AM RADIO RECEIVER

Fundamentals of AM Modulation and Demodulation

'In Amplitude Modulation the transmitted signal expressed mathematically as u(t)=Ac[1+m(t)]cos(2πfct+φc)Double-Sideband part: Acm(t)cos(2πfct+φc),Carrier Component part: Accos(2πfct+φc).

It is sometimes convenient to express m(t) as
m(t)=amn(t)
where mn(t) is normalized such that its minimum value is -1. This can be done, for example, by defining mn(t)=m(t)/max|m(t)|
The scale factor a is called the modulation index.'90

Amplitude Modulator

There are several different methods for generating AM modulating signal.Since the process of modulation involves the generation of new frequency components, modulators are generally characterized as nonlinear and, or, time-variant systems.In the figure below the Square Law Fet Amplitude modulator is depicted.

Amplitude Demodulator

At the receiver part of the AM radio, we can easily observe the carrier signal and carrier signal multiplied with message signal. Therefore we can easily separate the message from the carrier signal with the help of the combination of the rectifier and the low-pass filter which are called as an envelope detector.

In this project, first of all the antenna grabs all kind of signals from air then passes to the tuner. The tuner is used to adjust the desired frequency. For medium wave AM receiver, it is between 550kHz – 1600kHz. To achieve that it is necessary to use an inductor and a capacitor. After we obtain the signal which is at desired frequency, we should amplify this signal in order to get a sufficiently large signal. To do that, we used diode and a low-pass filter as a detector. At this state we have the pure message signal. As a next step we should amplify the signal again and again to he ar the audio signal from speaker more easily.

Tuner Design

LC resonant circuit comprising L1 and VC1 used to select or tune the required frequency and station. L1 is a coil of wire wound on a ferrite rod. 
This behaves like a transformer - currents flowing in L2 generate a changing magnetic flux which cuts L1 and induces an emf in it.

The Antenna Graps All Kind of Signals from the Air

RF Amplifier

An RF power amplifier is a type of electronic amplifier used to convert a low-power radio-frequency signal into a larger signal of significant power, typically for driving the antenna of a transmitter.

Figure 2.6 Process of Two Stage RF Amplifying

Detector

The receiver must extract or detect this audio signal by separating it from the carrier. 
One way of doing this is by means of a diode detector.

Audio Amplifier

An audio amplifier is an electronic amplifier that amplifies low-power audio signals (signals composed primarily of frequencies between 20 hertz to 20,000 hertz, the human range of hearing) to a level suitable for driving loudspeakers.
For comfortable listening with headphones an audio signal of about 1 volt amplitude is desirable, so that an amplifier with a gain of about 100 times is needed. 

Process of Audio Amplifying and Simulation of Two Stage Audio Amplifier

Power Amplifier and Speaker

The circuit is set to give a voltage gain of -20, which is more than enough.  The output produced by this circuit is quite limited because the device can only provide a current of about 15 mA maximum at its output terminal, simply not enough to drive a loudspeaker effectively.

LOUD SPEAKER

The large capacitor C17 is to prevent a substantial DC current flowing through the speaker, which could cause long-term damage.

IMPLEMENTATION

First of all, for the tuner part we found a medium wave inductor coil with a ferrite rod and a variable capacitor which can give us a capacitance value between 0 – 295 pF.

Implementation of RF Amplifiers

As a second step, we implemented RF amplifier part with a JFET called BF245C. For the design to work well with weak signals from more remote transmitting stations, or with a smaller antenna, additional amplification is needed. We used two stage amplifier to amplify the signal by a factor of 100.

IMPLEMENTATION OF DETECTOR

After we obtain the amplified medium wave signal, we make it to pass over the detector to seperate the message signal from the carrier signal.

Implementation of Audio Amplifiers

As a next step, we set the audio amplifier circuit with a gain of 100 to hear the voice from loudspeaker.

Power Amplifiers and Loudspeaker

After that, since the output of the audio amplifier is not enough to derive a loudspeaker we used power amplifier as a final step.

Making the Printed Circuit Board of the Am Receiver

Making the Plot of the Printed Circuit Board

The Overall Design of the Am Receiver

The Overall Implementation of the Am Receiver

CHALLANGES

**The first difficulty which we come up with was the finding proper inductor for the Medium Wave AM receiver. We first tried to make the inductor manually with turning the magnet wire with air core. We measured the inductance value at every step. However, we could not able to make sufficient inductance value. Then, we found an old radio to take its specially designed medium wave inductor coil with a ferrite rod. Therefore we overcome this difficulty.



**The second problem which we come up with was about the variable capacitor. The variable capacitors which are available on the market nowadays are not suitable in a two ways. The first one is about their capacitance range which is not sufficient enough for the AM receiver’s tuner part. The second one is that they are not easily adjustable. Because of these reasons we had to find the variable capacitor from an old radio. Since we had an old radio we overcame this problem by taking its variable capacitor which is not only gives desired capacitance value but also easily adjustable.

**The third challenge is about the finding proper components. The components which we could not find at the market were MC33172P op-amp and 2N3819 JFET. We investigated equivalent of these components via internet and we find UA741 instead of MC33172P and BF245C instead of 2N3819.


**The other difficulty is that before we implement this AM Receiver Circuit, we implemented three different circuit on the board and we did not accomplish to perform the circuits. We then find this AM Receiver Circuit Schematic for about two months later and implemented it.
**The final problem was the effect of the noise. Since we are building AM receiver and the effect of the noise for Amplitude Modulation is much higher than Frequency Modulation, our circuit was affected considerable. We tried to minimize this effect by using Printed Circuit Board instead of a regular board.

CONCLUSION

*First of all, we comprehend the theory of the Amplitude Modulation. This was the first type of modulation used for communicating signals from one point to another and is still the simplest to understand.
*In this project our goal is to build an AM radio Receiver which can work for Medium Wave Frequencies (550kHz – 1600kHz). Medium wave signals have the property of following the curvature of the earth (the groundwave) at all times, and also refracting off the ionosphere at night (skywave). This makes this frequency band ideal for both local and continent-wide service, depending on the time of day.
*After understanding the mathematical expression for the Amplitude Modulation and Demodulation, we started to study physical systems for AM Modulators and Demodulators.
*Then we implemented the circuit as we mentioned before.

REFERENCES

90'Communication Systems Engineering Textbook'
 
http://en.wikipedia.org/wiki/Amplitude_modulation
 
http://www2.eng.cam.ac.uk/~dmh/ptialcd/trf/trf.htm
 
http://physics.usask.ca/~angie/ep311/lab8.htm
 
http://www.st-andrews.ac.uk/~www_pa/Scots_Guide/RadCom/part9/page1.html
 
www.ee.buffalo.edu/faculty/paololiu/413/AM.ppt