Question E4B10:
Which of the following methods measures intermodulation distortion in an SSB transmitter?  

Choices:  
A: Modulate the transmitter using two RF signals having harmonically related frequencies and observe the RF output with a spectrum analyzer  
B: Modulate the transmitter using two AF signals having non-harmonically related frequencies and observe the RF output with a spectrum analyzer  
C: Modulate the transmitter using two AF signals having harmonically related frequencies and observe the RF output with a peak reading wattmeter  
D: Modulate the transmitter using two RF signals having harmonically related frequencies and observe the RF output with a logic analyzer    

Correct Answer is  B: Modulate the transmitter using two AF signals having non-harmonically related frequencies and observe the RF output with a spectrum analyzer  

Correct answer: B — Modulate the transmitter using two AF signals having non-harmonically related frequencies and observe the RF output with a spectrum analyzer.

Why this is correct
- SSB transmitters are modulated by audio (AF). A standard two-tone test uses two clean, equal-level, non-harmonically related audio tones (e.g., 700 Hz and 1900 Hz).
- Nonlinearity in the transmitter creates intermodulation products. With two tones at $f_1$ and $f_2$, the dominant third-order products appear at:
  - $2f_1 - f_2$ and $2f_2 - f_1$ (translated to RF around the suppressed carrier), so at $f_0 + (2f_1 - f_2)$ and $f_0 + (2f_2 - f_1)$.
- A spectrum analyzer at the RF output shows two main lines (the desired signals) and the IMD products. The difference in level (in dB) between a desired line and a 3rd-order spur is the IMD figure:
  - $$IMD_3\ (\text{dBc}) = P_{\text{tone}}(\text{dBm}) - P_{\text{IM3}}(\text{dBm})$$
- This is the accepted and practical method for SSB linearity testing.

Why the other choices are incorrect
- A: “Two RF signals” and “harmonically related” are both wrong for SSB IMD testing.
  - SSB IMD is evaluated by audio modulation, not by injecting two RF carriers into the PA.
  - Harmonically related tones cause some IMD products to overlap desired signals, obscuring measurement.
- C: AF tones are correct, but “harmonically related” and using a peak-reading wattmeter are not.
  - Harmonically related AF tones let IMD products land on top of fundamentals, making results ambiguous.
  - A wattmeter cannot resolve individual spectral components; a spectrum analyzer is needed.
- D: Two RF signals and a logic analyzer are inappropriate.
  - A logic analyzer is for digital logic timing, not analog RF spectra.

Useful formulas and rules of thumb
- 3rd-order IMD product frequencies from two tones $f_1, f_2$:
  - $$f_{\text{IM3, low}} = 2f_1 - f_2,\quad f_{\text{IM3, high}} = 2f_2 - f_1$$
  - In an SSB rig, these appear at RF offset from the suppressed carrier: $f_0 + f_{\text{IM3}}$.
- IMD measurement (relative):
  - $$IMD_3\ (\text{dBc}) = P_{\text{tone}} - P_{\text{IM3}}$$
  - Good transmitters often show $-30$ dBc to $-35$ dBc (or better).
- Two-tone PEP relationship (equal tones, linear operation):
  - $$PEP = 4\,P_{\text{one tone}} = 2\,P_{\text{avg,total}}$$
  - So PEP is 3 dB higher than the total average power of the two tones.
- Practical setup tips:
  - Use two clean, non-harmonically related AF tones (e.g., 700 Hz and 1900 Hz), equal amplitude.
  - Keep the ALC from engaging; drive only to the linear region.
  - Use sufficient attenuation into the spectrum analyzer to avoid overloading.