Am Radio Design
Essay Preview: Am Radio Design
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Objective:
The objective of this lab is to design and AM Radio.
Introduction:
Am radio uses Amplitude Modulation (AM), which works by varying the strength of the transmitted signal in relation to the information being sent. When you tune into an AM radio station, for example 1560 on the AM dial—the transmitter’s sine wave is transmitted the data at 1560,000 hertz. What this number essentially means is that the sine wave repeats every 1560,000 times per second. The data is modulated onto the carrier wave by changing the amplitude of this sine wave. The radio station uses and amplifier in order to amplify the signal. For large AM stations the signal is amplified to about 50,000 watts. After the signal has been amplified the radio station sends the radio waves out into space with the help of an antenna. This is essentially how data gets transmitted from an AM station onto a radio receiver. Now let’s take at the inner working of an AM receiver and required steps in order to take that signal from the air and into a speaker.
Provided below is a block diagram of an AM receiver.
Antenna:
The purpose of the antenna is to capture the radio signal from the air waves and into the tuner. An AM antenna is simply a wire or a metal stick.
Tuner:
The antenna will pick up many signals. The function of the tuner is to separate obtain the specific signal you need and ignore the others. For example, if you are tuned into AM 1560, we want the tuner to isolate every other signal and recover this signal. An AM tuner works on the principle called resonance. What this means is that an AM tuner resonate & amplify at certain frequency and avoids all the other frequencies.
RF Amplifier:
The reason we incorporate an RF amplifier into the AM receiver is because an RF Amplifier helps us further improve the signal. The major benefits of using RF amplification are the following:
Improved image frequency rejection
More gain and thus better sensitivity
Improved noise characteristics.
Detector:
Once the data has been obtained, we have to extract the voice out of that sine wave. This is where the detector comes into play. For an AM radio, a detector is made from a diode. The properties of a diode allows current to flow in one direction and not in the other. Therefore once the signal has been processed through the detector, it clips off one side of the sine wave. See figure below for illustration:
Amplifier:
The next stage is to amplify the clipped signal and send it to the speakers. The amplification stage can be accomplished by using op-amps and transistors.
Analysis
The input signal of this circuit was modeled by a multiplier and two input signals. A 5 kHz and 1 MHz were the intelligence and carrier frequencies. During construction of this circuit, a parallel inductor and capacitor would replace this model. Using the following formula we are able to tune to a desired frequency.
The Inductance value was 85 uH and a Capacitance of 300pf was used in order to tune the circuit at 1MHz.
In the next stage of the AM receiver a BJT transistor setup is used.
As the diagram shows, this stage included several components. The modulated signal is biased so that the transistors a working a class A amplifiers. The capacitor C1 is implementing in order to prevent any of the DC current from flowing tuning circuit. Q1 and Q2 are cascaded in order to provide high input resistance. Q3 is used to prevent loading from the peak detector. Both the input and output of this stage can be found in Figure 2 and 3. The calculations to component values can be found in the sample calculations.
In order to recover the modulated signal a peak detector was used. The following is circuit diagram of this stage.
This design was chosen due to it wide acceptance as peak detector in the communication field. The parallel RC component was chosen so that its О¤ will be less than the period of the intelligence signal but greater than the carrier frequency. The output of the peak detector can be found in figure 5. Calculations can be found in sample calculations.
A filter was included to strip the recovered signal of its DC component. The following circuit diagram displays our setup.
A Voltage follower was included in order prevent a change in the peak detectors О¤. The output of the filter can be found in Figure 6.
The final stage is audio amplification. We used a high power Op-Amp in conjunction with a push-pull transistor. The following circuit is a diagram of our design.
Analysis for voltage gain, current gain and heat calculation can be found in the sample calculations. Due to the number of transistors available in the student version, the audio amplifier was simulated in a new project. The output of this amplifier can be found in figure 8 and 9.
Calculations (continued):
Heat- sink Calculation:
In order to calculate the value of the heat sink, we first obtained the power dissipated across each transistor. The power dissipation across each transistor was obtained form P-Spice. The value turned out to be approximately 10 watt per transistor. Once we have the power dissipated across the transistors, we can perform the necessary calculations.
Power Dissipated = 10 watts.
Thermal Compound =
Junction to Case =
Heat Sink =
Therefore we need two heat sinks with 10.868 C/W
Conclusion:
In conclusion this design gave us a deep understanding the AM receiver. Constant design and redesign was needed due to overcompensation. One major learning experience was to go from a basic design to a complex one. Originally we attempted to build the