FM Demodulation Basics

FM demodulators convert frequency variations back into voltage signals for audio playback. A simple slope detector uses a tuned LC circuit to perform this conversion.

Core Concept

An FM receiver must extract the original message from a rapidly varying frequency. A frequency discriminator performs this essential task. It converts input frequency changes into proportional amplitude changes.

The simplest method uses an LC tuned circuit operated slightly off-resonance. As the input frequency moves back and forth, the circuit responds with varying amplitude. This is called slope detection.

An envelope detector then extracts this amplitude variation. This recovers the original audio signal from the high-frequency carrier. More advanced circuits use phase shifts to improve linearity.

Key Equations

Demodulator output voltage: Vout = Kd * Δf

Demodulator constant definition: Kd = dV / df

Here, Kd is the discriminator sensitivity constant. The variable Δf represents the instantaneous frequency deviation from the 10.7 MHz center carrier.

Given:
Discriminator constant Kd = 50 mV/kHz
Frequency deviation Δf = 20 kHz

Why this formula:
The output voltage is directly proportional to the instantaneous frequency deviation.

Formula:
Vout = Kd * Δf

Substitution:
Vout = 50 * 20

Calculation:
Vout = 1000 mV

Final Answer:
The peak output voltage is 1 V.

Exam Tip: GATE questions often test the discriminator constant unit. It is always specified in volts per hertz. Do not confuse it with phase sensitivity in volts per radian.

Key Properties

• Converts frequency variations into amplitude variations.
• Requires a completely linear transfer curve over the entire operating bandwidth.
• Must operate symmetrically around the 10.7 MHz intermediate frequency.
• Basic slope detectors suffer from severe non-linearity near the resonance peaks.
• Always requires a preceding amplitude limiter circuit to strip away noise spikes.

Quick Revision

• FM demodulators are also called frequency discriminators.
• They generate an S-curve voltage response.
• Linearity dictates audio distortion levels.
• Slope detection uses tuned circuit skirts.
• Kd controls the audio output volume.
• Exam trap: Forgetting that a pure discriminator cannot reject AM noise on its own.