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Hardware Implementation of AM Radio Transmitter and Receiver

For long-distance data transfer, the first condition of the signal power has to be enough strong. The voice signal frequency is almost below 3400Hz signal which has no good enough power to transmit long-distance. Amplitude modulation (AM) is a modulation technique in which the amplitude of a high-frequency sine wave (usually at a radio frequency) is varied in direct proportion to that of a modulating signal. The modulating signal carries the required information and often consists of audio data, as in the case of AM radio broadcasts or two-way radio communications. The high-frequency sine wave (the carrier) is modulated by adding the modulating signal to it in a mixer. A simplified AM radio transmitter system is shown below.A simplified AM radio transmitter system
A simple form of amplitude modulation was originally used to modulate audio voice signals onto a low-voltage direct current (dc) carrier on a telephone circuit. A microphone in the telephone handset acts as a transducer and uses the sound waves produced by the human voice to vary the current passing through the circuit. At the other end of the telephone line, a second transducer (in the form of a small loudspeaker mounted in the remote handset) uses the varying voltage to produce sound waves that are close enough to the original speech patterns to be recognizable as the voice of the caller. Although the human voice is composed of frequencies ranging from 300 to approximately 20,000 hertz, the public switched telephone system limits the frequencies used to between 300 and 3,400 hertz, giving a total bandwidth of 3,100 hertz. This bandwidth is perfectly adequate for purely voice transmission since the higher frequencies in the human voice (i.e. those above 3,100 hertz) are not really needed for recognizable speech reproduction. The use of a limited bandwidth also makes the telephone system much simpler from an engineering perspective.
Whereas telephone signals can be transmitted at audio frequencies, the same is not really a practical proposition for radio transmissions. The main reason for this is that the optimum length of a radio antenna is half or a quarter of a wavelength. Since a typical audio frequency of 3,000 hertz has a wavelength of approximately 100 kilometers, the antenna would need to have a length of 25 kilometers to be effective – not a realistic proposition. By comparison, a radio frequency of 100 megahertz would have a wavelength of approximately 3 meters and could use an antenna 80 centimeters long. therefore, it becomes necessary to use a radio frequency carrier signal to transmit audio signals, which are used to modulate the carrier waveform.
A typical amplitude modulated signal

AM transmitter Section:

Am transmitter Section consists of 3 sub-circuits.
  1. Carrier frequency generator
  2. Voice signal Amplifier
  3. Amplitude modulator
                                     Figure 4: Our designed Transmitter circuit in Multisim 14.0

Carrier Frequency generator:

Normally allotted AM Radio Range is 500kHz-1.6MHz and sinusoidal signal is normally used as the carrier frequency. There are many ways to generate carrier frequency within the allotted range. Few types of sinusoidal oscillators are

  1. Phase Shift Oscillator
  2. Colpitts Oscillator
  3. Hartley Oscillator
  4. Wien Bridge Oscillator

For oscillation to be sustained, certain conditions known as Bark-Hausen criteria must be fulfilled. These conditions are
The loop gain of the circuit must be equal to or greater than 1
Phase shift around the circuit must be 0 or 360 degree

In our project, we use the Colpitts oscillator for generating carrier frequency Fc. A brief description of the Colpitts Oscillator is given below.
An LC circuit generates a Sinusoidal signal at its resonant frequency. But it decays after few oscillations. So There requires an amplifier that keeps amplifier gain greater than 1 and maintains the second condition of Bark–Hausen Criterion.

                                                             Figure 6: Damping Sinusoidal Signal
We designed an amplifier using Transistor BC547.
                                                Figure 7: Colpitts Oscillator Schematic section
Here LC tank circuit oscillates in its resonant frequency.
The resonant frequency is f=12πLC.  In our circuits, we use L= L1=47uH and C1=C2=C=2200pF. After calculation, we get resonant frequency =494.94KHz≈500Khz.
For simulation, we used National Instrument Multisim 14.0. The oscillator output graph from the simulation result is given below.
                                       Figure 8: Simulated Carrier wave in Multisim 14.0
The oscillator Frequency found from the simulation is 528.653KHz.
But in the real circuit, the frequency differs from the calculated value. From the Oscilloscope output, we get the sinusoidal output is slightly distorted and the carrier frequency is 833KHz.
                               Figure 10: Experimental carrier wave in Oscilloscope

Voice Signal Amplifier:

We require a transducer to convert a human voice signal into an electrical signal,  The transducer is a device that converts energy from one form to other. A microphone is a transducer that converts sound energy to electrical energy. Here we use an electret microphone condenser for converting a human voice signal into an electrical signal.
The electrical signal from the electrical microphone is a current signal which is very week so we need necessary amplifiers to convert the voice current signal to an amplified voice voltage signal. Here we use a Transimpedance Amplifier. The schematic is given below.
This topology was selected for a few reasons.

it allows for the single-supply operation to be easily accommodated by biasing the non-inverting input of the op-amp to the mid-supply point.

The gain of the pre-amp is determined by R5, but the noise gain of the op-amp is determined by the ratio of R5 to R1. Therefore, it is possible to achieve lower noise with this topology than with a non-inverting amplifier. Finally, because capacitor C3 is chosen to have a very low impedance at audio frequency

Typical Sound waveform when we play music in front of Electret microphone is included here

Amplitude Modulator:

The amplitude modulator consists of a simple BJT and Bias circuitry. Here C6 is used to block the DC portion of the Audio waveform. It is selected in such a way so that its impedance is kept minimum so an audio wave of low frequency can pass through it. C6=12πflow(audio)R8=10uF
C5 is used to block the Dc portion of Colpitts Oscillator output. As it allows to pass high frequency, So its impedance can be lower enough for the low value of c5.  C5=12πflowColpitts)R5=2.2nF
                                                                   Figure 14:Amplitude Modulator Circuit
Simulation results found from NI Multisim 14.0 are shown below

                                    Figure 15: Message signal(200mVp-p) and modulated signal
So from the simulation, we see that the message signal is modulated in Double sideband (DSB) modulation.
R5=10k is just to give the base a fixed non-floating voltage.
100% modulation is found for the 1500mVp-p-value of the message signal. (From simulation value)
                                   Figure 16: Message signal(1500mVp-p) and modulated signal(100% modulation)
                                                              Figure 17: Observed Voice signal Modulation
To ensure the situation so that the signal does not distort we have designed a voltage divider to ensure the input voltage of the Amplitude modulator is less than 1.5V
                                                    Figure 18: Voltage divider to prevent Distortion of voice signal

AM Receiver:

The process of demodulation for DSB-AM is relatively straightforward. The radiofrequency carrier can be removed from the signal using a simple diode detector consisting of a diode, a resistor, and a capacitor. The incoming signal is rectified by the diode, which allows only half of the alternating waveform to pass through it. The capacitor removes the remaining radio frequency signal components to provide a smooth output, and the resistor allows the capacitor to discharge. THUS, an AM receiver can be produced relatively cheaply since there is no requirement for specialized components. The basic circuit diode detector circuit is shown below.
A basic diode detector circuit
Figure 19: Envelope Detector  circuit
Time constant τ=RC selected such a way so that it maintains the following conditions
1fc≪τ≪1B
Where fc=833kHz is the carrier frequency and B=6800Hz is the Bandwidth of the signal.
So1.2us≪τ≪147us. For this condition to fulfill we select the value of R=10Ω and C=4.7uF.
Diode Selection: In a diode detection circuit we use a germanium diode to detect the message signal.
Figure 20: Voice signal(Blue) &  Recovered voice signal(Yellow)
Here blue color is the original voice signal and the yellow color is the diode detected signal.

Power Supply:

We designed a 12V voltage power supply from a 220V AC system using an LM317 Linear regulator.

Discussions:

The designed circuit cannot generate DSD-SC Modulation
We were unable to design the power amplifier stage after the modulator circuit. As a result, we could not make this modulation process wireless
Our Electret Audio Amplifier Circuit works in the range of 2.7V-5.2V. Unfortunately, while using it for the first time, we biased it with the voltage of 12V. As a result, it was burnt. So as a precaution we designed a 5V power supply using a 7805 Linear Voltage Regulator.

As audio amplifier output is in the range of 0-5V and our modulator circuit takes a maximum of 1.5V for 100% modulation. So to prevent distortion a voltage divider circuit was designed.