Introduction to Modulation: Amplitude Modulation(AM)
Sharlene Katz James Flynn Overview Modulation Overview Basics of Amplitude Modulation (AM) AM Demonstration GRC Exercise
2 Flynn/Katz 7/8/10 Why do we need Modulation/Demodulation?
Example: Radio transmission
Voice Microphone Transmitter Electric signal, Antenna: 20 Hz – 20 Size requirement KHz > 1/10 wavelength
c 3×108 Antenna too large! 5 Use modulation to At 3 KHz: λ = = 3 =10 =100km f 3×10 transfer ⇒ .1λ =10km information to a higher frequency
3 Flynn/Katz 7/8/10
Why do we need Modulation/Demodulation? (cont’d)
Frequency Assignment Reduction of noise/interference Multiplexing Bandwidth limitations of equipment Frequency characteristics of antennas Atmospheric/cable properties
4 Flynn/Katz 7/8/10 Basic Concept of Modulation
The information source Typically a low frequency signal Referred to as the “baseband signal” X(f) x(t)
t f Carrier A higher frequency sinusoid baseband Modulated Modulator Example: cos(2π10000t) carrier signal Modulated Signal Some parameter of the carrier (amplitude, frequency, phase) is varied in accordance with the baseband signal
5 Flynn/Katz 7/8/10 Types of Modulation
Analog Modulation Amplitude Modulation, AM Frequency Modulation, FM Double and Single Sideband, DSB and SSB Digital Modulation Phase Shift Keying: BPSK, QPSK, MSK Frequency Shift Keying, FSK Quadrature Amplitude Modulation, QAM
6 Flynn/Katz 7/8/10 Amplitude Modulation (AM)
Block Diagram
x(t) m x + xAM(t)=Ac [1+mx(t)]cos wct
Ac cos wct Time Domain
Signal information is contained in the Frequency Domain sidebands X(f) XAM(f)
f f -fm fm -fc fc
7 Flynn/Katz 7/8/10 Traditional AM Demodulators
Synchronous Detector
Low Pass x (t)=A [1+mx(t)]cos w t AM c c x Filter XAM(f) w f cos ct -fc fc f -2f 2f Envelope Detector c c
8 Flynn/Katz 7/8/10 Receiving AM with SDR
USRP and Computer with GNU Radio Software
64Ms/s USRP 256Ks/s
Basic Rx A/D FPGA Shifts by -710KHz Computer or (GNU Radio)
Stored USRP File
9 Flynn/Katz 7/8/10 AM Modulation
Demonstration Receiving off-air signals Using a captured file source GRC Exercise
10 Flynn/Katz 7/8/10 Questions?
11 Flynn/Katz 7/8/10