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AC2 Frequency Modulation EN1081 Analogue Communications | Assessed Coursework 2 The aim of this assessed coursework is to setup and examine Frequency Modulation (FM) signals using SystemVue

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AC2 Frequency Modulation EN1081 Analogue Communications | Assessed Coursework 2 The aim of this assessed coursework is to setup and examine Frequency Modulation (FM) signals using SystemVue. Section 1 Students will create a wideband FM signal using direct VCO synthesis, investigate the signal bandwidth and confirm the distribution of power in the frequency spectrum. Section 2 Students will create an FM demodulator using Zero-Crossing Detection, and investigate frequency to voltage conversion. Section 3 Students will compare the effect of amplitude noise on FM signals and DSB-AM signals using their modulation subcircuits designed in previous coursework. Parts of this coursework will require you to use the last two digits from your student number, these will be written as X and Y. As an example, if your student number is C1234567 then X would be 6 and Y would be 7. All answers to questions should be recorded in the associated answer booklet. Table of Contents Section 1: Frequency Modulation (FM) .............................................................................................3 1.0 Introduction .............................................................................................................................3 1.1 Creating an FM Modulator using Direct VCO Synthesis ..........................................................5 1.2 Investigate: Power in an FM Signal.........................................................................................8 1.3 Investigate: Bandwidth of an FM Signal ................................................................................10 Section 2: Demodulating Frequency Modulated Signals .................................................................12 2.0 Introduction: FM Demodulation using ZCD............................................................................12 2.1 Implement: Repatching Frequency Modulation .....................................................................13 2.2 Implement: Adding the Comparator.......................................................................................14 2.3 Implement: Adding the Zero Crossing Detector.....................................................................15 Section 3: Noise Immunity of FM Signals........................................................................................19 3.0 Introduction ...........................................................................................................................19 3.1 Implement: Channel with Noise.............................................................................................19 3.2 Importing Subcircuits for DSB-AM.........................................................................................21 3.3 Investigate: Influence of Amplitude Noise on FM and DSB-AM Signals.................................24 Section 1: Frequency Modulation (FM) After completing this section, you should be able to: • Generate an FM signal using ideal components. • Analyse the signal using time and frequency domain plots. • Calculate the bandwidth and power distribution of an FM signal. 1.0 Introduction A disadvantage of the DSB-AM, DSB-SC and SSB-SC communication systems we covered previously is that they are susceptible to picking up electrical noise in the transmission medium (the channel). This is because noise changes the amplitude of the transmitted signal and the demodulators of these systems are designed to respond to amplitude variations. As its name implies, Frequency Modulation (FM) uses a message’s amplitude to vary the frequency of a carrier instead of its amplitude. This means that the FM demodulator is designed to look for changes in frequency instead. As such, it is less affected by amplitude variations and so FM is less susceptible to noise. This makes FM a better communications system in this regard. There are several methods of generating FM signals but they all basically involve an oscillator with an electrically adjustable frequency: • The oscillator uses an input voltage to affect the frequency of its output. Typically, when the input is 0V, the oscillator outputs a signal at its rest frequency (also commonly called the free-running or centre frequency). • If the applied voltage varies above or below 0V, the oscillator’s output frequency deviates above and below the rest frequency. • The amount of deviation is affected by the amplitude of the input voltage. That is, the bigger the input voltage, the greater the deviation. Figure 1 shows a bipolar square wave message signal and an unmodulated carrier. It also shows the result of frequency modulating the carrier with the message. There are a few things to notice about the FM signal: • The envelopes are flat (recall that FM doesn’t vary the carrier’s amplitude). • The period (and hence the frequency) changes when the amplitude of the message changes. • As the message alternates above and below 0V, the signal’s frequency goes above and below the carrier’s frequency. Figure 1 Sketch of a Bipolar (1-bit FSK) FM signal Note that here we have used a square wave message to more clearly show the change in frequency in an FM signal, from the lectures we know that a true FM signal will be far more complex and have many frequencies in the output spectrum. This highlights one of the important differences between FM and the modulation schemes discussed earlier. The mathematical model of an FM signal predicts that even for a simple sinusoidal message, the result is a signal that potentially contains many sinewaves. In contrast, for the same sinusoidal message, an AM signal would consist of three sinewaves, a DSBSC signal would consist of two and an SSBSC signal would consist of only one. This doesn’t automatically mean that the bandwidth of FM signals is wider than AM, DSBSC and SSBSC signals (for the same message signal). However, in the practical implementation of FM communications, it usually is. There’s another important difference between FM and the modulation schemes discussed earlier. The power in AM, DSBSC and SSBSC signals varies depending on their modulation index. This occurs because the carrier’s RMS voltage is fixed but the RMS sideband voltages are proportional to the signals’ modulation index. This is not true of FM. The RMS voltage of the carrier and sidebands varies up and down as the modulation index changes such that the square of their voltages always equal the square of the unmodulated carrier’s RMS voltage. That being the case, the power in FM signals is constant. 1.1 Creating an FM Modulator using Direct VCO Synthesis We will begin by setting up an FM modulator using direct VCO synthesis. As we learnt in the lectures, this can be set up very simply using just a VCO. 1. Add components to the schematic to implement the design shown in Figure 2. Note: Remember these can all be found in the part selector menu, try searching for part or all of the name. Figure 2 Basic Direct FM VCO Synthesiser 2. Setup the message signal by configuring the square wave to have a HiLevel of +5.0 V, a LoLevel of -5.0 V, and a frequency of 2.0 kHz. 3. Setup the VCO to have a resting frequency of 8X.Y kHz, an amplitude of 1.0 V and a sensitivity of 5.0 kHz/V. The VCO module generates an output at specific time intervals, if these time intervals do not align with our simulation we will encounter issues (aliasing, undersampling, oversampling) to fix this you can either check the value is equal to the Time_Spacing attribute in the analysis OR simply set it to be Time_Spacing, this will save us from having to update it each time we modify our sweep! 4. Set the VCO SampleInterval to be Time_Spacing. 5. Add a data sink to the schematic and wire the components as shown in Figure 2. Note: Remember that two arrows on an input indicate a bus, which can accept multiple inputs at the same terminal. This setup implements our FM modulator, from the lectures we know that the instantaneous frequency will be: ???(?) = ?0 + ?0??(?) Where ?0 is the resting frequency of the VCO, and ?0 is it’s sensitivity. 6. Open the Analysis window by double clicking its entry in the workspace tree. 7. Set the stop time to 2999 us, and the sample rate to 2.0 MHz. 8. Run the calculation, either from the analysis window or by clicking the green play arrow on the main window ribbon. 9. Looking at Figure 2 we can see our data sink is measuring both the original message signal and the output from the VCO; add a new graph and plot both of these signals together. 10. Format your graph axis so that the message and FM signal are plotted on different y-axis, and only two periods of the message signal are shown. 1-1 Using markers measure the frequency for both states of the output (FM) signal, what are they? 1-2 Take a snapshot of your graph and attach it to your report. The current design of our VCO synthesiser is able to produce FM perfectly well however we are unable to change carrier frequency or sensitivity as we are restricted by the VCO available to us. Obviously in SystemVue this is no obstacle, we could simply set these values to be whatever we liked however this misses the point and is not representative of how a real VCO would behave. Instead, we shall modify our design in a realistic way to make both the carrier frequency and sensitivity adjustable. Figure 3 Fully Configurable Direct VCO FM Synthesiser 11. Modify the design by adding a constant gain, a constant generator, and a multiple input adder and connect as shown in Figure 3. 12. Leaving the gain as 1 and the constant as 0, rerun the analysis and confirm the output is the same as before. With the modifications made the FM carrier frequency is now: ?? = ?0 + ?0??? Where V_DC is the value of the constant, f_0 the VCO resting frequency, and K_0 the VCO sensitivity. The overall sensitivity of the system is now: ?? = ???0 Where A_m is the message signal gain. 13. Convert your FM modulator into a subcircuit and name it something sensible like “Modulator (FM VCO)”. 14. By default, SystemVue assigns a data port for all outputs, we’re only interested in the vout output of the VCO so delete the ones for clockq, clock, and voutq. 15. Add parameters for the VCO resting frequency, VCO sensitivity, message signal gain, carrier amplitude, and DC voltage (f_0, K_0, A_m, V_c, and V_DC) and configure your subcircuit components to use these parameters. Figure 4 Parameters for Direct VCO FM Subcircuit Note: Your VCO resting frequency will be different to the value shown in Figure 4. With our new modified FM modulator, we are free to adjust carrier frequency and sensitivity factor as we see fit! We will try this now by setting values of V_DC and A_m to achieve a system sensitivity of 10.0 kHz/V, and a carrier frequency of 100 kHz. 16. Confirm that your VCO’s resting frequency (f_0) is still 8X.Y kHz, and its sensitivity factor (K_0) is still 5.0 kHz/V. These values should not be changed for the duration of this coursework! 1-3 Calculate the value of V_DC needed to achieve a carrier frequency of 100.0 kHz for your FM modulator. 1-4 Calculate the value of A_m needed to achieve a sensitivity (k_f) of 10.0 kHz/V for your FM modulator. 17. On the main design schematic, replace your VCO synth with your new subcircuit and configure it using the values you calculated in 1-3 and 1-4. Figure 5 Using the Direct FM VCO Subcircuit Given the new values for carrier frequency (100.0 kHz), sensitivity factor (10.0 kHz/V), and that the message amplitude is still 5 V… 1-5 Calculate the maximum frequency deviation and hence the max and min output frequency. 18. Following the same steps as before, run the analysis and use markers to measure the frequency for both states of the output signal and confirm they are the same as you calculated in 1-5. 19. Replace the square wave generator with a sine wave generator with the same frequency (2.0 kHz) and set the amplitude to 1.0 V. 1.2 Investigate: Power in an FM Signal As mentioned earlier, the power in an FM signal is constant regardless of its level of modulation. This part of the experiment lets you see this for yourself. 1. Set the message signal gain (A_m) on your FM Modulator subcircuit to zero. 2. Set the carrier amplitude (V_c) on your FM Modulator subcircuit to 5.0 V. From the lectures we know that the total power of an FM signal is given by the squared RMS voltage and the load resistance (in SystemVue this is assumed to be 50 Ohm): ? = (????) 2 ? = (?? ) 2 2? 3. Run the analysis and plot the output spectrum on a new graph. 4. Set the x axis limits to display from 50 to 150 kHz, and the y axis units to be Watts. You should see a single frequency at 100 kHz. 1-6 Using markers measure the power of this signal peak and record in your report. 1-7 Calculate the total signal power using the equation above. How does it compare with your measurement in 1-6? 5. While viewing the spectrum gradually increase the value of your FM modulators message signal gain (A_m), you should notice that sidebands begin to appear in the output spectrum. Note: You may find it easier to use tuning to adjust the value of A_m, be sure to keep the step size low (i.e. 0.01 increments). The carrier will now be frequency modulated by a low-level sinusoidal message signal. The spectrum plot should show between 3-4 sidebands spaced by the message signal frequency (2.0 kHz). The amplitude of these sidebands will be small, to see them clearer temporarily set the spectrum’s y-axis units to dBm rather than Watts. 6. Set the y-axis units back to Watts. 7. While paying attention to the spectrum adjust the FM modulators gain control (A_m) so that only five signals (carrier + 2 sideband pairs) are clearly visible in the signal spectrum. 1-8 Using markers measure the power of each of these signal peaks and record them in Table 1. 1-9 Add the power for each signal peak together to calculate total signal power, record this in Table 1. 8. Increase the FM modulators gain control (A_m) to increase the modulation of the FM signal until the carrier power drops to zero for the first time. 1-10 Using markers measure the power of the six most significant (i.e. biggest) peaks and record them in Table 2. 1-11 Add the power for each signal peak together to calculate total signal power, record this in Table 2. 1-12 How do the total powers calculated from Tables 1 and 2 compare with each other and the value measured in 1-6? 1-13 What do these measurements help to prove? Explain your answer. 1.3 Investigate: Bandwidth of an FM Signal The spectral composition of an FM signal can be complex and consist of many sidebands. Usually, many of them are relatively small in size and so an engineering decision must be made about how many of them to include as part of the signal’s bandwidth. There are several standards in this regard and two of the more common ones involve including all sidebands that are equal to or greater than 1% of either the unmodulated carrier’s power (in Watts) OR its amplitude (in Volts). In the lectures we have chosen to use the amplitude version, as it makes calculation from Bessel functions more straightforward. However, for this experiment, we will use power; as this is what you would normally measure with a signal analyser (and is how SystemVue displays spectrum). This part of the experiment lets you use this criterion to measure FM signal bandwidth. 1. Keeping the same setup as before, set the message signal gain (A_m) on your FM Modulator subcircuit to 1.XY. 2. If not already plot the spectrum of your FM signal, centred on 100.0 kHz with the y-axis units in Watts. 3. Using markers identify the lowest and highest frequency sidebands in the FM signal with a power ≥ 1% of the value measured in 1-6 (unmodulated carrier power). 1-14 Using these markers calculate the bandwidth of your FM signal. 1-15 Calculate the bandwidth of a DSB-AM signal with a 100 kHz carrier signal, and a 2.0 kHz message signal. Comment on how your measured FM bandwidth compares to this DSBAM bandwidth, calculated for the same input frequencies. Adjusting the message signal gain allows us to adjust our overall sensitivity factor. Recall that the modulation index of an FM system is given by: ? = ???? ?? (and ?? = ???0) Where ?? is the sensitivity factor, and ?? and ?? are the message signal amplitude and frequency, respectively. 4. Increase the message signal gain (A_m) on your FM Modulator subcircuit to 2.XY. 1-16 Using the same method as before, measure bandwidth of your FM signal. 1-17 How has the FM signal’s bandwidth changed? Explain your answer. Section 2: Demodulating Frequency Modulated Signals After completing this section, you should be able to: • Describe the zero-crossing detection method. • Discuss frequency to voltage translation. • Recover a variety of FM modulated messages. 2.0 Introduction: FM Demodulation using ZCD There are as many methods of demodulating an FM signal as there are of generating one. Examples include: • Slope detector • Foster-Seeley discriminator • Ratio detector • Phase-locked loop (PLL) • Quadrature FM demodulator • Zero-crossing detector It’s possible to implement several of these methods using SystemVue, but for an introduction to the principles of FM demodulation, the zero-crossing detector will be used here, and its block diagram is shown below. Figure 6 Block Diagram for Zero-Crossing Detection The zero-crossing detector is a simple yet effective means of recovering the message from FM signals. The received FM signal is first passed through a comparator to clip it heavily, effectively converting it to a square wave. This allows the signal to be used as a trigger signal for the zerocrossing detector circuit (ZCD). The ZCD generates a pulse of fixed duration every time the squared-up FM signal crosses zero volts (either on the positive or the negative transition but not both). Given the squared-up FM signal is continuously crossing zero, the ZCD effectively converts the square wave to a series of rectangular pulses with a fixed duration. As the FM signal’s frequency is changing (in response to the message), the time between pulses changes which can be represented by a changing duty cycle (ratio between pulse width and the signal period). This is shown in Figure 7 using an FM signal that only switches between two frequencies (generated by a square wave for the message for simplicity). Figure 7 ZCD Operation Timing Diagram Recall from the theory of complex waveforms that square waves can be represented as an infinite sum of odd harmonic sinewaves. If the square wave is also non-symmetric about zero, as is the case in Figure 7, there will also be a DC voltage given by: DC component = peak amplitude x duty cycle Hence when the FM signal switches between the two frequencies, the DC voltage that makes up the rectangular wave out of the ZCD changes between two values (due to the changing duty cycle). This changing DC component is a copy of the message signal that produced the FM signal in the first place. Recovering this copy is a relatively simple matter of picking out the changing DC voltage using a low-pass filter. 2.1 Implement: Repatching Frequency Modulation We will now follow the steps to create a ZCD FM demodulator in SystemVue. We will start by using a square wave message signal and our Direct FM VCO Modulator design from before. 1. Modify your schematic to resemble Figure 5, replacing the sinewave generator with a square wave generator with a 5 V amplitude. FM signal ZCD signal Comparator's output 0V 0V 0V 2. If not already configure your FM modulator so that the overall sensitivity factor is 10 kHz/V and the centre frequency is 100 kHz. Hint: If you are stuck on this go back and follow the steps in Section 1. Remember your VCO will have a unique resting frequency based on your student number. 2.2 Implement: Adding the Comparator The first step in implementing our ZCD detector is creating the comparator. From the discussion we know a comparator is a circuit which will take our analogue input signal and converts it to a square wave output signal. It does this by comparing the input signal amplitude to a reference voltage level (thresholding) and then, depending if the input signal is higher or lower than the reference, outputs a logic high or low signal. This is similar to how digital quantizing works; in that we are “rounding” our input signal to some discrete level (in this case only two: Hi and Lo). Since SystemVue does not have a comparator, we will approximate this behaviour using a 2 level quantizer. 1. Modify your circuit by adding a Quantizer as shown in Figure 8, by default the quantizer is already setup with 2 levels. Figure 8 Patching in the "comparator" 2. Plot the FM signal before and after the “comparator” (the quantizer) on the same graph. 3. Adjust the x axis on your graph so that individual periods are clearly shown and the transition between the two frequency states of the FM signal is at the centre. Hint: As our message signal is a 2.0 kHz squarewave, this will occur every 250 us. 4. Adjust the y axis so that both signals are clearly visible and at a sensible scale. Hint: Using a separate y axis for each signal may make this easier. 2-1 Take a snapshot of your graph and attach it to your report. Duty cycle in a rectangular waveform is the ratio between the pulse width and the period: ???? ????? = ????? ????? ?????? 2-2 Looking at your graph you will notice that when the frequency of the FM signal changes so too does the comparators output. Does the duty cycle also change? Explain your answer. 2-3 What does your answer for 2-2 tell us about the DC component of the comparator’s output? 2.3 Implement: Adding the Zero Crossing Detector Next, we will add our zero-crossing detection. In SystemVue we can achieve this using a pulse waveform generator, configuring it to output a single pulse each time it is triggered on a rising edge. The pulse generator in SystemVue can be configured in a number of different ways, opening its “Model Help” entry from its properties menu shows us that it can be configured to output either continuously, as a burst or even a single pulse in response to a trigger signal, or not! 1. Modify your schematic by placing a PulseGen between the quantizer and the datasink and connect as shown in Figure 9. Figure 9 Implementing the ZCD Function using a Pulse Generator 2. Add parameters to the main design schematic by right-clicking it in the workspace tree menu as shown in Figure 10. 3. Create a new parameter called T_zcd and give it a value of 1.0 us. 4. Configure your pulse generator as follows: a. Set the HiLevel to 5.0 V b. Set ShowAdvancedParams to YES, then set BurstMode to Single c. Set the Period, PulseWidth, and BurstLength to all be T_zcd. d. Set the EdgeTime to 0.1 us. Figure 10 Adding Parameters to the Main Design 5. Plot both the output of the comparator (quantizer) and the ZCD (pulsegen) on the same graph. 6. Adjust the x axis on your graph so that individual periods are clearly shown and the transition between the two frequency states of the FM signal is at the centre. Hint: As our message signal is a 2.0 kHz squarewave, this will occur every 250 us. 7. Adjust the y axis so that both signals are clearly visible and at a sensible scale. Hint: Using a separate y axis for each signal may make this easier. 8. Since our pulse width is so short you may find increasing the Sample_Rate in the analysis to 5 MHz makes the pulse shape clearer to see. Note: This will increase the number of point being simulated, depending on your laptop/PC this may cause the simulation to take longer to run. 2-4 When the frequency of the FM signal changes does the duty cycle of the ZCD’s (pulsegen) output also change? Explain your answer. 2-5 What does your answer for 2-4 tell us about the DC component of the ZCD’s output? 2-6 Take a snapshot of your graph and attach it to your report. The final step is to add the low-pass filter, in order to extract the DC component from the ZCD output and hence recover our message signal. 9. Add a generic filter design model to the output of the ZCD (pulsegen). 10. Configure the filter to be a low-pass Bessel filter with a pass frequency of 10.0 kHz. 11. Run the analysis and plot both the original squarewave message signal and the output from the filter together. 12. Set the x axis limits to plot from 1.0 ms, to 2.999 ms. 13. Adjust the y axis so that both signals are clearly visible and at a sensible scale. Hint: Using a separate y axis for each signal may make this easier. 2-7 Why is the recovered message signal only positive? 2-8 Comparing the recovered and original squarewave message signals, the recovered signal has a noticeable ripple and less sharply defined edges. What might have caused this? Explain your answer. 2-9 Take a snapshot of your graph and attach it to your report. 14. Select the quantizer, pulse generator and filter and convert your FM demodulator into a subcircuit, call it something sensible like “Demodulator (FM ZCD)” 15. On your subcircuit you will need to add your T_zcd parameter again, you should also remove the unwanted output port which has been added to the quantizer. 16. Your final subcircuit diagram should resemble that shown in Figure 11. Figure 11 Subcircuit diagram for ZCD FM Demodulator. 17. On the main schematic replace the ZCD demodulator components with your new subcircuit. 18. Replace the square wave generator with a 2.0 kHz sinewave generator with a 1.0 V amplitude. 19. Wire your FM modulator and FM demodulator as shown in Figure 12. Figure 12 FM Transmission with Sinusoidal Message Signal. 20. Run the analysis and plot the original and recovered message signals together. 2-10 Compared to the square wave signal, the recovered sinewave more closely resembles the original signal. Why might this be the case? Explain your answer. Section 3: Noise Immunity of FM Signals After completing this section, you should be able to: • Visualise how amplitude noise can change the transmitted signal. • Comment on the effect of amplitude noise on FM and DSB-AM. • Import subcircuit designs from another SystemVue workspace. 3.0 Introduction One of the more challenging aspects of transmitting a signal through a real channel is encountering noise. Noise can come from many different sources, both manmade and naturally occurring, and the presence of significant noise can often be the cause of severe signal degradation. Fortunately, there are many types of modulation schemes which can overcome these challenges. In this part of the experiment, we will create our own noisy channels to pass our signals through. We will then compare how both FM and DSB-AM signals are able to perform in this channel. To do this we will make use of one of SystemVue many powerful features: the ability to import designs from other workspaces! 3.1 Implement: Channel with Noise Adding noise to our signals in SystemVue is mostly straightforward. We will mainly make use of the “Add Noise Density to Input” component to do this. Since this generates noise evenly across all frequencies we also add a low-pass filter at it’s output. We do this to avoid aliasing due to noise at frequencies higher than our simulations sampling rate. 1. Create a new Subcircuit by clicking the “New Item” button in the workspace tree and selecting Designs > Sub-Network Model… Figure 13 Creating a blank subcircuit. 2. Call your new subcircuit “Channel with Noise”. 3. On your subscircuit schematic add two DataPorts, an AddNDensity, and a generic filter model. 4. Create two parameters, one called V_noise and one called f_c. Set the value of V_noise to 0, and f_c to 150 kHz. Figure 14 Noisy Channel Parameters. 5. Configure the Noise Density model by setting NDensity to be equal to (V_noise^2)/50. 6. Configure the anti-aliasing filter as a low-pass Butterworth filter and set the PassFreq value to be f_c. 7. Configure one of the ports as an output by setting its direction setting to output and wire as shown in Figure 15. Figure 15 Noisy Channel Subcircuit Diagram. Test your channel by adding it to the main schematic and passing a sinewave message through. 8. On the main schematic add your channel subcircuit, and connect it to the sinewave message signal and datasink as shown in Figure 16. 9. Configure your channel so that V_noise is tunable. Figure 16 Adding noise to a sinusoidal signal. 10. While viewing the output sinewave in the time domain, gradually increase the value of V_noise from 0 to 100 uV. 3-1 What happens to the shape of the signal as you add noise? 3.2 Implement: Importing Subcircuits for DSB-AM In the next part of the experiment, we will be using our noisy channel to add amplitude noise to our FM signal. One of the advantages to using FM over AM is that the signal amplitude does not contain any information, making it more resistant to amplitude noise. This part of the experiment lets you demonstrate this. One of the useful things with SystemVue is that it allows us to export subcircuit designs from one workspace and then import them into another one. This allows us to reuse circuits we’ve designed previously or the circuits of others, making collaborative work easier. In this case we will be reusing our DSB-AM modulator and demodulator from coursework 1 (AC1). 1. Save your current workspace (the one you have been using for this coursework). Before continuing it is very, very important that you saved the workspace as any unsaved work will be LOST. 2. Open the workspace you used for the amplitude modulation coursework. Note: If for whatever reason you no longer have access to the workspace from the first coursework skip ahead to the end of this section. There you will find subcircuit diagrams for you to re-construct the necessary DSB-AM modulator and demodulator. As part of the first coursework you will have made modulator subcircuits for all three AM schemes, but not the demodulators. 3. If your product detector is still setup on the main schematic convert it to a subcircuit, otherwise create a new blank subcircuit and setup as shown in Figure 20. 4. Add a parameter called f_c and configure the sinewave generator to use this value for its frequency. 5. Name your subcircuit “Demodulator (AM Product)” Next, we will export our subcircuits. 6. Export your DSB-AM Modulator subcircuit by right-clicking it and selecting Export as shown in Figure 17. 7. Save your subcircuit as a *.XML file in the same directory as your workspace, be sure to save the file name as something recognisable. Figure 17 Exporting subcircuit diagrams. 8. Repeat steps 6 and 7 for the Product demodulator. 9. Once done, save the current workspace (we may as well keep the work we did creating the AM Demodulator subcircuit). 10. Open the workspace you were using for this coursework (AC2 Frequency Modulation). 11. Import both of your subcircuits by clicking File > Import > XML File… and selecting them. Figure 18 Importing Subcircuit Design Files. 12. Once imported they should appear in your workspace tree, at this point if you have not already done so it may be worth creating a Folder within Designs specifically for all your subcircuits. Below are subcircuit diagrams for the DSB-AM modulator and the AM Product Detector. If you were unable to reuse your existing subcircuits you should recreate them based on these designs within the workspace we are using for THIS coursework (AC2). Figure 19 DSB-AM Modulator Subcircuit Design. Figure 20 Product Demodulator for AM Subcircuit Design. 3.3 Investigate: Influence of Amplitude Noise on FM and DSB-AM Signals We will now investigate the effect adding noise to the signal during transmission effects recovery. 1. On the main schematic add your FM modulator, your FM demodulator, the noisy channel, a sinewave generator and a data sink as shown in Figure 21. 2. Configure the message signal to be a 2.0 kHz sinewave with a 1.0 V amplitude. 3. Configure your FM modulator to have a carrier frequency of 100 kHz, a carrier amplitude of 5.0 V, and a sensitivity of 20.0 kHz/V. Note: Use the same value for V_DC calculated previously to set carrier frequency, for sensitivity factor recall the VCO sensitivity is 5 kHz/V so we need to use A_m = 4.0. 4. Configure the noisy channels V_noise to be tunable and initially set it to 0.0 V. 5. Configure your analysis as follows: a. Start_Time = 1000 us b. Stop_Time = 4999 us c. Sample_Rate = 4 MHz Figure 21 Patching for FM channel noise. 6. While viewing the output signal in the time domain gradually increase the level of noise voltage (V_noise) from 0 V in increments of 0.1 mV. 3-2 Describe how noise has affected the recovered FM message signal for noise levels of 0, 2, and 4 mV. 3-3 With reference to how ZCD demodulation works, describe how noise might interfere with recovering the message signal. 7. Modify your schematic by adding in the DSB-AM modulator and product detector and wiring as shown in Figure 22. 8. Configure the DSB-AM modulator to have a carrier frequency of 100 kHz, a carrier amplitude of 2.0 V and a DC voltage of 1.5 V. This gives us a DSB-AM signal with the same RMS voltage (and hence power) as our FM signal. Figure 22 Patching for FM and DSB-AM with Channel Noise. 9. Ideally, we want both channels to have the same noise voltage, so create a parameter on the main schematic called V_n and configure both channel’s to use this for their V_noise. 10. Set V_n to be tunable (on the main schematic parameters tab). 11. Configure both channels V_noise to NOT be tunable (we no longer need to tune them directly as we have control of V_n). 12. Set the value of V_n to be 0.0 V initially and run the analysis. 13. Plot both recovered message signals on separate graphs and arrange the two windows so you can see them both. Set the y axis for both to be auto scaled. 14. While viewing both recovered message signals gradually increase the value of channel noise from 0.0 mV in 0.1 mV steps. 3-4 At what value of noise amplitude did you first notice a distortion in the recovered DSB-AM message signal? 3-5 At what value of noise amplitude did you first notice a distortion in the recovered FM message signal? 3-6 Take a snapshot of both graphs for the noise level determined in 3-5 and attach them to your report. 3-7 Describe how the FM and DSB-AM recovered signals compare to each other at 0, 1, 2 and 4 mV noise levels. By about 4 mV noise the DSB-AM recovered signal should be heavily distorted, and the FM signal slightly less-so. We can improve the DSB-AM signals ability to resist noise by making it DSB-SC. 15. Convert the DSB-AM signal to DSB-SC by setting the V_DC to 0 V. 16. Since this changes the peak-peak voltage we will change the carrier amplitude to be 5.0 V so our signals still have the same power. 17. While viewing both recovered signals set the value of noise voltage to 4.0 mV. 3-8 How is the DSB-SC recovered signal different to the DSB-AM signal when the noise level is 4.0 mV? DSB-SC really just represents an extreme case of over modulating our DSB-AM signal, what we’ve done here is to effectively increase the amount of modulation which has improved the signals ability to handle noise. The catch, however, is that amplitude modulation hits a maximum modulation depth with DSB-SC; decreasing the value of V_DC further would actually decrease modulation depth and take us back to DSB-AM. FM has no such limit, and we can increase our modulation index so long as we have available bandwidth. Additionally, there are many more types of FM demodulators which offer superior noise immunity than the ZCD detector!