tag:blogger.com,1999:blog-60761862517878981042009-04-04T18:20:29.114-07:00Anandhttp://www.blogger.com/profile/02394166709582242135noreply@blogger.comBlogger1125tag:blogger.com,1999:blog-6076186251787898104.post-28162413529810190052009-03-29T07:41:00.001-07:002009-03-29T07:41:56.792-07:00<div style="text-align: justify;"><span style="color: rgb(0, 0, 0);font-size:100%;" ><span style="font-family: georgia;">An FM is a constant envelope signal, due to the fact that the envelope</span></span><br /><span style="color: rgb(0, 0, 0);font-size:100%;" ><span style="font-family: georgia;">of the carrier does not change with changes in the modulating signal. Hence the</span></span><br /><span style="color: rgb(0, 0, 0);font-size:100%;" ><span style="font-family: georgia;">transmitted power of an FM signal is constant regardless of the amplitude of the</span></span><br /><span style="color: rgb(0, 0, 0);font-size:100%;" ><span style="font-family: georgia;">message signal. The constant envelope of the transmitted signal allows efficient</span></span><br /><span style="color: rgb(0, 0, 0);font-size:100%;" ><span style="font-family: georgia;">Class C power amplifiers to be used for NP' power amplification of FM. In AM,</span></span><br /><span style="color: rgb(0, 0, 0);font-size:100%;" ><span style="font-family: georgia;">however, it is critical to maintain linearity between the applied message and the</span></span><br /><span style="color: rgb(0, 0, 0);font-size:100%;" ><span style="font-family: georgia;">amplitude of the transmitted signal, thus linear Class A or AB amplifiers, which</span></span><br /><span style="color: rgb(0, 0, 0);font-size:100%;" ><span style="font-family: georgia;">are not as power efficient, must be used.</span></span><br /><span style="color: rgb(0, 0, 0);font-size:100%;" ><span style="font-family: georgia;">The issue of amplifier efficiency is extremely important when designing</span></span><br /><span style="color: rgb(0, 0, 0);font-size:100%;" ><span style="font-family: georgia;">portable subscriber terminals since the battery life of the portable is tied to the</span></span><br /><span style="color: rgb(0, 0, 0);font-size:100%;" ><span style="font-family: georgia;">power amplifier efficiency. Typical efficiencies for Class C amplifiers are 70%,</span></span><br /><span style="color: rgb(0, 0, 0);font-size:100%;" ><span style="font-family: georgia;">meaning that 70% of the applied DC power to the final amplifier circuit is converted</span></span><br /><span style="color: rgb(0, 0, 0);font-size:100%;" ><span style="font-family: georgia;">into radiated RF power. Class A or AB amplifiers have efficiencies on the</span></span><br /><span style="color: rgb(0, 0, 0);font-size:100%;" ><span style="font-family: georgia;">order of 30-40%. This implies that for the same battery; constant envelope FM</span></span><br /><span style="color: rgb(0, 0, 0);font-size:100%;" ><span style="font-family: georgia;">modulation may provide twice as much talk time as AM.</span></span></div><div class="blogger-post-footer"><img width='1' height='1' src='https://blogger.googleusercontent.com/tracker/6076186251787898104-2816241352981019005?l=www.slatteryfile.com' alt='' /></div>Anandhttp://www.blogger.com/profile/02394166709582242135noreply@blogger.com0