Design and implementation of amplitude modulator and demodulator
Introduction
Communication is the most important needs from the very beginning of civilization. With the advancement of time and growth of technology various methods are introduced for communication. Amplitude Modulation (AM) is the most dominant method of broadcasting messages and information since the very beginning of 20th century. And the noteworthy thing is that AM is still widely used as a broadcasting medium.
Amplitude Modulation: Background
AM radio began with the first, experimental broadcast on Christmas Eve of 1906 by Canadian experimenter Reginald Fessenden, and was used for small-scale voice and music broadcasts up until World War I. San Francisco, California radio station KCBS claims to be the direct descendant of KQW, founded by radio experimenter Charles “Doc” Harold, who made regular weekly broadcasts in San Jose, California as early as June of 1909. On that basis KCBS has claimed to be the world’s oldest broadcast station and celebrated its 100th anniversary in the summer of 2009. The great increase in the use of AM radio came late in the following decade as radio experimentation increased worldwide following World War I. The first licensed commercial radio services began on AM in the 1920s. XWA of Montreal, Quebec (later CFCF, now CINW) claims status as the first commercial broadcaster in the world, with regular broadcasts commencing on May 20, 1920. The first licensed American radio station was started by Frank Conrad, KDKA in Pittsburgh, Pennsylvania. Radio programming boomed during the “Golden Age of Radio” (1920s–1950s). Dramas, comedy and all other forms of entertainment were produced, as well as broadcasts of news and music.
AM radio technology is simpler than FM radio, DAB, Satellite Radio and HD Radio. An AM receiver detects amplitude variations in the radio waves at a particular frequency. It then amplifies changes in the signal voltage to drive a loudspeaker or earphones. The earliest crystal radio receivers used a crystal diode detector with no amplification.
In North American broadcasting practice, transmitter power input to the antenna for commercial AM stations ranges from about 250 watts to 50,000 watts. Experimental licenses were issued for up to 500,000 watts radiated power, for stations intended for wide-area communication during disasters including Cincinnati station WLW, which used such power on occasion before World War II. WLW’s superpower transmitter still exists at the station’s suburban transmitter site, but it was decommissioned in the early 1940s and no current commercial broadcaster in the US or Canada is authorized for such power levels. Some other countries do authorize higher power operation (for example the Mexican station XERF formerly operated at 250,000 watts). Antenna design must consider the coverage desired and must direct the transmitted signal so as not to interfere with other stations operating on the same or adjacent frequencies.
Scope
An Amplitude modulation can be operated using differential amplifiers. The output of a differential amplifier can drive a dual differential amplifier for amplitude modulation and demodulation as well. An IC, MC1496, widely used as differential amplifier, serves this purpose of modulation-demodulation. An operational Amplifier (Op-Amp) is used to implement a demodulator circuit and an amplifier as an extension of the project. A power supply unit of ±5 volts and ±12 is constructed to drive all the circuits in this project.
- To understand the basic theory of amplitude modulation.
- To understand the waveform and frequency spectrum of amplitude modulation, also calculate the percentage of modulation.
- To design and implement the amplitude modulator.
- To understand the measurement and adjustment of amplitude modulation circuit.
- To understand the theory of amplitude demodulation.
- To design and implement the diode detection amplitude demodulation.
- To design and implement the product detection amplitude demodulator.
Amplitude Modulation Transmitter
Amplitude modulation is the variation of amplitude of the carrier signal in terms of message signal. Actually the frequency of the message signal we transmit in our real life is a low frequency signal. Most of cases, this is either voice or music signal. The frequency range of the message signal is in the audible range that is from 20 Hz to 20 kHz. This very low frequency signal can not be transmitted directly in wireless communication for several limitations:
Firstly, the antenna size of both the transmitter and receiver should be very large to efficiently transmit and receive the signal, and size of the antenna is inversely proportional to frequency; consequently directly proportional to wavelength. It is indeed beyond imagination to transmit message signal directly with antenna without carrier on account of the huge size of the antenna.
Secondly, receiving antenna picks up signals depending on frequency. If we transmit message signals in the same frequency range without carriers, the antenna will receive all the signals without discrimination. So, the receiver will hear a messed up sound.
Besides these above two there are several other limitations like high frequency transmitted signal is more immune to noise than low frequency signal.
So, before deciding transmitting message signals we have to shift them to high frequency. We will shift all the message signals to high frequencies different from each other. This is called frequency division multiplexing (FDM). Carrier will help us to this frequency shifting purpose. The three parameters of the carrier can be modulated to get frequency shift. These are amplitude, frequency, and phase. In this article amplitude modulation is our concern. A detail theoretical analysis followed by circuit diagrams and results will be provided later in this chapter.
The function of the amplitude modulation transmitter is to modulate the amplitude of the carrier with respect to message signal. This signal will be transmitted later. But before this the strength of this transmitted signal will be raised by the power amplifier. The power amplifier is followed by a band pass filter. This high power signal will be propagated as electromagnetic wave by the transmitting antenna. So, the transmitter consists of amplitude modulator, power amplifier, band pass filter, and transmitting antenna. A block diagram of the amplitude modulation transmitter is provided in Figure 2.1.
|
Figure 2.1: Amplitude modulation transmitter
But in this work the power amplifier and transmitting antenna portion is omitted from the amplitude modulation transmitter. Actually we have used no channel in our test. We have connected directly the output of the transmitter to the input of the receiver. So, in this work antenna was useless.
Amplitude Modulator
Amplitude modulator is a multiplier circuit. It multiplies the two input signals. IC MC1496 is used in conjunction with other passive circuit elements to serve this purpose. A detail theory on amplitude modulation followed by the modulator circuit is provided in the next subsections.
Amplitude modulation: theory
In amplitude modulation (AM), we utilize the amplitude of audio signal to modulate the amplitude of carrier signal, which means that the amplitude of the carrier signal will be varied with amplitude of audio signal. The waveform is shown in Figure 2.2.
Figure 2.2: Signal waveform of amplitude modulation
Let the amplitude of the audio signal be
And the carrier signal be
,
Then the amplitude modulation can be expressed as
(2.1)
Where
Am: Audio signal amplitude
Ac: Carrier signal amplitude
ADC: DC signal amplitude
: Audio signal frequency
: Carrier signal frequency
Among these three sub-equations in Eq. (2.1), we use the last one in our study. We can rewrite the last sub-equation of Eq. (2.1) as
(2.2)
Where modulation index, m = Am/ADC.
(2.3)
The first term of Eq. (2.3) represents scaled carrier signal and the second term represents double side band signal. From Eq. (2.3), we can sketch the frequency spectrum of amplitude modulation as shown in Figure 2.3.
Figure 2.3: Frequency spectrum of amplitude modulation signal
Since the audio signal is hidden in the double sidebands and the carrier signal does not contain any message, therefore power is lost in carrier during transmission of amplitude modulation signal. For this reason the transmission efficiency of AM modulation is lower than double sidebands suppressed carrier modulation but its demodulation circuit is much simpler. A double side band suppressed carrier amplitude modulation signal is shown in Figure 2.4.
Figure 2.4: A double sideband suppressed carrier amplitude modulation signal
There is an important parameter “m” in Eq. (2.3) called modulation index or depth of modulation. Normally it is represented in percentage, so we also call modulation percentage. The definition is as follow:
(2.4)
Generally the magnitude of the DC signal is not easy to measure; therefore we express the modulation index in another form
(2.6)
Where Emax = Ac + Am and Emin = Ac – Am.
We know that at amplitude modulation, the audio signal is hidden in the double sidebands, so if the double signals are getting stronger, the transmission efficiency is getting better. From Eq. (2.3), it is clear that double sideband signals are proportional to modulation index. Thus the larger is the modulation index, the better the transmission efficiency. Normally modulation index is smaller or equal to 1. It greater than 1, we call it over modulation. The effect of modulation index on the amplitude modulated signal is shown in Figure 2.5. Figure 2.5 (a) shows the amplitude modulated signal with low modulation index and Figure 2.5 (b) with high modulation index.
Figure 2.5: (a) Amplitude modulated signal with low modulation index.
(b) Amplitude modulated signal with high modulation index.
Amplitude modulation: circuits
Figure 2.6 is the circuit diagram of the amplitude modulator. We can see that carrier signal and message signal belongs to single ended input. Message and carrier signals are input to pins 1 and 10 respectively. R8 determines the gain of the whole circuit and R9 determines the gain of the bias current. If we adjust the potentiometer or change the amplitude of the message signal we can change the percentage modulation of the modulation index.
Figure 2.6: Circuit diagram of amplitude modulation by utilizing MC1496
In this work, we utilize the balanced modulator MC1496 to implement amplitude modulator. Following the variation of the input signal frequency, balanced modulator (MC1496) can become a frequency multiplier, amplitude modulator or double side band suppressed carrier modulator. Its input signal, output signal and circuit characteristics are shown in Table 2.1.
|
|
|
|
|
|
|
|
|
|
|
|
Figure 2.7: Internal circuit diagram of the balanced modulator, IC MC1496
Figure 2.7 is the internal circuit diagram of MC1496, where D1, R1, R2, R3 Q7, and Q8 comprise an electric current source, which can supply DC bias current for Q5 and Q6. Q5 and Q6 comprise a differential combination to drive the dual differential amplifiers constructed by Q1, Q2, Q3, and Q4. Pins 1 and 4 are the inputs of audio signal; pins 8 and 10 are the inputs of the carrier signal.
Table 2.1: Three different types of modulation signal produced by different signal frequency of balance modulator
| Input carrier signal | Input audio signal | Output balanced modulator | Circuit Characteristics |
| fc | fc | 2fc | Frequency multiplier |
| fc | fm | fc, fc+ fm, , fc– fm | Amplitude modulator |
| fc | fm | fc+ fm, , fc– fm | DSB-SC modulator |
The pin configuration of the MC1496 is provided in Figure 2.8. Pins 5 and 14 are the biasing pins. Pins 1 and 4 are reserve for the audio input signal and pins 8 and 10 for carrier input signal. Output is the difference of two signals of pins 6 and 12. There is no connection to pins 7, 9, 11, and 13.
Figure 2.8: Pin configuration of MC1496
Figure 2.9: The bread board circuit of the amplitude modulator using IC MC1496
As a test, we first implement our circuit diagram of the amplitude modulator on bread board on the laboratory of the United International University. Figure 2.9 is the circuit diagram of this amplitude modulator. To bias the circuit we have used the DC power supply of the laboratory. Signal generators of the laboratory are used to input both the message and carrier signals. Both the message and the carrier signals were sinusoidal.
Figure 2.10: PCB layout of amplitude modulator
After finding that our amplitude modulator is functioning properly, we have designed printed circuit board (PCB) of the same circuit using the specialized tool, Proteus. The PCB diagram of the amplitude modulator is shown in Figure 2.10. Proteus is a user friendly and helpful tool to PCB design. The demo circuit diagram of the amplitude modulator is shown in Figure 2.11.
Figure 2.11: Demo circuit diagram of amplitude modulator
Figure 2.12: Amplitude modulator implemented on the printed circuit board
Finally, on the designed PCB we have implemented the amplitude modulation circuit. Firstly, the implemented circuit on the PCB was not functioning properly. So, we redesign the PCB again. This time PCB works properly. The hardware circuit diagram implemented on PCB is shown in Figure 2.12. Again we have used separated DC power supplies from the laboratory. Signal generators are again also used from the laboratory to test the PCB implemented circuit diagram. As a proof that our circuit diagram was functioning properly we have provided proof through the picture of the signal in oscilloscope accompanied with circuit diagram later.
Amplitude Demodulation Receiver
For some limitation of the direct transmission of the message signal as stated in chapter 1, we have encoded the message signal in the amplitude of the carrier signal and transmitted it. We receive this signal in our practical life by the radio receiver through its antenna. In this project, we have used no antenna for transmission as well as reception. We have directly connected the output of the amplitude modulation transmitter to the input of the amplitude modulation receiver for testing purpose. But the amplitude modulated received signal at the amplitude modulation receiver input is useless and also meaningless if we can not decode or demodulate it. The amplitude modulated signal can not directly convey any information. Message is the only signal that carries information. So, first of all we demodulate the amplitude modulated signal by using amplitude modulation receiver to recover the message signal at the receiver side. There are two possible ways according to our knowledge to recover the message signal at the receiver side: firstly, diode detector or asynchronous detector and also known as non-coherent detector, and secondly, product detector or synchronous detector and also known as coherent detector. A detail theoretical analysis followed by circuit diagrams is provided in the next subsection to learn diode detector and after next subsection to learn product detector.
Amplitude modulation receiver: diode detector
Since amplitude modulation signal utilizes audio signal to modulate carrier signal, which means the variation of carrier signal amplitude is followed by the change of audio signal amplitude. Hence the objective of amplitude demodulator is to take out the variation envelop detection from amplitude modulation signal. Block diagram of the amplitude modulated signal using diode detector is provided in Figure 3.1. In general the amplitude modulated signal is picked by the receiving antenna of the amplitude demodulator at the input of the receiver. This weak signal is amplified several times to increase the amplitude of the voltage. A cascade amplifier stage instead of a single amplifier is used to improve filtering capacity of the amplifier as an auxiliary function.
Figure 3.1: Amplitude demodulator circuit using diode detector
Figure 3.2: (a) Amplitude modulated input signal to the amplitude demodulator (diode detector) and
(b) Reconstructed message signal at the output of the demodulator.
Figure 3.3: Asynchronous amplitude demodulation using diode detector
The rectifier in conjunction with the RC filter detects the envelope of the modulated signal. That is why it is also known as envelope detector. The time constant of the RC circuit should be large enough to follow the message signal or the envelope of the modulated signal instead of the carrier signal. If the time constant is very low then the RC circuit will follow the very high frequency signal. So, the inverse of the time constant should be approximately equal to the message signal frequency and very low to the carrier signal frequency for most accurate detection. DC blocking capacitor as the name implied is used to block the unwanted DC from the reconstructed message signal. Usually DC is available as additions at the output of the RC filter (LPF). The reconstructed signals will be fed to the power amplifier to achieve enough capacity to drive the loud speaker (not shown in the block diagram in Figure 3.1).
The circuit diagram of the diode detector is provided in Figure 3.3 to under the envelope detection. Two cascade amplifiers as stage are used exploiting OP-AMP as the active device. This stage acts as a voltage amplifier.
The circuit diagram of the diode detector is first tested through PSPICE simulation software. It is functioning properly. Then we implement the circuit on the bread board. The printed circuit board (PCB) design of this demodulator is shown in Figure 3.4. We take the help of the layout design and simulation software proteus.
Figure 3.4: PCB layout of the amplitude demodulator: diode detector
Figure 3.5: Demo circuit diagram of amplitude demodulator: diode detector
Figure 3.6: Diode detector circuit of amplitude demodulator implemented on PCB
After implementing the diode detector circuit on the PCB layout, we have tested the circuit whether it is functioning properly. It is pleased to say we get satisfactory performance. The hardwire circuit diagram implemented on the PCB is shown in Figure 3.6. As a proof our simulation we have shown later the reconstructed signal output from the diode detector accompanied with the oscilloscope.
Amplitude modulation receiver: product detector
The product detector is the alternative of the diode detector. The product detector is more sophisticated. Again it is costlier than diode detector. One important advantage of the product detector is it provides more precise output. Unlike diode detector, here balanced modulator is used as the preliminary stage for the detection purpose. Other things remain as before. Balanced modulator is a multiplier circuit. MC1496 is used as balanced modulator. We have also used balanced modulator for modulation purpose. In the block diagram provided in Figure 3.7 the balanced modulator multiplies the amplified modulated signal with the carrier signal. This carrier signal should be extracted in the demodulator circuit. Synchronization is very important of the carrier signal with the received modulated signal. Otherwise very poor reconstruction of the message signal takes place.
Figure 3.7: Amplitude demodulator circuit using diode detector
Figure 3.8: (a) Amplitude modulated input signal to the amplitude demodulator (product detector) and (b) reconstructed message signal at the output of the demodulator.
That is why this process is called synchronous detection. Balanced modulator is also known as synchronous detector. The internal circuit diagram of the balanced modulator (MC1496) is provided in the previous chapter. Let, xAM(t) is the amplitude modulated signal, c(t) is the carrier signal. That is,
(3.1)
(3.2)
When, these two signals input into two differential ports of balanced modulator, then the balanced modulator output signal is as follow
(3.3)
Where, k represents the gain of the balanced modulator. The first term is the DC signal, second term is the audio signal, and third term is the second harmonic modulated signal. If we can take out the second term from xout(t), we can obtain the exact amplitude demodulated signal or audio signal. The DC signal which is the first term of xout(t) in Eq. (3.3) can be blocked by the DC blocking capacitor. The second harmonic modulated signal will be filtered output by the low pass filter. Therefore the signal that we obtain at the output of the product detector is:
(3.4)
Equation (3.4) represents the scaled version of the audio signal or in other word the original amplitude modulated signal can be taken out by the product detector.
Figure 3.9: Circuit diagram of the Product detector in simulation software
We have first simulated the product detector circuit. The simulation results are quite good. The diagram of the simulated circuit is shown in Figure 3.10. After getting satisfactory results from the simulation we implemented the circuit in our laboratory. The circuit function works properly as well. Then we decide that we transfer the circuit in the PCB. The implemented circuit of the product detector on the bread board in shown in Figure 3.10. The PCB layout and demo are shown in Figures 3.11 and 3.12, respectively. The circuit diagram implemented on the PCB is shown in Figure 3.13.
Figure 3.10: Physical view of product detector on bread board
Figure 3.11: PCB layout of amplitude demodulator: product detector
Figure 3.12: Demo circuit diagram of amplitude demodulator: product detector
Figure 3.13: Circuit of product detector implemented on PCB
The above demodulator circuits are workable with bias voltage in the range of ±12 V. This is a very high voltage and practically infeasible to use in portable receiver. So, later we have decided to implement a diode detector circuit with bias voltage +3 V using high frequency BJT as amplifier circuit. But although we have designed the circuit for the lack of time we have not completed the work. The schematic diagram of this receiver is shown in Figure 3.14.
Figure 3.14: Circuit diagram of the AM diode detector receiver with low bias (bias voltage 3V)
Amplitude Modulation Transmitter and Receiver
Detail explanation of the amplitude modulation transmitter and receiver is provided before in chapter 1 for transmitter and in chapter 3 for receiver. So we will not explain or say new thing here. We will just show the combine diagrams of transmitter and receiver. We here also show some practical signals as a token of our work.
The printed circuit board design (PCB) were developed using the specialized software, Proteus. The PCB diagrams are shown in Figure 4.1.
Figure 4.1: PCB layout amplitude modulation transmitter and receiver
The circuit diagram of the whole circuit that is in combine the transmitter and the receiver is implemented on the PCB. The PCB diagram of the whole AM circuit is shown in Figure 4.2.
Figure 4.2: Implemented circuit diagram of the amplitude modulation transmitter and receiver on the PCB
Few selected signals of our experiment are shown in Figures 4.3, 4.4, and 4.5.
Figure 4.3: Input low frequency message signal by signal generator
Figure 4.4: Input high frequency carrier signal by signal generator
Figure 4.5: Amplitude modulated signal (Under modulation) in oscilloscope
Regulated DC power supply
Power supply Unit
Power supply unit is use for power up the whole block/unit of the system. We collect power from 220VAC. By using a step down transformer we convert this into around 13V AC. After that we use a bridge rectifier to convert this AC into DC. So after that we get around 12V DC. We can not ensure the voltage stability because of line voltage fluctuation. For this reason we use voltage Regulator IC. For 12V regulator we use 7812 IC and for 5V regulator we use 7805 IC. Also use IC7905 for -5V and IC7912 for -12V. Those are three pin chip looks like a transistor.
Bridge Rectifier
A diode bridge is an arrangement of four diodes in a bridge configuration that provides the same polarity of output for either polarity of input. When used in its most common application, for conversion of an alternating current (AC) input into direct current a (DC) output, it is known as a bridge rectifier. A bridge rectifier provides full-wave rectification from a two-wire AC input, resulting in lower cost and weight as compared to a rectifier with a 3-wire input from a transformer with a center-tapped secondary winding. The circuit diagram of the bridge rectifier is shown in Figure 5.1. The output of the bridge rectifier is the absolute value of input. Both the input and output waveform of the bridge rectifier is shown in Figure 5.2.
Pulsating DC, the output of the rectifier is not sufficient to drive properly the active devices. So we have low pass filtered the output of the bridge rectifier (Figure 5.3). Now we get output DC with ripple (Figure 5.4). To remove the ripple from the DC power supply we have to use voltage regulator.
Figure 5.1: Circuit diagram of the bridge rectifier
Figure 5.2: (a) Input and
(b) Output of the bridge rectifier
Figure 5.3: Circuit diagram of the bridge rectifier with filter
Figure 5.4: Ripple DC output of the bridge rectifier after low pass filtering
Regulator IC
Regulator IC is the device which keeps constant the output voltage what ever the input voltage is. If input voltage is more then the desired voltage, the regulator IC make the output constant.
General Description of 7812/7805
The LM78XXC monolithic 3-terminal positive voltage regulators employ internal current-limiting, thermal shutdown and safe-area compensation, making them essentially indestructible. If adequate heat sinking is provided, they can deliver over 1.0A output current. They are intended as fixed voltage regulators in a wide range of applications including local (on-card) regulation for elimination of noise and distribution problems associated with single-point regulation. In addition to use as fixed voltage regulators, these devices can be used with external components to obtain adjustable output voltages and currents. Considerable effort was expended to make the entire series of regulators easy to use and minimize the number of external components. It is not necessary to bypass the output, although this does improve transient response. Input bypassing is needed only if the regulator is located far from the filter capacitor of the power supply.
The 5V and 12Vregulator options are available in the steel TO-3 power package.
Features
- Complete specifications at 1A load
- Output voltage tolerances of ±2% at Tj = 25°C and ±4% over the temperature range (LM340A)
- Line regulation of 0.01% of VOUT/V of ?VIN at 1A load (LM340A)
- Load regulation of 0.3% of VOUT/A (LM340A)
- Internal thermal overload protection
- Internal short-circuit current limit
- Output transistor safe area protection
- P+ Product Enhancement tested
General Description of 7912/7905
NEGATIVE VOLTAGE REGULATORS Mostly available -ve voltage regulators are of 79xx family. You will use -ve voltage if you use IC741. For IC741 +12v and -12v will be enough, even though in most circuits we use +15v and -15v. You can get more information about 7905 from the following link. 7805 gives fixed -5V DC voltage if input voltage is in (-7V,-20V). The mainly available 79xx IC’s are 7905,7912 1.5A output current, short circuit protection, ripple rejection are the other features of 79xx and 78xx IC’s VARIABLE VOLTAGE REGULATORS Most commonly variable voltage regulator is LM317 although other variable voltage regulators are available. The advantage of variable voltage regulator is that you can get a variable voltage supply by just varying the resistance only.
Figure 5.5: Internal Circuit Diagram of 7805 and 7812 IC
Schematic Diagram of Power supply unit
The schematic diagram of the DC power supply unit is shown in Figure 5.6.
Figure 5.6: Schematic Diagram of Power supply unit.
Finally we are successful to develop a communication system based on amplitude modulation through the design of transmitter & receiver. It is functioning properly. While designing and implementing the system, we are always concerned about the economical design. The overall cost of the implementation is provided in Appendix A. This circuit diagram can be used as a trainer board. The total cost will be near 1200 BDT. So, it will be very much economical as a trainer board compare to abroad’s. We have also designed AM radio receiver to receive the broadcasting radio signal. But this design is not so much economical. For the lack of time we have not completed this properly.
References
| [1] | Man-Long Her, Fan-His Kung, “Digital and Analog Communication Systems, Application Measurements”, 1st edition,September,2003 |
| [2] | Communication systems, Simon Haykin- 4th edition. ISBN 9971-51-305-6. |
| [3] | Operational Amplifiers and linier integrated circuits, Robert F. Coughlin, Frederick F. Driscoll-6th edition. ISBN-81-203-2096-4. |
| [4] | Modern Digital and Analog Communication Systems, by Lath-4th Edition |
| [5] | “Digital and Analog Communication’ … Sharma Sanjay, |
| [6] | “Signals and Systems”, 4th Ed. (Revised), S K Kataria and Sons (2005) … |
| [7] | Modern Digital and Analog Communication Systems. Pakistan: Oxford, 1998. 3rd. Edition. ISBN: 0195473701. |
| [8] | http://www.national.com/ds/LM/LM7905.pdf |
| [9] | http://cache.national.com/ds/LM/LM7905.pdf |