Amplitude Modulation Radio Simulator Avelino, Anne Loraine L., Galang, Vincent N., Nañoz, Allona Jane M., Punzalan, Justine Roy A. College of Engineering School of Technology First Asia Institute of Technology and Humanities
— This paper aims to create an amplitude modulation radio simulation through different subVIs using LabVIEW.
Abstract
I. I NTRODUCTION An amplitude modulation radio simulation was generated in this experiment. It features an audio signal viewer to simultaneously display the frequency spectrum and time-domain representations of any desired signal in the system and play the signal on the sound card. Using trigonometric functions, it can form mathematical expressions for the carrier and the modulating signal and combine these to create a formula for the complete modulated wave. Modulators compute the product of the carrier and modulating signals. Amplitude modulation has been in use since the very earliest days of radio technology. The first recorded instance of its use was in 1901 when a signal was transmitted by a Canadian engineer named Reginald Fessenden. Amplitude modulation is defined as the process in which is the amplitude of the carrier wave is varied about a means values linearly with the base band signal. It is the most straightforward way of modulating a signal.
Figure 2.1: Radio Simulator Front Panel
The front panel of the radio simulator shown in Figure 2.1 has two displays, the output waveform of the signal in time domain and its equivalent form in spectrum analyzer. The frequency knob is the adjust for the carrier frequency. The modulated signal will then be displayed on the graph based on the carrier frequency set in the knob. B. Radio Simulator Block Diagram
Lastly, AM transmitter and receiver should be formed in this experiment through different subVIs. II. CIRCUIT DESIGN The Amplitude Modulation Radio simulator made in this machine problem features an audio signal viewer to simultaneously display the spectrum and time domain representations of any desired signal in the system and play the signal on the sound card. The radio simulator is composed mainl y of two main sections: the AM transmitter and the receiver sections, and every sections is built with different subVIs which will be discussed later in this section.
To further explain the operation of the radio simulator, the block diagram will be discussed in t his section. T he constants in the block diagram is placed to global variables. These global variables composed of signal duration dur[s], the sampling frequency f_s[Hz], maximum frequency f_max[Hz], and the intermediate frequency used by the receiver f_IF[Hz] which has default values shown in Figure 2.2.
A. Radio Simulator Front Panel
Figure 2.2: System Constants
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AM Transmitter a)
Three AM Transmitters
Figure 2.5: Triangle Wave Generator
Figure 2.5 shows the block diagram of the triangle wave generator. The only difference of this triangle wave to sine wave is the triangle wave node. The pr ocess of getting the samples and the normalized frequency is the same as the sine wave generator. d)
Dual Sine Wave Generator
Figure 2.3: AM Transmitters
Figure 2.3 shows the AM transmitter section of the radio simulator. There are three baseband signals with different frequencies namely sine, dual sine and the triangle each is pl aced in different subVI’s. These signals will go through the AM modulator (another subVI) that modulates the signal based on the carrier frequency set. The modulated output signals are then “mix together” by the use of the compo und arithmetic function which will be processed on the receiver section. b)
Sine Wave Generator
Figure 2.6: Dual Sine Wave Generator
In the program shown in Figure 2.6. The sine wave generator is just doubled to produce two sine signals. However, the other sine signal is set to have 0.7 amplitude and frequency of 1.7 times of the input signal. The two sine waves are added together to produce a dual sine signal. e)
AM Modulator
Figure 2.4: Sine Wave Generator
Figure 2.4 shows the block diagram of the sine wave generator. The product of the two global variables for sampling frequency, and the duration gives the number of samples that is converted to integer. To get the normalized frequency, the sampling frequency is divided by the input frequency. These parameters are processed by the sine wave node to produce the signal. c)
Triangle Wave Generator
Figure 2.7: AM Modulator
The AM Modulator is the vital part of the Radio simulator. Figure 2.7 shows the block diagram of the modulator. First, the signal must be modulated according to the equation: y (t ) = [A + x(t )]cos(2πf t), where A = |Xmin(t )|.
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To get the amplitude of the array input signal, the function Array Max and Min is used. This will get the minimum value of the input. This minimum value is added to the input array x(t). To get the function cos(2πf t), first is to make a sine wave. Like in sine wave generator, the normalized frequency and the number of samples is needed. The output of the sine wave is then process on the function Get Waveform components. This will get the components of the waveform sine producing a function sin(2πf t). Taking the derivative by the aid of the derivative node, the function cos(2πf t ) will be get. The amplitude and the function cosine is simply multiplied to get the modulating equation.
AM Receiver
Fig. 2.10: Local Oscillator with Mixer. (BPF input refers to received RF Signal)
The second stage of the receiver consists of Local Oscillator and Mixer as shown on Fig. 2.10. In this stage, RF Signal is down converted to Intermediate Frequency (IF). Before conversion, a local oscillator (sine wave) is created. This signal is then multiplied together with Bandpass’ output. After mixing with local oscillator, RF is now successfully converted to IF. Accordingly, although it is converted, envelope still remains the same thus preserving the intelligence signal.
Fig. 2.8: RF Filter Stage
Fig. 2.8 shows the complete block diagram and flow of data/ signal to create an AM Receiver. Details of each subVI and stage is shown below including their function and significance. Fig.2.11: IF Section
Fig. 2.9: RF Filter Stage
The first stage for AM Receiver is RF Filter. Accordingly, its primary functions include detecting, bandlimiting and amplifying the received RF Signals. SubVI BPF shown in Fig. 2.9 demonstrates a bandpass filter used in filtering RF Signal. Butterworth Order is a constant ten which is equivalent to a circuit of ten cascaded filter design. Control input Frequency[Hz] will serve as the tuning knob for the AM radio. Low cutoff frequency is defined to be fc – fmax. On the other hand, high cutoff frequency is defined to be its opposite fc + fmax.. Remembering that the input signals were RF, this stage enables all necessary frequency components to be passed onto its next stage based on the tuning desired by the user and likewise, reject others. A Select function is also included so that the circuit can be disabled when necessary.
Receiver’s third stage is the Intermediate Frequency section. IF Section is consisted of series of bandpass filters. For this simulation however, as shown on Fig. 2.11 onl y one bandpass filter is used with center frequency of 750 Hz which is a system constant. Value of intermediate frequency and bandwidth is constant for all stations and less than any of RF signals received. With IF as center frequency, filter’s selectivity would boil down close enough to the baseband signal. Also, this stage further clears the received of signal by ideally rejecting unnecessary parameters present in the signal.
Fig.2.12: Low Pass Filter
The fourth stage of AM Reception is the demodulator. Demodulation process extracts the information signal. Actual circuits use diode (detection), capacitors and resistors (both for filters) to completely define a simple demodulator. Filters stated
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above is referred as Low- Pass Filters. It is demonstrated on Fig. 2.11. Without LPF, output of demodulator (see Fig. 2.12) can have a lot unnecessary parameters. Signal output becomes unintelligible. Through LPF, information signal can be properly demodulated.
A triangle wave is used to provide another intelligence signal. Set with the same frequency as the sine wave, 500 Hz, a triangular modulating signal is shown in Figure 3.2.
Fig.2.12: Demodulator
AM Receiver’s last stage for this simulation is the DC Blocker. Fig. 2.13 shows how the group used an amplifier and AC & DC Estimator to finally output the transmitted signal. Ideally, signal transmitted must be received and decoded as it is before transmission. For this simulation, it was stated that the output waveform had DC components. To prevent offset caused by DC, the estimator is implemented. Moreover, due to Amplitude Modulation and many filtering processes, amplitude was attenuated. To solve the problem, an audio amplifier is added which is represented by multiplication operand (gain).
Figure 3.2
A third intelligence signal was made by combining two sine waves of different tone frequencies and amplitude. A visible output of the combined sine waves is shown in Figure 3.3, which is quite unique but a pattern i n the wave can be easily determined.
Fig.2.13: DC Blocker
III. R ESULTS AND DISCUSSION The AM radio simulator views the intelligence signal and the demodulated signal which allows comparison in the results, whether they’re the same or not. With the models presented and simulated in LabVIEW, the actual process of transmitting and receiving different signals can be easily understood. The first intelligence signal is a sine wave, simulated at a frequency of 500 Hz, as shown in Figure 3.1. A signal viewer was used to help in the visualization of the signal in time domain and its respective frequency in the spectrum.
Figure 3.3
The next step is to develop a sub VI that stands as the modulation stage of the signal that includes the carrier frequency. Setting the input of the modulator to be the three intelligence signal, a modulated signal is produced. And shown in Figure 3.4 are the three modulated signal with a carri er frequency calibrated to 1500 Hz.
Figure 3.1 Figure 3.4
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Another simulation of the modulated signals is carried out but the carrier frequency this time is 3 kHz. The signals in time domain is shown in Figure 3.5. A final simulation for the modulated signals is done by setting the carrier frequency to 4.5 kHz, shown in Figure 3.6. By experimenting with the carrier frequency and setting it to different values, it can be observed that as the carrier frequency increases, the frequency of the modulated signal also increases. It is evident in the fi gures that the lines in the graph are getting thicker and forming a clear envelope.
Adding the three modulated signal using compound arithmetic, a signal is derived and is shown in Figure 3.7. The combined signal has a resemblance in the modulated dual sine wave despite being combined with the sine and triangle waves. Nonetheless, it just resembled the dual sine wave because it is one of its origin, but they’re completely not equal. A requirement to keep the transmitter from stepping on each other’s signal is that they operate at different frequencies. By doing so, unique signals can be modulated without stepping on each other’s signal that will cause distortion.
Figure 3.7
Figure 3.5
After the transmitting the baseband signals, an AM receiver is done to demodulate the combined modulated signal. The first stage of a receiver is the RF filter, which is labeled in this experiment as the tuner. The output of the RF filter will be multiplied to the local oscillator which its frequency depends on the tuning knob. This stage serves as the mixer. During the ‘F’ condition of the bandpass filter (BPF), which is also the RF filter, with a frequency set to 1.5 kHz, the spectrum and time domain signal is shown in Figure 3.8.
Figure 3.8 Figure 3.6
From the waveform of the baseband signals and the output waveform when they’re entered in the AM modulator, an envelope was formed which connects the positive and negative peaks of the carrier waveform. It can be noticed that the positive values in the modulated signal is also the same as the intelligence signal. While the negative values of the modulated signal, is a mirror of the signal input.
At the same stage, tuning the frequency to 3 kHz, a change in the spectrum and time domain is quite noticeable in Figure 3.9.
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Enabling the bandpass filter by setting it to ‘T’, a drastic change happened and can be viewed in Figure 3.11. With a frequency set to 1.5 kHz, just like the calibration when the condition is ‘F’, the spectrum doesn’t show any frequency that is allowed to pass through it. Only the time domain has a signal and has a very low amplitude.
Figure 3.9
Different band of frequencies are allowed to pass through the filter but compared to the previous output, the filtered frequencies has a greater interval. The frequencies in the spectrum of the 1.5 kHz frequency is closer to each other compared to the 3 kHz. Figure 3.11
For the last trial, the knob is calibrated to a 4.5 kHz frequency, and the results in spectrum and time domain is shown in Figure 3.10.
Setting the frequency to 3 kHz, frequencies appeared in the spectrum, which means that now, there are frequencies allowed to pass through the filter but with a less amplitude. Still, a change in the signal in the time domain is evident as shown in Figure 3.12.
Figure 3.10
Notice that the spectrum of a 4.5 kHz has a distinct change compared to the previous two. There are certain frequencies that are allowed to pass through but a very low amplitude. This affects the amplitude in the time domain signal. Simulating these when the bandpass filter is in ‘F’ condition, the output of the random frequencies is similar to the output when the baseband signals are added. This is due to the false condition of the filter. It’s because, during this condition, the input in the bandpass filter, will also be its output. Then multiplying it with the signal produced by the local oscillator, only a change in amplitude will happen.
Figure 3.12
Figure 3.13 shows the results in the spectrum and in time domain when the frequency is changed to 4.5 kHz. Just like the previous result, certain band of frequencies are allowed. Comparing their amplitude, a 4.5 kHz is slightly greater than a 3 kHz. This is also true when the filter is set to false condition; that the higher the input frequency, the higher its amplitude.
The spectrum of a desired station can be translated to its corresponding intermediate frequency when the bandpass filter is enabled, which is the true purpose of the mixer. With these three simulation, it can be said that as the frequency of the local oscillator increases, the greater interval of the frequencies passed through the filter are set in false condition. Figure 3.13
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It can also be observed that the signal in the ti me domain of Figure 3.13 looks like the modulated signal of the triangle wave, although the reception of this signal is not as clear as the modulated triangular signal. The primary benefit of the bandpass filter with the translation of the desired station to the intermediate frequency is that it limits the frequency that will pass through it. With the help of the tuning knob that will serve as the center frequency of the filter, only limited frequencies will be mixed with the local oscillator and be converted to intermediate frequency. Having said that, the bandpass filter only operates one signal at a time.
Figure 3.16
Figure 3.16 shows the output signal when the frequency is set to 5 kHz, the same as the carrier frequency of the triangle wave. It can be easily noticed that the modulating signal in t his is a triangle signal and its modulation index is almost equal to 1. The benefit of the IF filter is that it allows a narrow bandwidth to pass through which in return, improves the accuracy of the signals allowed to pass through it without any distortion and interfering of other signals in neighboring frequencies.
Figure 3.14
The next stage after the mixer is the IF filter. After converting the radio frequency to its corresponding intermediate frequency, it be filtered again, but this time, the frequency that will determine its high-cutoff and low-cutoff frequency is set to be the f_IF equivalent to 750 Hz. Shown in Figure 3.14, is the result of the IF filter when then knob is calibrated to the carrier frequency of the sine wave which is 2.5 kHz. Though, the sine wave is not that clear for the process of the receiver is not completed yet.
Figure 3.17
Figure 3.15
Shown in Figure 3.17 are the outputs of the demodulator stage when it is in ‘F’ condition. It can be seen that the output signal are only positive values and the observation of the modulating signal is getting clearer. This is because of the disabled lowpass filter. Similar to the previous filters with an ‘F’ condition, the demodulator (LPF) will only output what its input. And after the IF filter, the absolute value of the output is used to be the input of the demodulator that is why only positive values appear on the output of the demodulator.
Adjusting the frequency to 3.75 kHz, the carrier frequency of the dual sine signal, Figure 3.15 shows band of frequencies in the spectrum but compared to the output of the previous stage, this one is more limited in terms of t he frequency that is allowed to pass through, which is a good deal.
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Figure 3.18
Figure 3.20
Enabling the demodulator (LPF), another drastic change happened in its output shown in Figure 3.18. Notice that the output of the enabled demodulator is the outline of the output when the demodulator is disabled. Which means that the role of the lowpass filter is to reject all frequencies that are above the low-cutoff frequency.
After the demodulator stage, the audio amplifier is the next and last stage of an AM receiver. One feature of the audio amplifier is the DC blocker which blocks the DC component of the signal. Figure 3.21 shows the output of the receiver when the knob is calibrated to 2.5 kHz, the carrier frequency of the sine wave. The spectrum shows that the sine wave has a frequency of 300 Hz, which is the exact value in the transmitter.
Shown in Figure 3.19 is the spectrum of the disabled demodulator (sine – dual sine – triangle) and comparing it to the spectrum of the enabled demodulator (sine – dual sine – triangle) shown in Figure 3.20, it can be perceived that the enabled lowpass filter only allows the first set of frequencies from the disabled filter, which is the reason why the signal in the time domain of the enabled is only the outline of the disabled.
Figure 3.21
Figure 3.22 shows the output when the knob is set to 3.75 kHz, the carrier frequency of the dual sine wave. It is seen in the spectrum analyzer the two frequency tones that the dual sine wave contains. A delayed dual sine wave is shown in the time domain, similar to the delay of the sine wave in Figure 3.21.
Figure 3.19
Figure 3.22
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Adjusting the knob to 5 kHz, the carri er frequency of the triangle wave, the frequency of the triangle wave in the spectrum is shown in Figure 3.23, which is approximately 80 Hz just like the frequency in the transmitter.
signals is modulated with the given intelligence frequency and carrier frequency. The frequency knob is a representation of tuning process where it has to be set on the frequency of the carrier you want to see in the Graphs. Although there is no noise introduce in the exercise, there is still other matters happen like the attenuation of the signal when it passes through the bandpass filters. The importance of the amplifier in the latter part of the circuit is quite an important part of the receiver.
Vincent N. Galang Figure 3.23
From the results of the AM receiver, it can be noticed that the amplitudes are not the same as the amplitudes of the baseband signals. It is because certain losses occurred when the signal is passed through the filters. That is why there is a need for an audio amplifier, but still, fidelity cannot be achieved for the amplitude of the output in the receiver varies on the frequency set in the knob. IV. CONCLUSIONS Simulating the AM transmitter and the receiver gives a clearer knowledge and understanding in the application of it in the real life. Three different baseband signals are created to represent the modulating signal that will be transmitted and demodulated. These signals are added and it can still be demodulated by tuning the control knob in its corresponding carrier frequency. So the knob basically, is the carrier frequency. The spectrum i n the front panel will display the frequency of the modulating signal, while the time domain shows the actual signal. Different stages in the receiver was passed through and upon seeing the final output of the receiver, it was then noticed that the amplitudes of the baseband signals and the amplitudes of the demodulated signals are not the same. Figuring out what may be the cause of this, the viewer is connected to every output stages in the receiver. Then, a conclusion was derived that every time the signal passes through a filter, the amplitude decreases. That is why an audio amplifier is needed. The group added a constant amplifier to the part of the DC blocker to achieve an output amplitude of 1 but the outcome didn’t made all the signal amplitudes to 1. Because the amplitude of the demodulated signal depends on the frequency set in the knob, which was also discovered when experimenting in the different output stages of the receiver.
Anne Loraine L. Avelino The Machine Problem demonstrates the basic process of AM Modulation from the input baseband signals, to transmission and reception. The machine problem clearly demonstrates how the
In our finished machine problem, I learned the principles of amplitude modulation and the fact that it can be divided into AM transmitter and receiver. Modulating an AM signal can be achieved in a number of ways. In terms of transmission, we need to create an AM modulator that modulates its baseband input signal array according to the equation given. While in terms of reception, essentially, the first stage is the RF section where in it amplifies the received RF signals. Second, the mixer section, where it down-converts the received RF frequencies to intermediate frequencies. Next is the IF section, the stage where it amplifies and select the signal. AM envelope detector takes place after it amplifies the selected signals. It demodulates the AM wave and converts it to the original information signal. I t can be implemented by an absolute value operator followed by a lowpass filter. After that, a DC blocker is used to eliminate the dc component of the demodulator output. Lastly, the audio amplifier which is the final stage of the receiver. The resulting output will be the required amplitude modulated signal.
Allona Jane M. Nañoz AM Radio Simulator emphasizes the function a nd significance of each block in the process. Baseband signals which represent the AM radio stations in real life is modulated in terms of amplitude then transmitted as RF Signals. On the other hand, receiver must be able to amplify and demodulate. After series of stages in the reception, signal’s amplitude is degraded, therefore an amplifier is needed. In an ideal reception, transmitted signal is the same when it is received and demodulated. However, based on this Machine Problem/ Simulation, it is impossible to attain such accuracy. The signal goes a lot of process before it is fed to the amplifier. Simulation revealed that the baseband signal as viewed from output has some difference compared to its state before transmission.
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This MP also showed the importance of filters in accepting and rejecting signals specially in the demodulation process. However, more filter means degradation of amplitude signal. Therefore, it is important to consider the number of filters in cascade to balance its effect on the amplitude of transmitted signal.
Justine Roy A. Punzalan
R EFERENCES [1]
[2]
Amplitude Modulation. [Online]. Available at: http://www.radio-electronics.com/info/rf-technology-design/amamplitude-modulation/what-is-am-tutorial.php Amplitude Modulation. [Online]. Available at: http://www.pa2old.nl/files/am_fundamentals.pdf
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