Question E7E07:
What is meant by the term “baseband” in radio communications?  

Choices:  
A: The lowest frequency band that the transmitter or receiver covers  
B: The frequency range occupied by a message signal prior to modulation  
C: The unmodulated bandwidth of the transmitted signal  
D: The basic oscillator frequency in an FM transmitter that is multiplied to increase the deviation and carrier frequency    

Correct Answer is  B: The frequency range occupied by a message signal prior to modulation  

Correct answer: B — The frequency range occupied by a message signal prior to modulation.

1) Why B is correct
- “Baseband” is the original message spectrum centered near 0 Hz before it is shifted up to an RF carrier. Examples:
  - Voice baseband ≈ 300–3000 Hz
  - Digital symbols shaped by a low-pass filter around DC
- In modulation, the baseband signal $m(t)$ is used to create an RF signal, e.g. AM:
  $$s(t) = [A + m(t)]\cos(2\pi f_c t)$$
  Here $m(t)$ is the baseband.

2) Why the other choices are incorrect
- A: “The lowest frequency band the radio covers” describes a tuning range, not the message spectrum prior to modulation.
- C: “The unmodulated bandwidth of the transmitted signal” is not meaningful—an unmodulated carrier ideally has no bandwidth. Baseband refers to the message before RF modulation.
- D: The “basic oscillator frequency in an FM transmitter” is a carrier-generation detail. While frequency multipliers scale deviation and carrier, that is not what baseband means.

3) Useful formulas and rules of thumb
- AM (DSB with carrier): $$s(t) = [A + m(t)]\cos(2\pi f_c t)$$
- FM approximate occupied bandwidth (Carson’s Rule):
  $$B_{FM} \approx 2\left(\Delta f + f_m\right)$$
  where $f_m$ is the highest baseband (modulating) frequency and $\Delta f$ is peak deviation.
- For linear digital modulation with root-raised-cosine filtering:
  $$B \approx (1+\alpha)R_s$$
  where $R_s$ is symbol rate and $\alpha$ is roll-off. The shaped symbol stream is the digital baseband.