Lecture 22 Threshold effects in FM.pptx

Lecture 22 Threshold effects in FM.pptx

1. Threshold effects in FM Lecture 22

2. 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

3. 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.

4. 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.

5. 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

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

7. Qs