The document discusses the threshold effect in FM receivers. As the carrier-to-noise ratio decreases at the discriminator input, the FM receiver will break down. First, individual clicks will be heard in the output. As the ratio decreases further, the clicks will rapidly merge into a crackling or sputtering sound. The threshold effect is defined as the minimum carrier-to-noise ratio that provides an output signal-to-noise ratio not less than predicted by the usual formula, assuming small noise power. Techniques like FM feedback, phase-locked loops, and frequency-locked loops can delay the onset of threshold effects.

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