ones), which can broaden/splatter your transmitted spectrum.
- IMD measurement (two-tone test):
  $$\text{IMD3 (dBc)} = P_{\text{fund}}(\text{dBm}) - P_{\text{IM3}}(\text{dBm})$$
- Drive-level rule of thumb:
  - Fundamentals rise $\approx 1$ dB per 1 dB of drive.
  - Third-order IMD rises $\approx 3$ dB per 1 dB of drive.
  - Therefore, backing off drive by $1$ dB improves IMD3 by about $2$ dB.
- Operating tip: Reduce AFSK audio so ALC action is minimal and only two clean lines appear on a panadapter; aim for IMD3 ≤ −30 dBc (−35 to −40 dBc is better).

Why the other choices are incorrect:
- A: Signal-to-noise ratio — SNR quantifies noise relative to signal, not nonlinear distortion from overdrive.
- B: Baud error rate — Not a standard RF/transmitter linearity metric; symbol/bit errors reflect link quality, not specifically transmitter audio overdrive distortion.
- C: Repeat Request Rate (RRR) — A higher retransmission rate indicates link problems but is an indirect, protocol-level outcome, not a direct measure of AFSK distortion.

Takeaway:
- Excessive AFSK audio drive causes nonlinearity; evaluate it with IMD, especially third-order products (IMD3). Back off audio drive to improve IMD.