AM Transmission Analog Communications Exercise 1: Amplitude Modulation EXERCISE OBJECTIVE When you have completed this exercise, you will be able to describe the generation of amplitudemodulated signals and explain how the message signal affects the shape of the AM signal. You will be able to calculate the modulation index and percentage of modulation from AM signal parameters. You will oscilloscope to make AM signal measurements. DISCUSSION There are many ways to produce an AM signal, but all of them must allow the amplitude variations of the message signal to be impressed onto the carrier signal. A balanced modulator integrated circuit (IC), which is a nonlinear device, can function as the amplitude modulator. The message and carrier signals are inputs to the amplitude modulator. 84 FACET by Lab-Volt
Analog Communications AM Transmission The amplitude modulator mixes the message and carrier signals to frequency translate the message signal: it shifts the message signal to the carrier signal s frequency. The envelope of the AM signal is a copy of the message signal and varies at the same frequency as the message signal. If the carrier signal frequency (f c ) is 1000 khz and the message signal frequency (f m ) is 2 khz, how many frequencies are present in the AM signal? a. 1 b. 2 c. 3 FACET by Lab-Volt 85
AM Transmission Analog Communications If the carrier signal frequency (f c ) is 1000 khz and the message signal frequency (f m ) is 2 khz, what is the lower sideband (LSB) frequency? a. 1002 khz b. 998 khz c. 1000 khz PROCEDURE The following procedure is divided into three sections. AM Signals Modulation Index and Percentage of Modulation Each section starts with an explanation of the AM signals that you will observe and the parameters that you will measure and calculate. AM Signals In this PROCEDURE section, you will observe the effect of the message signal on the AM signal. If you must restart this section at a later time, setup the circuit by completing the steps in the Adjust the Modulator for a Less-Than 100% AM Signal section in the Resources of this manual. Circuit setup guides and the switch function guide are also located in the Resources. Increasing or decreasing the amplitude of the message signal causes higher and lower peaks and valleys in the envelope of the AM signal. 86 FACET by Lab-Volt
Analog Communications AM Transmission The message signal shown produces which AM signal shown? a. A b. B c. C shown. Be sure to place a two-post connector in the 1000 khz position on the VCO-LO circuit block. Set switches S1, S2, and S3 to OFF. Connect the oscilloscope channel 1 probe to the message signal input (M) of the MODULATOR. While observing the signal on channel 1, adjust the signal generator for a 0.2 V pk-pk, 2 khz sine wave signal at M. NOTE: When making oscilloscope measurements or observations, be sure that you connect the probe s ground clip to a ground terminal on the circuit board. Scope probes should be set on X10 to prevent the AM signal from being loaded down. FACET by Lab-Volt 87
AM Transmission Analog Communications Connect the channel 2 probe to the carrier signal input (C) of the MODULATOR. While observing the signal on channel 2, adjust VCO-LO for a 0.2 V pk-pk, 1000 khz signal at C. Adjust the carrier frequency with the NEGATIVE SUPPLY knob on the base unit, and adjust the carrier amplitude with the knob on the VCO-LO circuit block. Connect the oscilloscope channel 2 probe to the output of the MODULATOR. Set the oscilloscope vertical mode to ALT, and trigger on channel 1(the message signal). NOTE: When making oscilloscope measurements or observations, be sure that you connect the probe s ground clip to a ground terminal on the circuit board. Scope probes should be set on X10 to prevent the AM signal from being loaded down. 88 FACET by Lab-Volt
Analog Communications AM Transmission Adjust the MODULATOR potentiometer knob so that the AM waveform on oscilloscope channel 2 has 2.0 V between the upper and lower peaks, as shown. Does the AM signal envelope (channel 2) have the same shape and frequency as the message signal (channel 1)? a. yes b. no You have set the carrier signal frequency (f c ) to 1000 khz and the message signal frequency (f m ) to 2 khz. What frequencies are present in the frequency spectrum of the AM signal? a. 2 khz and 1000 khz b. 1000 khz and 1002 khz c. 998 khz, 1000 khz, and 1002 khz FACET by Lab-Volt 89
AM Transmission Analog Communications Change the signal generator function from a sine wave to a square wave. Did the envelope of the AM output signal change from a sine wave to a square wave? a. yes b. no Return the signal generator function to a sine wave. While observing the AM output signal on channel 2, vary the signal generator s AF FREQUENCY control knob to vary the message signal frequency. Did the frequency of the AM signal envelope change to correspond to the frequency of the message signal? a. yes b. no Readjust the message signal frequency to 2 khz. While observing the AM output signal, vary the signal generator s AF LEVEL control knob to vary the amplitude of the message signal. Did the amplitude of the AM signal envelope change to correspond to the amplitude of the message signal? a. yes b. no 90 FACET by Lab-Volt
Analog Communications AM Transmission Modulation Index and Percentage of Modulation In this PROCEDURE section, you will make oscilloscope measurements of AM signals and calculate modulation index (m) and percentage of modulation (% Mod.). If you are not proceeding directly from the AM Signals portion of this procedure or if you must restart this section at a later time, setup the circuit by completing the steps in the Adjust the Modulator for a Less-Than 100% AM Signal section in the Resources of this manual. Circuit setup guides and the switch function guide are also located in the Resources. Modulation index (m) and percentage of modulation (% Mod.) are the parameters that quantify the peaks and valleys in the AM signal. The modulation index is the ratio of the message signal peak amplitude to the carrier signal peak amplitude. The calculation method for determining the modulation index from the AM signal is shown. Percentage of modulation is the modulation index expressed as a percentage. If A equals 4 V and B equals 1 V, what is the modulation index (m)? m = (Recall Value 1) FACET by Lab-Volt 91
AM Transmission Analog Communications What is the percentage of modulation (% Mod.) when m is 0.6? % Mod. = % (Recall Value 2) When the message signal is constantly changing, as in a voice or music signal, the frequency of the modulating envelope and the percentage of modulation are constantly changing. This constant change makes it practically impossible to determine the modulation index from the waveform. To get an oscilloscope display more useful for measuring modulation characteristics, use the trapezoidal method. This method permits a uniform display when the message signal changes. To use the trapezoidal method, put the oscilloscope in the X-Y mode to obtain a trapezoidal pattern of the signals. Then connect the message signal to the X input and the modulated signal to the Y input. You can calculate the modulation index (m) from the measurements of A and B as shown on the trapezoidal oscilloscope display. 92 FACET by Lab-Volt
Analog Communications AM Transmission If A equals 5 V and B equals 0.5 V, what is the modulation index (m)? m = (Recall Value 3) S1, S2, and S3 should be OFF. On oscilloscope channel 1, adjust the peak-to-peak voltage of the message signal to 0.2 V pk-pk. If necessary, adjust the MODULATOR potentiometer knob so that the AM waveform shown on channel 2 has 2.0 V between the upper and lower peaks, as shown. FACET by Lab-Volt 93
AM Transmission Analog Communications On oscilloscope channel 2, measure (in volts) the vertical height between the upper and lower valleys (measurement B) of the modulated waveform. B = V (Recall Value 4) Calculate the modulation index (m). B = V (Step 3, Recall Value 4) m = (Recall Value 5) Calculate the percentage of modulation (% Mod.). m = (Step 4, Recall Value 5) % Mod. = % (Recall Value 6) 94 FACET by Lab-Volt
Analog Communications AM Transmission While observing the AM signal on channel 2, increase the amplitude of the message signal until the AM signal envelope waveform touches the zero reference line, as shown. The distance represented by B on the AM signal waveform is now 0 V. On oscilloscope channel 2, measure (in volts) the vertical distance between the upper and lower peaks of the modulated waveform (measurement A). A = V (Recall Value 7) Calculate the modulation index. A = V (Step 7, Recall Value 7) m = (Recall Value 8) FACET by Lab-Volt 95
AM Transmission Analog Communications Calculate the percentage of modulation (% Mod.). m = (Step 8, Recall Value 8) % Mod. = % (Recall Value 9) Adjust the amplitude of the message signal on channel 1 to 0.20 V pk-pk. Adjust the MODULATOR potentiometer knob counterclockwise (CCW) so that the AM waveform on channel 2 is not 100% modulated and has 2.0 V between the upper and lower peaks, as shown. Place the oscilloscope in X-Y mode. Adjust the X and Y attenuators to obtain a trapezoidal pattern as shown. On the oscilloscope screen, measure A (in volts). A = V (Recall Value 10) 96 FACET by Lab-Volt
Analog Communications AM Transmission On the oscilloscope screen, measure B (in volts). B = V (Recall Value 11) Calculate the modulation index (m). A = V (Step 11, Recall Value 10) B = V (Step 12, Recall Value 11) m = (Recall Value 12) Calculate the percentage of modulation (% Mod.). m = (Step 13, Recall Value 12) % Mod. = % (Recall Value 13) FACET by Lab-Volt 97
AM Transmission Analog Communications Are your trapezoidal method measurements of modulation index (m) and percentage of modulation (% Mod.) similar to the results you obtained using the AM signal? a. yes b. no Trapezoidal method Using AM signal m (Step 13, Recall Value 12) (Step 4, Recall Value 5) % Mod. (Step 14, Recall Value 13) (Step 5, Recall Value 6) In this PROCEDURE section, you will observe 100% modulation, observe overmodulation, and calculate If you are not proceeding directly from the AM Signals or Modulation Index and Percentage of Modulation portion of this procedure, or if you must restart this section at a later time, setup the circuit by completing the steps in the Adjust the Modulator for a Less-Than 100% AM Signal section in the Resources of this manual. Circuit setup guides and the switch function guide are also located in the Resources. The AM waveform shown is 100% modulated (modulation index equal to 1.0); the valleys touch the zero reference line. modulation is desirable in AM communications for maximum sideband power. 98 FACET by Lab-Volt
Analog Communications AM Transmission Which AM waveform illustrates 100% modulation? a. A b. B c. C When overmodulation occurs (greater than 100%), both sides of the modulation envelope cross over the zero reference line. In AM communications, overmodulation causes fake sideband frequencies called sideband splatter. This splatter causes distortions in the receiver and interference with other radio stations. As the modulation index is increased, the power level of the sidebands increases while the carrier power remains constant. Because useful information contained in the radio frequency (RF) signal is located in the sidebands, maximizing the sideband power by increasing the modulation index is desirable. However, in AM, the modulation index must not be greater than 1, or distortion and interference will occur. The total power in an AM signal (P T ) is the sum of the carrier power (P C ) and the lower and upper sideband power (P SB ). P T = P C + P SB If P C is 40 kw and P SB is 10 kw, what is P T? P T = kw (Recall Value 1) ) is the fraction of total power that is contained in the sidebands. = P SB T FACET by Lab-Volt 99
AM Transmission Analog Communications It is also related to the modulation index. = m 2 2 ) )? = m 2 2 ) = (Recall Value 2) If the total power (P T is the sideband power (P SB )? (Recall Value 2), what P SB = x P T = kw (Recall Value 3) S1, S2, and S3 should be OFF. as shown. Increase the message signal amplitude on channel 1 by adjusting the AF LEVEL knob on the signal generator until the AM signal appears as shown. The AM signal is a. not modulated. b. overmodulated. Is the modulation index of the AM signal greater than or less than 1? a. >1.0 b. <1.0 100 FACET by Lab-Volt
Analog Communications AM Transmission Is an overmodulated signal desirable in AM communications? a. yes b. no Reduce the message signal so that the AM signal on channel 2 is 100% modulated, as shown. ). = m 2 2 ) = (Recall Value 4) ) is (Step 6, Recall Value 4), calculate the sideband power (P SB ). P SB = x P T = kw (Recall Value 5) FACET by Lab-Volt 101
AM Transmission Analog Communications CONCLUSION The AM signal envelope has the same shape and frequency as the message signal. Modulation index (m) and percentage of modulation (% Mod.) are parameters that quantify the peaks and valleys in the AM signal envelope. Using the trapezoidal method on an oscilloscope, you can determine the modulation index from the message and AM signals. An AM signal with over 100% modulation is not desirable because it causes receiver distortion and interference with other stations. ), which is the ratio of sideband power (P SB ) to total signal power (P T ). REVIEW QUESTIONS 1. When the message signal frequency and amplitude change, what happens to the envelope of the AM signal? a. Only the amplitude of the AM signal envelope changes. b. Only the frequency of the AM signal envelope changes. c. The frequency and amplitude of the AM signal envelope change. d. The frequency and amplitude of the carrier signal change. 2. If a 2000 khz carrier signal (f c ) is amplitude modulated by a 3 khz message signal (f m ), what frequencies are present in the frequency spectrum of the AM signal? a. 3 khz, 6 khz, and 2000 khz b. 1997 khz, 2000 khz, and 2003 khz c. 2000 khz and 2003 khz d. 1997 khz and 2003 khz 3. What is the modulation index of the AM signal shown? a. 1.5 b. 1.0 c. 0.5 d. 0.1 102 FACET by Lab-Volt
Analog Communications AM Transmission 4. What is the percentage of modulation (% Mod.) of an AM signal with a trapezoidal pattern as shown? a. 100% b. 150% c. 20% d. 33% 5. ) of an AM signal? a. = m x P T b. = P T x P SB c. = P SB T d. = P T SB FACET by Lab-Volt 103