Question E7E11:
Which type of detector is used for demodulating SSB signals?  

Choices:  
A: Discriminator  
B: Phase detector  
C: Product detector  
D: Phase comparator    

Correct Answer is  C: Product detector  

To demodulate SSB, you need to reinsert a carrier and multiply (mix) the signal with it, then low-pass filter to recover audio. That is exactly what a product detector does.

Why C (Product detector) is correct:
- SSB has a suppressed carrier, so an envelope detector won’t work.  
- A product detector multiplies the incoming SSB signal by a Beat Frequency Oscillator (BFO) at (approximately) the carrier frequency and then low-pass filters to recover the baseband (audio).
- Model: if the received SSB slice is approximated as $v_{in}(t) \approx m(t)\cos(\omega_c t)$, and the BFO is $v_{LO}(t) = 2\cos(\omega_c t)$, then
  $$v_{out}(t) = v_{in}(t)\cdot v_{LO}(t) = m(t)\cos(\omega_c t)\cdot 2\cos(\omega_c t) = m(t) + m(t)\cos(2\omega_c t)$$
  After low-pass filtering, the $m(t)$ term (audio) remains.

Why the others are incorrect:
- A: Discriminator  
  - Used for FM detection (converts frequency deviations to voltage). Not appropriate for SSB, which needs coherent (synchronous) detection with a BFO.
- B: Phase detector  
  - Generic element measuring phase difference (often inside a PLL). Used for PM/FM demod or synchronization, not the standard SSB audio recovery method.
- D: Phase comparator  
  - Similar to a phase detector within PLLs; again for phase relationships (FM/PM), not SSB baseband recovery.

Useful formulas and rules of thumb:
- Product (synchronous) detection principle:
  $$\text{Multiply by BFO} \Rightarrow \text{LPF} \Rightarrow \text{Audio}$$
- Rule of thumb:
  - AM (with carrier): envelope detector (or synchronous detector for distortion-free).
  - SSB/CW: product detector + BFO.
  - FM/PM: discriminator, quadrature detector, or PLL (phase/frequency detector).