Question E7E03:
What is a frequency discriminator?  

Choices:  
A: An FM generator circuit  
B: A circuit for filtering closely adjacent signals  
C: An automatic band-switching circuit  
D: A circuit for detecting FM signals    

Correct Answer is  D: A circuit for detecting FM signals  

To demodulate FM, a receiver needs a circuit that converts instantaneous frequency deviation into a voltage. That circuit is a frequency discriminator (also called an FM detector).

Why D is correct:
- A frequency discriminator produces an output voltage proportional to how far the incoming signal’s instantaneous frequency is from the IF center frequency, thus recovering the original modulating (audio/data) signal.
- In its linear region, the discriminator behaves like:
  $$v_o(t) = K_d \cdot \Delta f(t)$$
  where:
  - $K_d$ = discriminator sensitivity (V/Hz)
  - $\Delta f(t) = f_i(t) - f_0$ = instantaneous frequency deviation from the center frequency
- Common discriminator types: Foster-Seeley, ratio detector, quadrature detector, and PLL FM demodulator.

Why the other choices are incorrect:
- A: An FM generator circuit
  - That’s a modulator (e.g., VCO) that creates FM, not a detector that recovers it.
- B: A circuit for filtering closely adjacent signals
  - That describes selectivity elements (e.g., crystal/ceramic/DSP IF filters). A discriminator is not a filter for adjacent-channel rejection.
- C: An automatic band-switching circuit
  - That’s a control/logic function to change bands; unrelated to FM detection.

Useful formulas and rules of thumb:
- For narrowband FM, if the modulating signal is $m(t)$ and frequency sensitivity is $k_f$, then:
  $$\Delta f(t) = k_f \, m(t) \quad \Rightarrow \quad v_o(t) \approx K_d \, k_f \, m(t)$$
- The discriminator’s transfer curve is “S-shaped”; operation is linear only near the center frequency $f_0$.
- A limiter stage is typically used ahead of the discriminator to remove AM noise; a ratio detector also provides inherent AM rejection.