Q18Principle of Electronic Communication

(a) FM Generation and Demodulation

FM is generated by using the message signal to directly vary the instantaneous frequency of a carrier oscillator, most commonly using a Voltage Controlled Oscillator (VCO), whose output frequency shifts above or below its free-running center frequency in direct proportion to the applied message voltage - a larger message amplitude produces a larger frequency deviation, while the message's own frequency determines how rapidly the carrier frequency swings back and forth. An alternative indirect method (Armstrong method) generates a narrowband FM signal via phase modulation of a highly stable crystal oscillator followed by frequency multiplication stages to achieve the desired wideband FM and final carrier frequency, avoiding the frequency-stability difficulties of a directly-modulated VCO.

FM demodulation recovers the message signal from the frequency variations of the received FM carrier. A widely used modern approach uses a Phase-Locked Loop (PLL): the PLL's internal VCO continuously tracks and locks onto the instantaneous frequency of the incoming FM signal, and the PLL's error (control) voltage - which must continuously adjust to keep the VCO frequency locked to the changing input frequency - is itself directly proportional to the original message signal, so this error voltage, taken as the demodulator output, reconstructs the message. Simpler alternative demodulation techniques include the slope detector (using the steep, roughly linear slope of a tuned circuit's frequency response to convert frequency variation into amplitude variation, then applying ordinary envelope detection) and the Foster-Seeley/ratio detector (using a more balanced discriminator circuit for improved linearity and amplitude-noise rejection compared to a simple slope detector).

(b) Amplifier Gain Calculation

Given input power Pin=50mW and gain=20dB, solving for the power ratio:

So the output power of the amplifier is 5 Watts.

It is worth noting the key distinguishing advantage of FM over AM that motivates its widespread use for broadcast audio: since FM encodes information in frequency variation rather than amplitude variation, it is inherently much more resistant to amplitude-based noise and interference (which primarily affects the amplitude, not the frequency, of the received signal) - a limiter stage is typically included ahead of the FM demodulator specifically to strip away any residual amplitude variations (noise) from the received signal before demodulation, ensuring only the frequency information is passed to the discriminator or PLL, a noise-immunity advantage that is not available to AM's simple envelope detector, which cannot distinguish genuine amplitude modulation from amplitude-domain noise.

It is also worth noting that dB-based gain calculations, as performed in part (b), are used throughout communication system link-budget analysis precisely because cascaded stages (amplifiers, attenuators, cables) simply add algebraically in dB, rather than requiring successive multiplication of linear power ratios - this additive property makes dB units especially convenient for quickly computing the overall gain or loss of a complete multi-stage communication chain.

In summary, FM's frequency-domain noise-rejection advantage, combined with straightforward dB-based gain arithmetic used throughout communication link analysis, together illustrate two foundational concepts that recur constantly across analog and digital communication system design.

Mastering both the qualitative FM noise-immunity argument and the quantitative dB gain calculation equips a student to handle both conceptual and numerical exam questions on analog modulation systems confidently.