Lecture 22 Threshold effects in FM.pptx

1. Threshold effects in FM
Lecture 22

3. Threshold Effect
• The output signal to noise ratio of FM receiver is
valid only if the carrier to noise ratio measured at the
discriminator input is high compared to unity
• It is observed that as the input noise is increased so
that the carrier to noise ratio decreased, the FM
receiver breaks

4. Threshold Effect – Receiver Output
• At first individual clicks are heard in the receiver
output
• And as the carrier to noise ratio decreases still further,
the clicks rapidly merge in to a crackling or sputtering
sound
• In video applications "click noise" is used to describe
short horizontal black and white lines that appear
randomly over a picture.

5. Threshold Effect
• Near the break point (SNR)o equation begins to fail predicting
values of output SNR larger than the actual ones.
• This phenomenon is known as the threshold effect.
• Threshold effect is defined as the minimum carrier to noise ratio
that gives the output SNR not less than the value predicted by the
usual signal to noise formula assuming a small noise power.

6. FM Threshold Reduction
• The onset of FM threshold effects can be
delayed by using techniques such as FM
feedback, phase locked loops and frequency
locked loops

7. FM Applications
• Radio broadcasting
• Audio transmission in TV
• Microwave communication
• Cellular communication
• Satellite communication

9. Qs