Question E4A03:
Which of the following test instruments is used to display spurious signals and/or intermodulation distortion products generated by an SSB transmitter?  

Choices:  
A: Differential resolver  
B: Spectrum analyzer  
C: Logic analyzer  
D: Network analyzer    

Correct Answer is  B: Spectrum analyzer  

Correct answer: Spectrum analyzer

1) Why it’s correct
- A spectrum analyzer displays signal amplitude vs. frequency, making it the right tool to see spurious emissions (harmonics, birdies) and intermodulation distortion (IMD) products from an SSB transmitter.
- With a two-tone test, IMD products show up as additional peaks near the test tones; the spectrum analyzer lets you measure their levels in dBc.

2) Why the others are incorrect
- Differential resolver: An electromechanical angle/position sensor, not an RF measurement instrument.
- Logic analyzer: For digital logic timing/levels in the time domain; it does not analyze RF spectra.
- Network analyzer: Measures linear network parameters (S-parameters, impedance, return loss) vs. frequency with a low-level stimulus. It is not used to view a transmitter’s spurious outputs or IMD products.

3) Useful formulas and rules of thumb
- Two-tone IMD product frequencies:
  - 3rd order: $2f_1 - f_2$ and $2f_2 - f_1$ (these lie close to the wanted tones and are most critical)
  - 5th order: $3f_1 - 2f_2$ and $3f_2 - 2f_1$
- Harmonics: $2f$, $3f$, $4f$, … (spurious emissions)
- Relative level in dBc:
  $$\text{dBc} = 10 \log_{10}\!\left(\frac{P_{\text{spur}}}{P_{\text{carrier or tone}}}\right)$$
- Rule of thumb: Keep 3rd-order IMD products at least about $-30$ dBc or better for clean SSB; verify with a two-tone test on a spectrum analyzer.