AMPLITUDE MODULATION

Amplitude modulation (AM) radio is a commonplace technology today, and is standard in any type of commercial stereo device. Because of the low cost of the parts necessary to implement AM transmission and the simplicity of the underlying technology, using amplitude modulations is a cheap and effective way to perform many tasks that require wireless communication.

The most well-known application of an AM transmitter is in radio. Am radio receivers are available in numerous devices, from automobile stereos to clock radios. However, the usage of AM transmitters is not restricted to professional radio stations

Transmit information bearing (message) or baseband

signal (voice music) through a communication channel Baseband = is a range of frequency signal to be

transmitted. eg: Audio (0 - 4 kHz), Video (0 - 6 MHz).

Communication channel: Transmission without frequency shifting. Transmission through twisted pair cable, coaxial cable and

fiber optic cable. Significant power for whole range of frequencies. Not suitable for radio/microwave and satellite

communication.

Carrier communication Use technique of modulation to shift the

frequency. Change the carrier signal characteristics

(amplitude, frequency and phase) following the modulating signal amplitude.

Suitable for radio/microwave and satellite communication.

the instantaneous amplitude of a carrier wave is varied in accordance with the instantaneous amplitude of the modulating signal. Main advantages of AM are small bandwidth and simple transmitter and receiver designs. Amplitude modulation is implemented by mixing the carrier wave in a nonlinear device with the modulating signal. This produces upper and lower sidebands, which are the sum and difference frequencies of the carrier wave and modulating signal. The carrier signal is represented by

c(t) = A cos(wct)

The modulating signal is represented by m(t) = B sin(wmt)

Then the final modulated signal is [1 + m(t)] c(t) = A [1 + m(t)] cos(wct)

= A [1 + B sin(wmt)] cos(wct)

= A cos(wct) + A m/2 (cos((wc+wm)t)) + A m/2 (cos((wc-wm)t))

Form of amplitude modulation(AM); carrier is suppressed (typically 40 – 60dB below carrier)

For short we call it SSB since the carrier is usually not transmitted

Advantages Conservation of spectrum space (SSB signal occupies

only half the band of DSB signal) The useful power output of the transmitter is greater

since the carrier is not amplified SSB receiver is quieter due to the narrower bandwidth

(receiver noise is a function of bandwidth)

Disadvantages The carrier must be reinserted the receiver, to down

convert the sideband back to the original modulating audio

Carrier must be reinserted with the same frequency and phase it would have if it were still present

This is why clarity control on a CB radio is so fiddly, if carrier not inserted correctly the received output will be like donald duck voice.

SSB-LSB SIGNAL

SSB-USB SIGNAL

SSB signal frequency spectrum

SSB-LSB signal

SSB-USB signal

Hartley modulator A direct approach for creating a single sideband AM signal

(SSB-AM) is to remove either the upper or lower SB by filtering the DSBSC-AM signal (frequency discriminator method)

SSB modulator using the frequency discrimination approach

Magnitude spectra : (a) baseband ; (b) DSBSC-AM; (c) upper SSB; (d) lower SSB

Quadrature modulator can be used to create a SSB-AM signal by selecting the quadrature signal to coherently cancel either the upper /lower SB from the inphase channel

SSB-AM signal given:

Using the minus sign in equation (1) results in upper SSB,whereas selection of the plus sign yields lower SSB

The hilbert transform is a wideband -90° phase shifter

(1)

(2)

Hartley modulator for SSB; -sign gives upper SSB, +sign gives lower SSB