EXPERIMENT 4 - Part I: DSB Amplitude Modulation

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1 OBJECTIVE To generate DSB amplitude modulated signal. EXPERIMENT 4 - Part I: DSB Amplitude Modulation PRELIMINARY DISCUSSION In an amplitude modulation (AM) communications system, the message signal is used to electrically vary the amplitude of a carrier. The carrier usually has a frequency that is much higher than the message's frequency. Figure 1 below shows a simple message signal, an unmodulated carrier, and amplitude modulated signal. Figure 1 The dotted lines in Figure 1 are known as the signal's envelopes. If you look at the envelopes closely you'll notice that the upper envelope is the same shape as the message. The lower envelope is also the same shape but inverted. The mathematical model that defines the DSB signal is given as DSB = (DC + message) x the carrier THE EXPERIMENT In this experiment you'll use Emona Telecoms-Trainer 101 to generate a real DSB signal by implementing its mathematical model. You'll examine the DSB signal using the scope and compare it to the original message. You'll do the same with speech for the message. Following this, you'll vary the message signal's amplitude and observe how it affects the modulated carrier. You'll also observe the effects of modulating the carrier too much. Finally, you'll measure the AM signal's depth of modulation using a scope. 1

2 Equipment Emona Telecoms-Trainer 101 (plus power-pack) Dual channel 20MHz oscilloscope two Emona Telecoms-Trainer 101 oscilloscope leads assorted Emona Telecoms-Trainer 101 patch leads Procedure Part A - Generating a DSB signal using a simple message 1. Gather a set of the equipment listed above. 2. Set up the scope. Ensure that: the Trigger Source control is set to the CH1 (or INT) position. the Mode control is set to the CH1 position. 3. Set the scope's Channel 1 Input Coupling control to the DC position. 4. Locate the Adder module and turn its G and g controls fully anti-clockwise. 5. Locate the Variable DCV module and turn its DC Voltage control almost fully anti-clockwise. 6. Implement the block diagram shown in Figure 2 below. Figure 2 At the moment, the scope should just be showing a flat trace because the Adder module's output is 0V. 7. Set the scope's Channel 1 Vertical Attenuation control to the 0.5V/div position. 8. Use the scope's Channel 1 Vertical Position control to move the trace so that it lines up with the horizontal line in the middle of the scope's screen. 9. While watching the Adder module's output on the scope, turn its g control clockwise until the DC level is 1V. 10. While watching the Adder module's output on the scope, turn its G control clockwise to obtain a 1Vp-p sinewave. 11. Modify the set-up as shown in Figure 3 below. 2

3 Figure 3 The set-up in Figure 3 can be represented by the block diagram in Figure 4 below. Figure 4 With values, Figure 4 implements the following equation: DSB = (1VDC + 1Vp-p 2kHz sine) x 4Vp-p 100kHz sine. 12. Set the scope's Mode control to the DUAL position. 13. Set the scope's Channel 2 Vertical Attenuation control to the 1V/div position. 14. Draw the two waveforms to scale in the space provided. 15. Use the scope's Channel 1 Vertical Position control to overlay the message with the AM signal's envelopes and compare them. 3

4 Question 1 What feature of the Multiplier module's output suggests that it's a DSB signal? Question 2 The DSB signal is a complex waveform consisting of more than one signal. Is one of the signals a 2kHz sinewave? Explain your answer. Question 3 For the given inputs to the Multiplier module, how many sinewaves does the DSB signal consist of, and what are their frequencies? Part B - Generating a DSB signal using speech This part of the experiment lets you see what a DSB signal looks like when modulated by speech. 16. Disconnect the plug on the Master Signals module's 2kHz SINE output that connects to the Adder module's A input. 17. Connect it to the Speech module's output as shown in Figure 5 below. Remember: Dotted lines show leads already in place. Figure Set the scope's Timebase control to the 2ms/div position. 19. Talk, sing or hum while watching the scope's display. Question 4 Why is there still a signal out of the Multiplier module even when you're not talking, whistling, etc? 4

5 Part C - Investigating depth of modulation& It's possible to modulate the carrier by different amounts. This part of the experiment let's you investigate this. 20. Return the scope's Timebase control to the 0.1ms/div position. 21. Disconnect the plug on the Speech module's output. 22. Reconnect the Adder module's A input to the Master Signals module's 2kHz SINE output. 23. Vary the message signal's amplitude a little by turning Adder module's G control left and right and notice the effect on the DSB signal. You probably noticed that the size of the message signal and the modulation of the carrier are proportional. That is, as the message's amplitude goes up, the amount of the carrier's modulation goes up. The extent that a message modulates a carrier is known as the modulation index (m). Modulation index is an important characteristic of a DSB signal for several reasons including calculating the distribution of the signal's power between the carrier and sidebands. Figure 6 below shows two key dimensions of an amplitude modulated carrier. These two dimensions allow a carrier's modulation index to be calculated. Figure 6 The next part of the experiment lets you practice measuring these dimensions to calculate a carrier's modulation index. 24. Adjust the Adder module's G control to return the message signal's amplitude to 1Vp-p. 25. Measure and record the AM signal's P dimension. Record your measurement in Table Measure and record the AM signal's Q dimension. 27. Calculate and record the AM signal's depth of modulation using the equation below. P Q m P Q A problem that is important to avoid in AM transmission is over-modulation. When the carrier is over-modulated, it can upset the receiver's operation. The next part of the experiment gives you a chance to observe the effect of over-modulation. 28. Increase the message signal's amplitude to maximum by turning the Adder module's G control fully clockwise and notice the effect on the DSB signal. 29. Use the scope's Channel 1 Vertical Position control to overlay the message with the DSB signal's envelopes and compare them. 30. Draw the two waveforms to scale in the space provided. Question 5 What is the problem with the DSB signal when it is over-modulated? What do you think is a carrier's maximum modulation index without over-modulation? 5

6 EXPERIMENT 4 - Part II: DSB Amplitude Demodulation OBJECTIVE To demodulate DSB amplitude modulated signal with an envelope detector and with a product detector. PRELIMINARY DISCUSSION Recovering the original message from a modulated carrier is called demodulation and this is the main purpose of communications and telecommunications receivers. In Part I, we see that a key characteristic of a DSB signal - its envelopes are the same shape as the message (though the lower envelope is inverted). Thus, the circuit used to demodulate DSB signals is called an envelope detector. The block diagram of an envelope detector is shown in Figure 1 below. Figure 1 In the above figure, the rectifier stage chops the DSB signal in half letting only one of its envelopes through (the upper envelope in this case). This signal is fed to an RC LPF which tracks the peaks of its input (the signal's envelope). Importantly, as the envelope is the same shape as the message, the RC LPF's output voltage is also the same shape as the message and so the DSB signal is demodulated. A limitation of an envelope detector is that it cannot accurately recover the message from over-modulated DSB signals. Recall that when a carrier is over-modulated the DSB signal's envelope is no-longer the same shape as the original message. Hence, the envelope detector produces a distorted version of the message. THE EXPERIMENT In this experiment, you'll generate the DSB signal as in Part I.A. Then you'll set-up an envelope detector using the Rectifier and RC LPF on the trainer's Utilities module. Once done, you'll connect the DSB signal to the envelope detector's input and compare the demodulated output to the original message and the DSB signal's envelope. You'll also observe the effect that an over-modulated DSB signal has on the envelope detector's output. Finally, you'll demodulate the DSB signal by multiplying it with a local carrier instead of using an envelope detector. Equipment Emona Telecoms-Trainer 101 (plus power-pack) Dual channel 20MHz oscilloscope two Emona Telecoms-Trainer 101 oscilloscope leads assorted Emona Telecoms-Trainer 101 patch leads one set of headphones (stereo) 6

7 Procedure Part A - Setting up the DSB modulator To experiment with DSB demodulation you need a DSB signal first. Thus, generate a DSB signal with a depth of modulation 0.5 as in the Part I.A of the experiment. Part B - Recovering the message using an envelope detector 1. Modify the set-up as shown in Figure 2 below. Remember: Dotted lines show leads already in place. Figure 2 Solid lines in Figure 2 can be represented by the block diagram in Figure 3 below. As you can see, it's the envelope detector explained in the preliminary discussion. Figure 3 2. Adjust the scope's Vertical Attenuation controls to appropriate settings for the signals. 3. Draw the two waveforms to scale in the space provided leaving room to draw a third waveform. 4. Disconnect the scope's Channel 2 input from the Rectifier's output and connect it to the RC LPF's output instead. 5. Draw the demodulated DSB signal to scale in the space that you left on the graph paper. Question 1 What is the relationship between the original message signal and the recovered message? 7

8 Part C - Investigating the message's amplitude on the recovered message 6. Vary the message signal's amplitude up and down a little (by turning the Adder module's G control left and right a little) while watching the demodulated signal. Question 2 What is the relationship between the amplitude of the two message signals? 7. Slowly increase the message signal's amplitude to maximum while watching the demodulated signal. Question 3 Why does over-modulation cause the distortion? Part D - Transmitting and recovering speech using DSB 8. If you moved the scope's Channel 1 input, reconnect it to the Adder module's output and return the scope's Trigger Source control to the CH1 position. 9. Adjust the message signal's amplitude back to 1Vp-p (by turning the Adder module's G control anti-clockwise). 10. Modify the set-up as shown in Figure 4 below. Figure Set the scope's Timebase control to the 2ms/div position. 12. Turn the Buffer module's Gain control fully anti-clockwise. 13. Without wearing the headphones, plug them into the Buffer module's headphone socket. 14. Put the headphones on. 15. As you perform the next step, set the Buffer module's Gain control to a comfortable sound level. 16. Talk, sing or hum while watching the scope's display and listening on the headphones. 8

9 Part E - Recovering the message using a product detector In this part of the experiment, the DSB signal is demodulated using a product detector. 17. Modify the set-up to return it to just a DSB modulator with a 2kHz sinewave for the message. 18. Modify the set-up as shown in Figure 5 below. Figure 5 The additions to the set-up can be represented by the block diagram in Figure 6 below. The Multiplier module models the mathematical basis of DSB demodulation and the RC Low-pass filter on the Utilities module picks out the message while rejecting the other sinewaves generated. Figure Compare the Multiplier module's output with the Rectifier's output that you drew earlier. Question 4 Given the DSB signal is being multiplied by a 100kHz sinewave, how many sinewaves are present in the Multiplier module's output? What are their frequencies? 9

10 20. Disconnect the scope's Channel 2 input from the Multiplier module's output and connect it to the RC LPF's output instead. 21. Compare the RC LPF's output with the message and the output RC LPF's that you drew earlier. A common misconception about DSB is that, once the signal is over-modulated, it's impossible to recover the message. However, this is only true for the envelope detector. The product detection demodulation method doesn't suffer from this problem as it's not designed to recover the message by tracking one of the DSB signal's envelopes. The final part of this experiment demonstrates this. 22. Connect the scope's Channel 1 input to the DSB modulator's output. 23. Set the scope's Trigger Source control to the CH 2 position. 24. Slowly increase the message signal's amplitude to produce a near 100% modulated DSB signal by adjusting the Adder module's G control. Note: Resize the DSB and demodulated message signals on the scope's screen as necessary. 25. Slowly increase the message signal's amplitude to produce a DSB signal that is modulated by more than 100% while paying close attention to the demodulated message signal. Note: Notice that the demodulated message signal is not distorted even though the carrier is overmodulated. As an aside, the commercial implementation of DSB modulation commonly involves a Class C amplifier for efficiency (that is, to minimize power losses). When a Class C amplifier is operated at depths of modulation above 100% the circuit's operation no-longer corresponds with the model of an DSB modulator as in Figure 4 (Part I). Importantly, in addition to producing an envelope that is not the same as the original message, the over-modulated Class C circuit produces extra frequency components in the spectrum. This means that neither the envelope detector nor the product demodulator can reproduce the message without distortion. 10