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Lec08ClassC - ECE 488 RF Circuits and Systems Fall 2008...

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1 1 Swartz 06/1/28 ECE 488 – RF Circuits and Systems – Spring 2008 Lecture 9: ECE 488 RF Circuits and Systems Basics of AM Modulation Class-C Amplifiers Class-D Amplifiers Vacuum Tubes Fall 2008 Lecture 8 2009/2/4 3 Swartz 06/1/28 ECE 488 – RF Circuits and Systems – Spring 2008 Lecture 9: A pure sinusoidal signal transmits no information CW radar can measure Doppler shift (e.g., velocity of target) Only two ways to modulate transmitted signal: Amplitude Telegraphy (simplest) Broadcast AM (530 kHz – 1700 kHz) NTSC Television Pulse Codes Phase (or frequency) Broadcast FM, provision for stereo Phase coded) Radars can use both amplitude (pulses) and phase modulation (e.g., Barker, complementary codes) Modulation of Radio Frequencies 4 Swartz 06/1/28 ECE 488 – RF Circuits and Systems – Spring 2008 Lecture 9: Modulation by a Single Frequency (tone) The carrier amplitude varies at the audio frequency. This is AM. ) } cos({ 2 1 ) } cos({ 2 1 ) sin( )] sin( )[ sin( t V t V t V t V V t a C a a C a C m a a m C ω + + = + 5 Swartz 06/1/28 ECE 488 – RF Circuits and Systems – Spring 2008 Lecture 9: The left side of the previous plot is expanded out here. The RF amplitude varies only a little bit in one cycle. Modulation by a Single Frequency (tone) 6 Swartz 06/1/28 ECE 488 – RF Circuits and Systems – Spring 2008 Lecture 9: Here the audio is made up of more than one frequency. Music or voice produces amplitude variations that look almost random. More General AM Now, how do we create AM? 7 Swartz 06/1/28 ECE 488 – RF Circuits and Systems – Spring 2008 Lecture 9: Basics of AM Modulation Simple AM radio station block diagram: MIC (Audio Source) LO DC Offset Multiplier (MIXER) Filter (Required!) Antenna H(f) Amplifier ) } cos({ 2 1 ) } cos({ 2 1 ) sin( )] sin( )[ sin( t V t V t V t V V t a C a a C a C m a a m C + + = +
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2 8 Swartz 06/1/28 ECE 488 – RF Circuits and Systems – Spring 2008 Lecture 9: Three frequencies: Carrier at : Power = Carrier is always present! Lower sideband at : Power = Upper sideband at : Power = At 100% modulation, Maximum Average Power is = times carrier power, Frequency Analysis of AM Modulation by a single frequency: ) } cos({ 2 1 ) } cos({ 2 1 ) sin( )] sin( )[ sin( t V t V t V t V V t a C a a C a C m a a m C ω + + = + m a V V C a C a C + R V P m C 2 2 = L a P R V = 8 2 2 3 m a V V = U a P R V = 8 2 9 Swartz 06/1/28 ECE 488 –
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This note was uploaded on 07/09/2009 for the course ECE 4880 taught by Professor Swartz during the Spring '06 term at Cornell.

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Lec08ClassC - ECE 488 RF Circuits and Systems Fall 2008...

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