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Am Emitter

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Submitted By marcattieh
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AM emitter
Introduction:
This is the second project done in Communication Systems Lab, the first one was an FM transmitter and it was to communicate via radio transmission using FM modulation technique, and this one is the AM emitter, and the purpose of this project is to create an AM emitter at a bandwidth around 1000Hz. Now what is an AM emitter? An AM emitter is a circuit builds to modulate the message signal and transmit it, where the amplitude of the carrier signal varies in direct propagation with the message signal. It is limited in power but provides amplitude modulation of voice over the medium waveband.
Circuit:

Procedure:
We connect the circuit as shown, but first we de attach the audio circuit that is connected to the oscillator by 4.7uF capacitor, and using a capacitor of 100pf, and then we connect the oscilloscope at the antenna. When I didn’t obtain a sine wave on the oscilloscope, I by the help of the instructor Dr Amin Abillamaa, connect the inductor to the emitter of the first transistor. After this I got a very high sine wave so I substituted that capacitor from 100pf to 1000pf (1nf), and then we got a carrier signal, and also we replace the variable capacitor by a 330pf.
Before connecting the audio part, we connect the function generator to the 4.7uF capacitor and we set the parameters to 3Vpp and 800Hz. Then we remove the oscilloscope and we connect the alligator as an antenna and we change in the radio frequency to het a pure whistle.
Then we connect the audio part, and we were working to get the purest audio signal that can be transmitted about 1m, for this we connect the capacitor to the potentiometer and we rise the potentiometer to the minimum or we can connect directly the collector of the second transistor to the capacitor. As a result, the audio signal was pure, but it wasn’t traveling further, so I started searching for ideas, to I finally realize that we need to increase the current supplied at the collector of the second transistor, and to do this we replace the 10k resistor by a 4.7k and we get the desired audio signal that can travel purely 1m by talking on the microphone.

Analysis of the circuit:
This circuit is composed in 2 halves and they are the audio amplifier and an RF oscillator. The RF oscillator is build around the first transistor, and the tank L and VC determines the frequency of oscillation and that is between 500 KHz and 1600 KHz. This circuit has a limit in the output power, but it will provide AM modulation. The capacitor C3 couple signals from the base to the top of the L, and the capacitor C4 ensures that the oscillation is passed from the collector to the emitter, and resistor R1 ensure that the oscillation will not be shunted to ground, and transistor one is to opposite ends of the tank of the circuit, however transistor two is a wired as a common emitter amplifier, and Capacitor C2 has a role of decoupling the emitter resistor and realizing full gain of this stage.
Materials needed & parameters:
We are asked to make an inductor or a coil. So, we did a coil of 62 turns and a diameter 10cm with a width of 0.5cm with taps every 6turns as said in the instruction in the book, and his inductance will be between 260uH and 265uH.
Set the parameters to 3Vpp and to 800Hz.
Materials needed for the circuit like capacitors, resistors, potentiometers and transistors as shown in the figure:
Capacitors (2.2uF 1nF 4.7uF 100uF 100pf or 1000pf)
Resistors (47K (2) 10K (2) 1K (2))
Potentiometer (4.7K)
Transistors (BC109C (2))

Dynamic microphone for the audio signal.

An antenna is not needed for the circuit, only a 30 cm wire or as we used an alligator.

A 9V DC voltage is needed.

Results:

After I finished building the circuit, and I did all the modifications to improve the power of transmission, and using the coil of 263uH and getting the purest audio signal, as a result we got a pure sine wave with very high frequency of:
Frequency= 1MHZ 1000000HZ
Amplitude=3.7V
Inductance=220uH
Capacitance=330pf
With the help of a small radio in the lab I started tuning to get a max pure sound but unfortunately I couldn’t expect by varying the resistor R3 from 10K to 4.7K, with this variation I got a pure wave and very acceptable sound.

Problems faced:

Building the coil needed for the project took a lot of time to be done perfectly.

The circuit needs a lot of modifications based on accurate calculations to improve the power of transmission, for example we replace the variable capacitor with a 330pf and other modifications.

We faced a problem that there was some distortion in the wave and that was explained by the usage of a power supply which has a ground connected with the output, that can add noise to my wave, and that can be solved by using a 9V battery.

The modifications done to get a pure sine wave before connecting the audio signal, and that when testing the carrier signal and before transmitting the AM signal.

Advantages and Disadvantages of AM modulation:
Advantages:
Easy to design and cheap.
BW=2fm or 2W.
No need for a synchronous receiver.

Disadvantages:
If m (t) was a single tone function of many terms we have to expect bigger number of sidebands (BW very higher).
Power loss (there is a part of the power in the carrier component).

Graph of the Circuit & Result on Oscilloscope:

Conclusion:

This project AM emitter consists of 4 parts: making the inductor – testing the oscillator – modulating the carrier – transmitting AM signals. The projects assigned and experiments done in class not only improved our logic understand of communication system but also made us touch the joy of communication.

Parts list in the lab:
Oscilloscope – Power supply – function generator – small radio.

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