EEE 3032 Communication Systems Lab #1class.icc.skku.ac.kr/~dikim/teaching/3032/handouts/lab1... · 2010-10-25 · 오류!여기에 표시할 텍스트에 Heading 1을(를) 적용하려면

EEE 3032 Communication Systems Lab #1

Amplitude Modulation

sample report

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Abstract

In the modulation part of this lab, we found the modulation index for the default sin and

square wave setups to be the same at a=71.4%. By varying modulation indices, we

noticed carrier power decrease as a approached unity. We sampled 79.1%, 87.6%,

91.6%, 97.1%, and 100% modulation to reach this conclusion. We also confirmed

efficiency at modulation index 1.000 to be 33.33%, and noticed that different frequencies

of the message signal had no effect on the carrier band power, but did change the

position of the sidebands, with them moving farther away from the carrier as the

message frequency increased.

In the demodulation part of this lab, we demodulated the AM input signals using the

LAB-Volt AM/DSB Receiver (Model 9411), and found the output has an amplitude

distortion and a phase deviation due to interference caused by the local oscillators and

the non-ideal filters.



Table of Contents

LIST OF FIGURES .................................................................................................................................... IV

LIST OF TABLES ........................................................................................................................................ V

SECTION 1. OBJECTIVES AND INTRODUCTION .........................................................................6

SECTION 2. THEORY ...........................................................................................................................6

2.1. AM MODULATION .........................................................................................................................6

2.2. AM DEMODULATION .....................................................................................................................7

SECTION 3. EXPERIMENT PROCEDURES & APPARATUSES ....................................................9

3.1. APPARATUS ...................................................................................................................................9

3.2. AM MODULATION PROCEDURE .....................................................................................................9

3.3. AM DEMODULATION PROCEDURE ............................................................................................... 10

SECTION 4. RESULTS & DISCUSSION ........................................................................................... 11

4.1. AM MODULATION WITH LAB-VOLT ......................................................................................... 11

4.2. AM DEMODULATION WITH LAB-VOLT ....................................................................................... 17

SECTION 5. CONCLUSION & RECOMMENDATIONS ................................................................ 27

SECTION 6. REFERENCES ................................................................................................................ 27



List of Figures

FIGURE 2.1: FREQUENCY SPECTRUM OF AM SIGNAL .................................................................................6

FIGURE 2.2: SYSTEM SCHEMATICS OF LAB-VOLT AM/DSM RECEIVER .................................................7

FIGURE 4.1: DEFAULT SETUP ..................................................................................................................... 11

FIGURE 4.2: EMAX AND EMIN MEASUREMENTS OF THE DEFAULT SETUP .................................................. 12

FIGURE 4.3: EMAX AND EMIN MEASUREMENTS OF THE SQUARE WAVE MESSAGE SIGNAL SETUP .......... 12

FIGURE 4.4: THE REFERENCE SIDEBAND POWER MEASUREMENTS (CONSTANT AMONG TRIALS) ........ 14

FIGURE 4.5: CARRIER BAND POWER LEVEL VS. MODULATION INDEX .................................................... 15

FIGURE 4.6: FREQUENCY SPECTRUM WITH MESSAGE SIGNAL FREQUENCY AT 10KHZ .......................... 16

FIGURE 4.7: FREQUENCY SPECTRUM WITH MESSAGE SIGNAL FREQUENCY AT 5KHZ ............................ 16

FIGURE 4.8: FREQUENCY SPECTRUM WITH MESSAGE SIGNAL FREQUENCY AT 11KHZ .......................... 17

FIGURE 4.9: BODE MAGNITUDE PLOT OF RF BAND-PASS FILTER ............................................................ 19

FIGURE 4.10: SPECTRUM ANALYZER CAPTURE AT 974.14KHZ ............................................................... 19



FIGURE 4.13: (A) ORIGINAL AM MODULATED SIGNAL SPECTRUM/AM RECEIVER INPUT (LEFT) ....... 21

FIGURE 4.14: TIME-DOMAIN IF OUTPUT .................................................................................................. 21

FIGURE 4.15: TIME-DOMAIN ENVELOPE DETECTOR OUTPUT (DEMODULATED SIGNAL) ...................... 22

FIGURE 4.16: DISTORTED DEMODULATION OUTPUT FROM AM ‘OVER-MODULATION’......................... 24

FIGURE 4.17: IF FILTER OUTPUT (LEFT, CH.1) & DEMODULATED OUTPUT (RIGHT, CH.2) WITH ¼ RF

GAIN TURN AND ACF ON ................................................................................................................. 24

FIGURE 4.18: IF FILTER OUTPUT (LEFT, CH.1) & DEMODULATED OUTPUT (RIGHT, CH.2) WITH 2/4 RF


FIGURE 4.19: IF FILTER OUTPUT (LEFT, CH.1) & DEMODULATED OUTPUT (RIGHT, CH.2) WITH 3/4 RF


FIGURE 4.20: IF FILTER OUTPUT (LEFT, CH.1) & DEMODULATED OUTPUT (RIGHT, CH.2) WITH FULL RF


FIGURE 4.21: DEMODULATED OUTPUT (CH.2) WITH AND ACF OFF AND WITH (A) 1/4 RF GAIN TURN

(LEFT) & (B) 2/4 RF GAIN TURN (RIGHT) .......................................................................................... 25

FIGURE 4.22: DEMODULATED OUTPUT (CH.2) WITH AND AGC OFF AND WITH (A) 3/4 RF GAIN TURN

(LEFT) & (B) FULL RF GAIN TURN (RIGHT) ........................................................................................ 26



List of Tables

TABLE 4.1 AM ENVELOPES, SPECTRA, AND MODULATION INDICES FOR FIVE DIFFERENT TRIALS .......... 13

TABLE 4.2 MODULATION INDEX AND EFFICIENCY ...................................................................................... 15

TABLE 4.3 FREQUENCY AND OUTPUT FOR RF FILTER INPUT OF 1V PEAK-TO-PEAK ................................. 18

TABLE 4.4 DEMODULATION VOLTAGE OUTPUT FOR DIFFERENT MESSAGE SIGNAL FREQUENCY .............. 23

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Section 1. Objectives and Introduction

In this laboratory, we use Lab-Volt modularized equipment to modulate a sin and square wave,

observing the percentage modulation, resulting AM waveforms, and power levels (with the

Spectrum Analyzer). We then verify efficiency for a unity modulation index and note the

effects of changing the modulation percentage, as well as the frequency of a sinusoidal message

signal. On the other end, we then used the demodulator block to examine the band-pass RF

filter (with both the scope and spectrum analyzer), as well as examine the characteristics of

AGC. We also vary the percentage modulation on the demodulator side and observe the

effects, as well as noting the results when the message signal frequency is varied. Finally, we

look at the effects of changing the RF gain control in quarter-turn steps, with AGC either

enabled or disabled. Ultimately, the big picture is that we modulate a signal, and demodulate

it to recover the original message signal.

Section 2. Theory

2.1. AM Modulation

The frequency spectrum of a modulated AM signal consists of a central carrier band and two

sidebands, as shown in Figure 2.1.

Figure 2.1: Frequency Spectrum of AM Signal

The position of the central band is determined by the frequency of the carrier signal while that

of the two sidebands are determined by the frequency of the message signal, as shown in Eq. 2.1

and Eq. 2.2.



mc ffUSB

.

(Eq. 2.1)

mc ffLSB

.

(Eq. 2.2)

The modulation efficiency is defined in Eq. 2.3, which describes, in percentage, the proportion

of the carrier signal being utilized to carry the message signal. The equation also shows that the

modulation efficiency is proportional to the modulation index.

)(1

)(22

22

tma

tmaEfficiency

n

n

.

(Eq. 2.3)

2.2. AM Demodulation

Figure 2.2: System Schematics of LAB-VOLT AM/DSM Receiver

To study the fundamental process of AM demodulation, the LAB-Volt AM/DSB Receiver

(Model 9411), which is a super heterodyne receiver with three different detection circuits, is

used in the second part of this laboratory. Figure 2.2 illustrates the system schematics of the

receiver. The supersonic heterodyne receiver, or superhet, was invented by Edwin Armstrong

in 1918 (Douglas, 1990), to overcome the poor frequency stability and selectivity of tuned radio

frequency receivers or other types of regenerative receivers.



Let xc(t) =Ac [1+amn(t)] cos(ωct) be the transmitted AM modulated signal to the input of the

superhet receiver. The xc(t) signal from the receiver input will first pass through a RF filter with

a center frequency equal to the carrier frequency, ωc, eliminating all image signals and other

signals with frequencies different from the carrier. Then, the carrier signal is translated to a

preset frequency or the intermediate-frequency (IF) by multiplying a local oscillator to the input

signal. The frequency of the local oscillator signal is determined by,

IFcLO . (Eq. 2.4)

Assuming the oscillator uses high-side tuning, the translated input signal after multiplication by

the oscillator’s signal of 2cos[(ωc + ωc)t] is,

])2cos[()](1[)cos()](1[)( ttamAttamAte IFcncIFnc .

(Eq. 2.5)

The undesired term on the second right-hand side term of Eq. 2.5 is then removed by an IF filter

with a center frequency of ωIF. As a result, the superhet filter has successfully translated the

input signal to the predetermined IF frequency by the IF filter, which can provide most of the

pre-detection and complex filtering as a standalone filter with a fixed frequency. This filter thus

allows a greater flexibility in frequency selecting and more stable frequency filtering.

After the IF filter removes the unwanted frequency, the new translated signal is then passed

through the AM demodulator or an envelope detector to retrieve the original modulated signal,

At the output, the LAB-Volt AM/DSB Receiver has an additional filter for smoothing unwanted

high frequency ripple. However, the message signal must be ensured to have the same or lower

bandwidth compared to the IF filter; otherwise, the retrieved message would be weak and

distorted.

In addition to the standard superhet receiver configuration, the LAB-Volt AM/DSB Receiver

(Model 9411) has a built-in automatic gain control (AGC) feedback circuit, which adjusts the

gain of the receiver’s RF stage. By switching AGC on, the gain of RF filter will be changed

according to the output of the envelop detector. As the output voltage of the envelope detector

increases, the gain will decrease to ensure the gain is constant and the input of the envelope

detector is not over-amplified and distorted.



Section 3. Experiment Procedures & Apparatuses

3.1. Apparatus

The following apparatus are to be used in this lab:

- LabVolt Power Supply and Dual Audio Amplifier Model 9401

- LabVolt Dual Function Generator Model 9402

- LabVolt Frequency Counter Model 9403

- LabVolt AM/DSB/SSB Generator Model 9410

- LabVolt AM/DSM Receiver Model 9411

- Scope

- Spectrum Analyzer

3.2. AM Modulation Procedure

First, connect all the different models to the power supply. Before turning on the power supply,

make sure all the gain and level controls of the LabVolt modules are at minimum. After the

power has been turned on, push in the carrier level knob on the AM/DSB/SSB Generator

module in order to enable the linear amplification and linear overmodulation. Next, set the

carrier level and RF gain control to maximum, and adjust the RF tuning control to set the carrier

frequency from the RF output port to 1,100 kHz.

Set up the function generator to produce a 0.5V peak-to-peak sine wave with frequency of 10

kHz as the message signal. Next, use a T-connector to connect the function generator output to

the input of the AM/DSB/SSB Generator module and Channel 2 of the scope. Set the scope to

be triggered by Channel 2 and connect the AM/DSB/SSB Generator module output to Channel

1 of the scope. After both Channel 1 and 2 of the scope have been connected, compute and

record the percent modulation. After everything has been done, change the message signal to

square wave. Compute and record the percent modulation for the square wave message signal.

Again, set the message signal to 10 kHz sine wave and generate 5 different AM waveforms that

have percent modulation between 20 – 100% and compare their spectra. Next, use the spectrum

analyzer to record the power levels of the carrier signal and each of the two sidebands. From the

record, use the frequency domain formula to compute the modulation index. After that, use the



power measured to compute the efficiency of the modulation. Once the efficiency has been

calculated, set the message signal to sinusoidal with a modulation index of 1 and verify that the

efficiency of this signal is 33%. Lastly, vary the frequency of the sinusoidal message signal and

observe the change in the signal spectrum.

3.3. AM Demodulation Procedure

Connect the AM/DSB receiver module in the system configuration if it has not been done so.

Next, use the frequency counter, the scope, or the spectrum analyzer to set the LO (local

oscillator) to 1.455 MHz at point 4 of the receiver and then adjust the frequency analyzer so that

it has a center frequency at 1 MHz and a span of 50 kHz/div. After the spectrum analyzer has

been set, disconnect the message signal and connect a BNC cable to the input of the receiver.

Make sure that the AGC (automatic gain control) is off (pulled out). Next, set the generator

carrier level to maximum in linear modulation mode and adjust the RF gain (amplifier A2)

control to approximately 11 o’clock.

With the previous settings, connect the RF output signal at point 3 of the receiver to the scope

and observe while tuning the carrier frequency of the generator from 950 kHz to 1,050 kHz.

After that, connect the RF output signal at point 3 to the spectrum analyzer. Observe the signal

and record the frequency at which the peak signal is obtained on the spectrum analyzer. In

order to find out the response of the RF filter, record the frequency that is 3 dB below the peak

of the signal.

Next, connect the AM generator output to the AM receiver input. Turn AGC on and connect the

frequency counter to IF output (point 7) of the receiver and adjust the LO for an exact reading of

455 kHz. Apply a 100 mV peak-to-peak, 2.5~5.0 kHz sinusoidal to the input of the AM

generator. After finished setting up, set the spectrum analyzer to have a center frequency of 455

kHz and a span of 10 kHz/div. Observe and sketch the IF output signal in both time and

frequency domains.

After finishing the sketches, connect the detector output (point 8) of the receiver to the scope

and select the envelope detector. Next, use a T-connector from the function generator and

trigger the scope using Channel 2. Vary the modulation audio input from 1-10 kHz in steps of 2

kHz and record the output levels. Observe the effects of varying the modulation index have on

the output levels.



Next, set the modulator frequency to 2.5 kHz at around 100% modulation and adjust the RF

gain control from the generator in 0.25 turn steps from minimum to maximum. Record the

demodulated output voltage. Lastly, switch off the AGC and adjust the RF gain control from

minimum to maximum again. Record the demodulated output voltage as well.

Section 4. Results & Discussion

4.1. AM Modulation with LAB-VOLT

The first modulation index calibration is made with a carrier frequency of 1,100kHz and a sinusoidal

message signal of 10kHz. The input and the output spectra of the AM Modulator are shown in Figure 4.1.

Figure 4.1: Default Setup

Next, we zoom in on the modulated output and determine the maximum and minimum of the modulation,

from which we calculate the percentage modulation as shown in Figure 4.2 and Eq.4.1.



Figure 4.2: EMAX and EMIN Measurements of the Default Setup

%4.71714.0

0.96576

0.96576

mVmV

mVmV

EE

EEa

MINMAX

MINMAX.

(Eq. 4.1)

Holding the frequency the same, we use a square-wave message signal instead of the original sinusoidal

one, and compute the percentage of modulation shown in Figure 4.3 and Eq.4.2, which we note is

identical to Eq. 4.1.

Figure 4.3: EMAX and EMIN Measurements of the Square Wave Message Signal Setup

%4.71714.0

0.96576

0.96576

mVmV

mVmV

EE

EEa

MINMAX

MINMAX.

(Eq. 4.2)

Next, we switch back to sinusoidal message signal and produces modulation signals with 5 different

modulation indices. The envelopes of the 5 different signals are presented in the first column of Table 4.1,



and the frequency spectrum with carrier band power measurements in the second column. The reference

sideband power measurement, which should roughly remain constant among different trials, is shown in

Figure 4.4. With all the power measurements obtained in the frequency spectra, we apply the power

formula to determine the modulation index for each trial as shown in the third column. As shown in

Table 4.1, as the modulation index approaches unity, the power level of the carrier band approaches

100%.

Table 4.1 AM Envelopes, Spectra, and Modulation Indices for Five Different Trials

AM Envelope Frequency Spectrum Modulation Index Calculation

791.0

10

2

20

)21.8(16.0

dBdBa

Mod. % = 79.1%

876.0

10

2

20

)21.8(04.1

dBdBa

Mod. % = 87.6%

916.0

10

2

20

)21.8(43.1

dBdBa

Mod. % = 91.6%



971.0

10

2

20

)21.8(93.1

dBdBa

Mod. % = 97.1%

000.1

10

2

20

)21.8(19.2

dBdBa

Mod. % = 100%

Figure 4.4, or as previously discussed, the reference sideband power measurement, is shown below.

Figure 4.4: The Reference Sideband Power Measurements (Constant Among Trials)

Clearly, as the modulation index increases, the peak height of the carrier band reduces and the power

measurement for the carrier band decreases as well. It reflects the fact that when modulation percentage is

below 100%, not all of the carrier amplitude is being used to carry the message signal. As the modulation

index approaches unity, the entire amplitude of the carrier becomes occupied with the message signal, as

shown in the AM Envelope column in Table 4.1. To furthermore visualize the change of power of the

carrier band with respect to the modulation index, Figure 4.5 is presented to show the detail trend.



P owe r of the Ca rrie r Ba nd vs Modula tion Inde x

-2.5

-2

-1.5

-1

-0.5

0

0.6 0.7 0.8 0.9 1 1.1

Modula tion Inde x (a )

P owe r (dB)

Figure 4.5: Carrier Band Power Level vs. Modulation Index

We have tried to reduce the amplitude of the carrier band even further to produce a modulation index

greater than 1. The resultant AM signal turns out to be over-modulated (the upper envelope and the lower

envelope overlap), and cannot be demodulated properly at the demodulator end.

When modulation index reaches 1, the modulation efficiency is numerically determined to be 33% with

Eq. 4.1. The comparison calculations are shown in Table 4.2.

Table 4.2 Modulation Index and Efficiency

Modulation Index Efficiency Calculation, <mn2(t)> = 0.5

0.791 %83.32%100

5.0791.01

5.0791.02

2

Efficiency

0.916 %55.29%100

5.0916.01

5.0916.02

2

Efficiency

1.000 %33.33%100

5.0000.11

5.0000.12

2

Efficiency



Finally, we attempted to vary the frequency of the sinusoidal message signal and observe the change in

modulation index. The results (Figures 4.6 to 4.8) show that the carrier band power level, which is

proportional to modulation index, is independent of the frequency of the message signal. The sidebands in

the frequency spectrum, however, move away from the central carrier band as the frequency increases.

This phenomenon is also within expectation since the change of frequency of the message signal would

change the position of the sidebands in the frequency spectrum.

Figure 4.6: Frequency Spectrum with Message Signal Frequency at 10kHz





4.2. AM Demodulation with LAB-Volt

Here we examine the demodulation side of AM radio by studying the RF and IF filters, as well

as the effects of input frequency, percentage modulation, and RF gain with and without AGC.

4.2.1 Study of the RF Filter

As outlined in the procedure section, we take numerous scope data points from around 950kHz

to 1050kHz, assuming that the input voltage is held constant at 1V, and measuring the

magnitude of the output using cursors. From our data it is immediately obvious that the

center frequency, or the largest peak-to-peak magnitude, occurs around 975kHz. Another

observation is that the upper and lower cutoff frequencies exhibit an extremely sharp roll-off, as

expected. However, by increasing the resolution of our data point acquisition, we were able to

verify that the upper cut-off frequency occurs around 950 and 1000kHz, respectively, by

verifying the 3dB points as shown in Table 4.3 below.



Table 4.3 Frequency and Output for RF Filter Input of 1V Peak-to-Peak

Frequency (kHz)

Voltage

Obtained

(mV)

Input

(mV) Gain Gain (dB)

900 92 1000 0.092 -20.724243

923 108 1000 0.108 -19.331525

945 142 1000 0.142 -16.954233

947 146 1000 0.146 -16.712943

949 152 1000 0.152 -16.363128

950 158 1000 0.158 -16.026858

955 170 1000 0.17 -15.391022

960 192 1000 0.192 -14.333975

965 200 1000 0.2 -13.9794

970 214 1000 0.214 -13.391725

972 216 1000 0.216 -13.310925

975 222 1000 0.222 -13.072941

981 212 1000 0.212 -13.473283

985 204 1000 0.204 -13.807397

990 180 1000 0.18 -14.89455

998 162 1000 0.162 -15.8097

1000 148 1000 0.148 -16.594766

1025 119 1000 0.119 -18.489061

1050 90 1000 0.09 -20.91515

Furthermore, graphing the magnitude Bode plot, we can get a good picture of the behavior of

the RF band-pass filter, as shown in Figure 4.9 below.



Figure 4.9: Bode Magnitude Plot of RF Band-pass Filter

Given these results, we can use the spectrum analyzer to verify the correctness of our

assumptions. Figure 4.10 below shows a capture taken at the hypothesized center frequency of

the filter, or 975kHz.

Figure 4.10: Spectrum Analyzer Capture at 974.14kHz

As we can see, the peak occurs at -34.36dBm. We can then compare this to similar captures

taken around 1MHz and 950kHz, and verify that the resulting drop is indeed 3dB. Figure 4.11

shows a capture taken around 950kHz.




Here the peak is at -37.73dBm, which is approximately 3dBm away from our center peak at

-34.36dBm. Figure 4.12 shows a capture taken at around 1Mhz.


Here, the peak is at -37.66dBm, which once again is about 3dBm away from our center peak.

So this measurement affirms our initial observations based on peak-to-peak voltage levels.

4.2.2 Study of the IF Filter

By connecting an AM modulated input with a 0.100mV peak-to-peak, and a 3.25kHz sinusoidal

message signal to the Lab-Volt AM receiver, we successfully retrieved the frequency-translated

modulated signal to the IF frequency from the original carrier frequency. The translation



method used by the superhet filter is explained in Section 2. Figure 4.13 (a) illustrates the

modulated signal in frequency domain with a carrier frequency of 974.38 kHz at the input of

receiver, and Figure 4.13 (b) shows the corresponding IF filter output spectrum with a carrier

frequency of 456.13kHz.

Figure 4.13: (a) Original AM Modulated Signal Spectrum/AM Receiver Input (Left)

(b) Translated AM Modulated Signal Spectrum /IF Filter Output) (Right)

Figure 4.13 (a) and (b) both show the message signal is on both sidebands of the carrier, which

are approximately 3kHz away from the center frequency. These sideband frequencies match the

3.25KHz frequency of the original message signal inputted at the AM generator. The magnitude

of the message signal has also been amplified by approximately 18dbM through the RF and IF

filters.

Figure 4.14: Time-Domain IF Output

From Figure 4.14, the time-domain IF output confirms the modulated signal has successfully

translated to the carrier frequency of around 455kHz without much distortion on the message



envelope.

By sending the IF output to the envelope detector, the original message signal was successfully

retrieved at the detector’s output as shown in Figure 4.15.

Figure 4.15: Time-Domain Envelope Detector Output (Demodulated Signal)

[Ch.1 = Demodulated Input, Ch.2 = Original Message Signal/AM Generator Input]

However, compared to the original message signal, the demodulated output has a change in the

amplitude and the phase. These amplitude distortion can be explained by the uncoordinated

amplitude scaling between various filters, including RF and IF filters. The presence of the phase

deviation can be attributed to the fact that the local oscillator’s frequency has a phase error ө(t)

due to signal interferences that yields an IF filter output of,

)](cos[)](1[)( tttamAtx IFncIF . (Eq. 4.3)

As a result, the envelope detector carries over the phase error of the translated modulated

output.

4.2.3 Effects of Input Frequency and Percentage Modulation

By varying the frequency of the 0.1V peak-to-peak message signal and the modulation index,

we obtained the following demodulated voltage output at the AM receiver:



Table 4.4 Demodulation Voltage Output for Different Message Signal Frequency

Message Input Signal

Frequency (kHz)

Demodulated

Output Voltage (mV)

Demodulated Output

Voltage (mV)

Modulation Index (a) 1 0.7

1 392 304

2 480 160

4 420 80.6

6 480 40.8

8 320 48

10 132 34

As the frequency of the message signal increased, the amplitude of the demodulated signal

decreased due to the narrow bandwidth of the IF filter. Since the message signal is carried on

the sideband of the IF’s carrier frequency, the narrow bandwidth of the IF filter limited the

allowable bandwidth of the input message. Thus, as the input message frequency increases, the

message sideband signals will move farther away from the IF carrier frequency, causing an

attenuation of the message signal at the IF filter’s output. When the message signal has a

frequency greater than the IF filter’s bandwidth, the message signal will be attenuated and

disappeared at the demodulated output.

When the index modulation decreased, the amplitude of the demodulated signal was also

lowered for the same message signal with the same frequency as shown in Table 4.4. This

observation can be explained by the fact that the magnitude of the message envelope decreased

as the modulation index decreased. If the index modulation was increased greater than 1, an

over-modulation was resulted and therefore a distorted demodulated output was obtained as

shown in Figure 4.16.



Figure 4.16: Distorted Demodulation Output from AM ‘Over-modulation’

4.2.4 Effects of RF Gain with and without Auto-Gain Control

By switching ACF on and varying the RF gain, we obtained the following IF filter outputs and

the corresponding demodulated outputs as illustrated in Figure 4.17 to Figure 4.20.

Figure 4.17: IF Filter Output (left, Ch.1) & Demodulated Output (Right, Ch.2) with ¼ RF Gain Turn and ACF ON

Figure 4.18: IF Filter Output (left, Ch.1) & Demodulated Output (Right, Ch.2) with 2/4 RF Gain Turn and ACF ON



Figure 4.19: IF Filter Output (left, Ch.1) & Demodulated Output (Right, Ch.2) with 3/4 RF Gain Turn and ACF ON

Figure 4.20: IF Filter Output (left, Ch.1) & Demodulated Output (Right, Ch.2) with Full RF Gain Turn and ACF ON

As the AM generator’s RF gain increased, the AM receiver input increased and therefore the

corresponding demodulated output increased. However, when RF gain was near its maximum,

the amplification reached its output limit and generated a distorted demodulated output shown

in Figure 4.20.

With ACF switched off, Figure 4.21 and Figure 4.22 illustrate the resulting demodulated outputs.

Figure 4.21: Demodulated Output (Ch.2) with and ACF OFF and with (a) 1/4 RF Gain Turn (Left) & (b) 2/4 RF Gain Turn (Right)



Figure 4.22: Demodulated Output (Ch.2) with and AGC OFF and with (a) 3/4 RF Gain Turn (Left) & (b) Full RF Gain Turn

(Right)

Figure 4.21 and 4.22 indicate the outputs were already distorted at low values of generator’s RF

gains when AGC was switched off. This observation confirms that AGC is working. Without

the AGC feedback circuit, the gain of the receiver’s RF filter would not be regulated to ensure

that the output is not over-amplified and distorted. As a result, the AGC keeps the

demodulated signal undistorted when a high voltage level is inputted at the receiver. On the

other hand, when the generator’s RF gain increased to be near the maximum and AGC is turned

on, the receiver’s gain could not be reduced anymore because AGC has reached its gain

reduction limit.



Section 5. Conclusion & Recommendations

In the modulation part of this lab, we had no problems correlating theory and our lab results.

We noted that both a sin and square wave message signal produce the same modulation index.

We also observed the AM envelope for different modulation indices and confirmed that as the

percentage modulation increases, the power decreases until unity, when the entire carrier is

used to carry the message signal. We also observed over-modulation distortion. We then

confirmed the efficiency of 33% with modulation index a=1, and finally varied the frequency of

the sinusoidal message signal, observing the change in modulation index, and noted that

although the carrier power band level is frequency independent, the sidebands will move away

from the central carrier band as frequency increases.

In the demodulation part of this lab, we successfully demodulated the AM input signal and

retrieved the original signal. However, the uncoordinated gains between filters, the narrow

bandwidth of IF filter and the inaccuracy of the local oscillation frequency caused amplitude

scaling and phase margin error. To ensure the output is not attenuated by the IF filter, the

modulated input should have a modulation index near 1 and the bandwidth of the message

signal should be minimized as much as possible. We also studied and verified the operation of

the AGC circuit, which keeps the demodulated signal undistorted when the high-voltage input

is too high at the receiver.

Section 6. References

Ziemer, Rodger and Tranter, William. Principles of Communications: Systems Modulation

and Noise, 5e. Wiley 2002.

Douglas, A. (1990, November). Who Invented the Superheterodyne?. Antique Radios. Retrieved

October 9, 2006, from http://antiqueradios.com/superhet/

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