Before you conduct the Week 1 Lab, read the following tutorial on AM and FM modulation.
Part 1—Amplitude Modulation | | Amplitude Modulation - Overview | | * AM signals are transmitted over several bands. AM transmission is governed and regulated by the International Telecommunications Union (ITU). * Commercial broadcast AM uses the range from 530 KHz to 1,710 KHz. * Short waves use the range 2.3–30 MHz. * Short wave transmission can be received worldwide as frequencies up to 30 MHz. These are reflected back by the ionosphere and make several bounces before being received. * Commercial broadcast AM has a bandwidth of 10 KHz. The maximum intelligence frequency in commercial broadcast AM is 5 KHz. * The AM family includes side bands.
The above equation represents a sinusoidal carrier.
1. If A (peak amplitude) is varied in step with amplitude variations in intelligence, we have an AM (amplitude modulation) signal.
2. If fc (carrier frequency) is varied in step with amplitude variations in intelligence, we have an FM (frequency modulation) signal.
3. If (carrier phase angle) is varied in step with amplitude variations in intelligence, we have a PM (phase modulation) signal.
Amplitude modulation is a process in which the peak amplitude of a high-frequency signal (carrier) is made to vary in direct proportion to the amplitude variations in a low-frequency signal (information).
Modulation Index a. Modulation index is defined as follows. m = Ei / Ec B = Maximum peak-to-peak amplitude of AM signal A = Minimum peak-to-peak amplitude of AM signal B = 2Ec + 2Ei A = 2Ec - 2Ei B + A = 4 Ec B - A = 4 Ei m = 4Ei / 4Ec = Ei / Ec = (B-A)/(B+A) m = (B-A) / (B+A)
b. There are three classifications of modulation index. (i) m = 1 i.e. Ei = Ec 100% modulation (ii) m > 1 i.e. Ei > Ec overmodulation (iii) m < 1 i.e. Ei < Ec
a. Trapezoidal patterns of an AM signal are used to observe the modulation characteristics of an AM transmitter.
b. The AM signal is applied to the vertical input of the oscilloscope.
c. The modulating signal is applied to the external horizontal input of the oscilloscope with the input horizontal sweep disabled.
d. The horizontal sweep rate is determined by the modulating signal frequency.
e. The magnitude of horizontal deflection is proportional to the amplitude of the modulating signal.
f. The vertical deflection is proportional to the amplitude and rate of change of the modulated signal (AM).
Mathematical Analysis of AM Signal
a. The mathematical analysis of the AM signal shows three frequency components of the AM signal. i. Carrier ( fc) ii. Lower sideband (fc- fi) iii. Upper sideband (fc+fi)
b. The amplitude of each sideband is equal to (mEc)/2.
c. The information is contained in the sidebands.
Frequency Domain Representation of AM Signal
Generation of AM Signal (Nonlinear Operation)
a. Intelligence (fi) and carrier (fc) are applied to the inputs of a mixer (multiplier).
b. The nonlinear operation of mixer produces sum and difference frequencies of the input signals and harmonics of each of the input frequencies.
c. The resonant tuned circuit is tuned at carrier frequency (fc). It allows only three frequencies (fc-fi, fc, and fc+fi) to pass. These three frequencies constitute the AM signal.
AM Signal Generation Circuit
a. Intelligence (fi) is applied at the base of Q1.
b. The carrier is applied at emitter of Q1.
c. The two signals are mixed in the nonlinear operation of Q1.
d. Q1 is used as a class C amplifier for maximum power output due to the high efficiency of class C amplifiers.
e. C1 and L1 form the resonant tuned circuit at the output (collector) of the nonlinear device (class C amp). The resonant circuit is tuned at fc.
f. The resonant