Question E7E04:
What is one way to produce a single-sideband phone signal?  

Choices:  
A: Use a balanced modulator followed by a filter  
B: Use a reactance modulator followed by a mixer  
C: Use a loop modulator followed by a mixer  
D: Use a product detector with a DSB signal    

Correct Answer is  A: Use a balanced modulator followed by a filter  

To produce SSB with the filter method:
- A balanced modulator multiplies the audio (modulating) signal by the RF carrier, suppressing the carrier and creating a double-sideband suppressed-carrier (DSB-SC) signal.
- A sharp band-pass filter (crystal/mechanical) then removes one sideband, leaving only the desired SSB signal.

For a single audio tone $m(t)=M\cos(2\pi f_m t)$ and carrier $\cos(2\pi f_c t)$, the balanced modulator output is:
$$
v_o = m(t)\cos(2\pi f_c t) = \frac{M}{2}\big[\cos 2\pi(f_c+f_m)t + \cos 2\pi(f_c-f_m)t\big]
$$
This has only the upper and lower sidebands (no carrier). The filter passes either $f_c+f_m$ (USB) or $f_c-f_m$ (LSB).

Why A is correct (balanced modulator followed by a filter):
- Balanced modulator → creates DSB-SC.
- SSB filter (≈2.1–2.8 kHz wide at the IF) → removes the unwanted sideband and residual carrier.
- This is the classic “filter method” of SSB generation.

Why the other choices are incorrect:
- B: Reactance modulator followed by a mixer
  - A reactance modulator varies an oscillator’s reactance to produce phase or frequency modulation (PM/FM), not SSB. A mixer only shifts frequency; it does not suppress a sideband.
- C: Loop modulator followed by a mixer
  - A loop (feedback) modulator is used for AM or carrier control schemes. A single mixer cannot create SSB from AM/DSB; you still need sideband suppression via a filter or phasing.
- D: Product detector with a DSB signal
  - A product detector is a receiver demodulator (multiplies RF by a BFO to recover audio). It does not generate an SSB RF signal.

Useful rules of thumb:
- Two standard ways to make SSB:
  1) Filter method: balanced modulator + narrow SSB filter (crystal/mechanical) at an IF.
  2) Phasing method: two balanced modulators with 90° audio and RF phase shifts to cancel one sideband.
- Typical SSB filter bandwidth ≈ 2.4 kHz for voice.
- After SSB is created at an IF, mixers/oscillators can translate it to the desired RF band without changing sideband content.