Modulations Analog Modulations Amplitude modulation (AM) Linear modulation Frequency modulation (FM) Phase modulation (PM) cos Angle modulation FM PM Digital Modulations ASK FSK PSK MSK MFSK QAM PAM Etc.
AM Modulators/Demodulators AM Modulators Multiplication modulator Non linear modulator Switching modulator Ring modulator AM Demodulators Coherent demodulator Rectifier Envelope Detector
Multiplication Modulator In early day, multiplication of two signals over a sizable dynamic range was a challenge to circuit designer. A multiplier is obtained from a variable gain amplifier. The gain is controlled by the input message. cos Variable gain Amplifier cos
Nonlinear DSB SC Modulator Add input message and carrier, then perform non linear operation cos cos 2 4 cos
Switching Modulation
Switching Modulation Any periodic signal can be expressed by a trigonometric Fourier Series. cos 1 2 2 1 cos 2 1 2 1 1 2 2 cos 1 3 cos 3 1 5 cos 5 Output of bandpass filter 2 cos
Switching Modulator During positive half of cycle, All the diodes conduct a and b are shorted During negative half of cycle, All the diodes open. a and b are opened. (a) Diode bridge electronic switch. (b) Series bridge diode modulator. (c) Shunt bridge diode modulator.
Ring Modulator During positive half of cycle, D 1 and D 3 conduct. a is connected to c. b is connected to d. During negative half of cycle, D 2 and D 4 conduct. a is connected to d. b is connected to c.
Ring Modulation Any periodic signal can be expressed by a trigonometric Fourier Series. cos 4 1 cos 2 1 2 1 4 cos 1 3 cos 3 1 5 cos 5 Output of bandpass filter 4 cos
Demodulation of DSB SC Signals The receiver must generate a carrier that synchronous in phase and in frequency with the incoming carrier. (coherent demodulator) Not easy in practice Delay Doppler effect cos cos cos
Amplitude Modulation Signal The alternative to coherent demodulator is for the transmitter to send a carrier cos along with the modulated signal cos. cos cos + Transmitter needs to transmit at a higher power level. Receiver circuit can be simpler and less expensive. Benefit for a broadcast system with a number of receivers for each transmitter.
AM Signal and Its Envelope
Envelope Detection Envelope detection condition 1. bandwidth of 2. 0 Modulation index For envelope to be distortionless, 0 1 with zero offset Let be the maximum and minimum of. min with non zero offset (rare case), min max max min 2 max min
Modulation Index Tone modulation, cos For modulation indices ( ) of 0.5 and 1. cos cos 1 cos cos (a) 50% modulation. (b) 100% modulation.
Sideband and Carrier Power Carrier power Sideband power Power efficiency useful power total power cos cos 100% For tone modulation, 2 100% 2 In case of 50% modulation, 11.11%
Rectifier Detector for AM
Envelope Detector for AM For proper operation, the discharge time constant RC must be chosen properly. 1 1 2 Discharge rate
Single Sideband (SSB) Spectra
SSB Spectra for Tone Modulation
SSB Modulation Conceptually, the generation of a SSB signal is straightforward. Generate a DSB signal. Apply an ideal band pass filter. Practically, the construction of ideal filter is very difficult. Similar to a DSB signal, coherent demodulation is required to detect an SSB signal.
Quadrature Amplitude Modulation (QAM) Because SSB SC signals are difficult to generate accurately, QAM offer an alternative to SSB SC. QAM can be generated without requiring sharp cutoff band pass filters. Two baseband signals, and, each of bandwidth Hz, can be transmitted simultaneously over a bandwidth by using DSB transmission and quadrature multiplexing. Upper channel = in phase (I) channel Lower channel = quadrature (Q) channel
Quadrature Amplitude Modulation (QAM) cos sin cos 2 sin 2 cos 2 sin 2
Quadrature Amplitude Modulation (QAM) QAM demodulation must be synchronous. An error in phase or frequency of the carrier at the demodulator will result in loss and interference between the two channels. If the carrier at the demodulator is, the output of the upper receiver branch will become This is called co channel interference.