Efficiency of AM modulation The Modulation Efficiency is the percentage of the total power of the modulated signal that conveys information. Therefore given the equation for power of an AM waveform, the efficiency is: 2 2 2 100 % Highest efficiency for a 100% AM signal : 50% - square wave modulation It can be seen from this equation that the efficiency of AM modulation increases as the modulation index, μ (m), increases. Normalized Peak Envelope Power (PEP) of the AM signal: 2 PPEP Ac 2 1 max m 2 Importance of High Percentage Modulation Let R be the resistance into which the current flow. T hen, H ence It 1 Ic It = Ic It Ic m 2 2 Pt I R I t m 1 Pc I R I c 2 2 t 2 c m 2 2 1+ m 2 = Total transmitted current = Carrier current = Modulating index 2 Effective Modulation Index If a carrier is modulated by more than a single sine wave, the effective modulation index is given by: m eff m1 m 2 m 3 ...... 2 2 2 Note that the total modulation index must not exceed 1 or distortion will occur. meff can be used in all previously developed equations using m. Usage • Amplitude is susceptible to interference – This technique in not normally used in modems • Also used for two-way mobile radio communications, such as citizens band (CB) radio. • Use in commercial broadcasting of both audio and video signals. • A variation of this technique is used in AM radio transmission – Analog-to-analog modulation takes place AM Advantages • Low bandwidth • Easy to modulate, demodulate signal AM Disadvantages • Must maintain linearity of message signal and transmitted signal. Need to use special filters which are not as efficient as those that do not maintain strict linearity; relates to battery lifetime • Interfering signals received additively discriminated during demodulation and must be • Hard to lock frequency of receiver local oscillator to carrier frequency (esp. in S.C. systems) • Can use pilot tone AM to mitigate these effects – Transmit known pilot tone either in-band or out-of-band – PLL can detect pilot tone and lock the frequency and amplitude of the local oscillator Types of Amplitude Modulation • Double Sideband with full carrier (AM DSBFC): This is the most widely used type of AM modulation. In fact, all radio channels in the AM band use this type of modulation. • Double Sideband Suppressed Carrier (DSBSC): This is the same as the AM modulation above but without the carrier. • Single Sideband (SSB): In this modulation, only half of the signal of the DSBSC is used. • Vestigial Sideband (VSB): This is a modification of the SSB to ease the generation and reception of the signal. Linear carrier wave (CW) modulation • • • • • • Bandpass systems and signals Lowpass (LP) equivalents Amplitude modulation (AM) Double-sideband modulation (DSB) Modulator techniques Suppressed-sideband amplitude modulation (LSB, USB) • Detection techniques of linear modulation – Coherent detection – Non coherent detection AM DSB LSB USB Question: Find the percentage of an AM wave whose total power content is 2500W and whose side bands contain 400W. Solution: Given, Pt = 2500W, Plsb =Pusb = 400W Pt = Pc + Plsb + Pusb Pc = 1700W Plsb =Pusb = m2Pc/4 Thus, m = 0.970 M= 0.970X100 = 97% M=? Generation of AM There are two types of devices by which amplitude modulated signals can be generated: – The AM transmitter, which generates such high powers that its prime requirement is efficiency, and so is a quite complex means. – The AM generator in lab, where AM is produced at such a low power level that simplicity is more important requirement than efficiency. Basic Requirements To generate the AM wave: It is necessary merely to apply the series of current pulses to a tank (resonant) circuit. Each pulse initiates a damped oscillation in the tuned circuit. The oscillation would have an initial amplitude proportional to the size of the current pulse and decay rate dependent on the time constant of the circuit. Since a train of pulses is fed to the tank circuit here, each pulse will cause a complete sine wave proportional in amplitude to the size of this pulse. This is followed by the next sine wave, proportional to the size of the next applied pulse, and so on. A very good approximation of AM wave will result if the original current pulses are made proportional to the modulating voltage. This process is known as flywheel effect of the tuned circuit, and it works best with a tuned circuit whose Q is not too low. In AM transmitter, amplitude modulation can be generated at any point after the radio frequency source. AM Transmitters A transmitter not only performs the modulation process, but also raises the power level of a modulated signal to the desired extent for effective radiation. The AM transmitters are divided into two categories, which depends on the transmitted circuit arrangements. • High level: If the output stage in a transmitter is plate modulated (or collector modulated in low power transmitter), the system is called high level modulation. • Low level: If modulation is applied at any other point, including some other electrode of the output amplifier, then it is low level modulation.