Fm demodulation using zero crossing detector

1. FM Demodulation using Zero
Crossing detector
Unit -1
Presented By
Mrs. M.P.Sasirekha

2. Block Diagram of Zero Crossing Detector
Zero
Crossing
Detector
Pulse
Generator
DC Block
Low Pass
Filter

5. Zero Crossing detector
• The zero crossing detector operator on the principle that the
instantaneous frequency of an FM wave is approximately given
by,
• where Δt is the time difference between the adjacent zero
crossover points of the FM wave as shown

6. • Let us consider a time-duration T as shown
• The time T is chosen such that it satisfies the following two
conditions:
• (i) T should be small compared to (1/W) wheel, W is the
bandwidth of the message signal.
• (ii) T should be large as compared to (1/fc) where fc is the carrier
frequency of the FM wave.

7. Zero Crossing detector
• Let the number of zero crossings during interval T be denoted
by n0. Hence, Δt i.e., the time between the adjacent zero
crossing points is given by
• By definition of the instantaneous frequency, we know that there
is a linear relation between fi and message signal x(t). Hence,
we can recover x(t) if n0 is known. This can be achieved by
using a zero crossing detector .

8. Advantage and Disadvantage

9. Thank you