Generation of AM wave ?

Introduction 

Purpose of a communication system: convey information through a medium or communication channel.

◊    The information is often represented as a baseband signal, i.e. a signal whose spectrum extends           from 0 to some maximum frequency.

◊     Proper utilization of the communication channel often requires a shift of the range of baseband         frequencies into other frequency ranges suitable for transmission, and a corresponding shift back to

    the original frequency range after reception.

◊    A shift of the range of frequencies in a signal is accomplished by using modulation, which is             defined as the process by which some characteristic of a carrier is varied in accordance with                 modulating (signal)

◊      A common form of the carrier is a sinusoidal wave, in which case we speak of continuous-wave     modulation.

◊      The baseband signal is referred to as the modulating wave, and the result of the modulation              process is referred to as the modulated wave.

◊      Modulation is performed at the transmitting end.

◊      At the receiving end, we require the original baseband signal to be restored. This is                            accomplished by using a process known as demodulation, which is the reverse of the modulation        process.

Amplitude Modulation :

      In AM modulation the amplitude of the carrier is modified in proportion to the baseband signal. A baseband signal is used to designate the band of frequencies of the signal delivered by the source.

Amplitude Modulation: Double Sideband Suppresed Carrier (DSB-SC)

In amplitude modulation, the amplitude A_c of the unmodulated carrier A_ccos(ω_ct + θ_c) is varied in proportion to the baseband signal (known as modulating signal). The frequency ω_c and the phase θ_c are constant. We can assume θ_c=0 to simplify the analysis.

If the carrier amplitude A_c is made directly proportional to the modulating signal m(t), the modulated carrier is

m(t ) cos(ω _c t )

This type of modulation simply shifts the spectrum of m(t) to the carrier frequency (see figure 1; that is, if

m(t ) ↔ M (ω )

m(t ) cos(ω _c t ) ↔ ¹ [M (ω + ω _c ) + M (ω − ω _c )] 2

Figure 1

The bandwidth of the modulated signal is 2B Hz, which is twice the bandwidth of the modulating signal m(t). From the figure, we observe that the modulated carrier spectrum centered at ω_c is composed of two parts: a portion that lies above ω_c, known as the upper sideband (USB), and a portion that lies below ω_c, known as the lower sideband (LSB). Similarly, the spectrum centered at -ω_c, has upper and lower sidebands.

For intance, if m(t)=cos(ω_mt), then the modulated signal m(t ) cos(ω _c t ) = cos(ω _m t ) cos(ω _c t )

=    ¹ [cos((ω _c + ω _m ))t + cos((ω _c − ω _m ))t ] 2

The component of frequency ω_c + ω_m is the upper sideband and that of frequency ω_c - ω_m is the lower sideband. Thus, each component of frequency ω_m in the modulating signal gets translated into two components, of frequencies ω_c + ω_m and ω_c - ω_m, in the modulating signal. Note that the modulated signal m(t)cos(ω_ct), from the above equation, has components of frequencies ω_c ± ω_m but not have a component of the carrier

 frequency ω_c. For this reason, this scheme is referred to as double-sideband-suppress-carrier.

Amplitude Modulation: Double Sideband Transmitted Carrier (DSB)

We now explore a modification of the AM DSB-SC modulation where we add a portion of the pure sinusoidal carrier to the modulated waveform. We will see that this addition greatly simplifies the demodulation process. The block diagram is shown in figure 2.

Figure 2

The resulting waveform is given by

s_m (t ) = s(t ) cos(ω _c t ) + A cos(ω _c t )

The Fourier transform of transmitted carrier AM is the sum of the Fourier transform of suppressed carrier AM with the Fourier transform of the pure carrier. The transform of the carrier is a pair of impulses at ±f_c in frequency. The complete transform of the AM wave is therefore as shown in figure 3. Figure 4 shows the time waveform.

                     Figure 03

Figure 4

Modulation Factor

A modulating wave that has low amplitude will produce a smaller amplitude variation in the modulated wave than a high amplitude modulating wave will. This fact gives to a need for expressing the degree of modulation produced by a wave of some particular amplitude. This is expressed by a ratio called the modulation factor, M_a. The modulation factor is simply the ratio of the peak amplitude variation used (A_m) to the maximum design variation (A_c). Under proper operating conditions A_m will always equal or less than A_c, therefore, the modulation factor, M_a will not be allowed to exceed unity. See figure 5.

                                     Figure 5

AM Spectrum

It has been shown that when modulating with a single frequency, a modulated wave is generated. This complex wave consists of three frequencies: the carrier, a sum

  frequency (upper side band) and a difference frequency (lower sideband). These three components of the modulated wave constitute the spectrum of the AM modulated wave when the modulating wave is but a single frequency. See figure 6.

When the modulating wave consists of more than a single frequency (which is the usual case), each of the components of the complex modulating wave must create its own pair of side frequencies during the modulation process. See figure 7.

Figure 7

The upper sideband is an exact replica of the spectrum of the modulating wave. The spacing between spectral components is the same in the upper sideband as the modulating wave. Since the spacing of all components of the upper sideband are identical to those same relationship among the harmonics in the modulating wave, it can be readily concluded that both the modulating wave and the upper sideband contain exactly the same bandwidth.

The lower sideband is an inverted replica of the spectrum of the modulating wave. This fact makes everything that was said about the upper sideband true of the lower sideband also, except that the order of components is reversed.

AM Generation Circuits

In this experiment we are going to use a Balanced modulator to modulate our input signal. Figure 8 shows a typical balanced modulator block diagram. We can generate the DSB-SC signal using two such generators in a balanced configuration that will suppress the carrier term.

Figure 8

PROCEDURE

1.      Connect the power supply to the trainer with all equipment turned off. Follow the following diagram.

2.  Select the following conditions in the trainer
a.	Audio Input Select		:	EXT
b.	Mode		:	DSB
c.	Output Amplifier Gain		:	Maximum in clockwise direction
d.	Speaker Switch		:	OFF

3.      Set the signal generator to a sinusoidal wave with frequency of 10 kHz and amplitude of 2 Vpp. This signal will be the modulating signal. Connect the signal generator to test point 16.

4.      In the BALANCED MODULATOR AND BANDPASS FILTER CIRCUIT I block set the Balance dial to the maximum position. This secure maximum power to the carrier.

5.      Connect the oscilloscope’s probe to test point 9. This signal is the carrier. Determine its frequency and draw oscilloscope output indicating the signal frequency. Determine the peak voltage value.

V _p   = ______________

f   = _______________

6.      Disconnect the oscilloscope of test point 9. Connect the Spectrum analyzer in test point 1 and ground. Set Center frequency to 10 kHz and Span to 10 kHz. Did you see the modulating frequency spectrum? Determine the peak power level (dBm). Print the spectrum.

P_peak   = ____________

7.      Connect the Spectrum analyzer to test point 9. Set Center frequency with the frequency determined in step 5 and Span to 50 kHz. . Did you see the carrier frequency spectrum? Determine the peak power level (dBm).

P_peak   = ____________

8.      Connect the spectrum analyzer in test point 11. Set the Center frequency to 1 MHz and Span of 50 kHz. Did you see the modulated signal frequency spectrum? Determine the frequency and power level of the USB and LSB components. Also, determine the power level of the carrier. Record in table 1.

9.      Connect the Spectrum analyzer in test point 12. Set the Center frequency to 1 MHz and Span of 50 kHz. Determine the power level of the carrier and the sideband components. Record in table 1.

10.  Connect the oscilloscope’s probe to test point 11. Determine the percentage modulation.

A = ___________

B = ___________

%m = _________

11. Connect the spectrum analyzer in test point 11. Determine the percentage modulation.

%m = _________

12.  Set the BALANCE of the BALANCED MODULATOR & BANDPSS FILTER CIRCUIT 1 to the middle position. This is the case of DSB-SC modulation.

13.  Repeat steps from 8 and 9. Complete table 2.

14.  Turn off the power supply and connect the microphone with the external audio input and the trainer. Connect the power supply to the external audio amplifier. Change the AUDIO INPUT SELECT switch to EXT. Connect oscilloscope channel one to point

test 1 and channel 2 to point test 3. Try to tune a note with your voice and determine the maximum frequency. Look how speech looks like in the oscilloscope.

F_max = _______________

Thanking you, 

Yogesh Darade 

BE_Q-11