The Physics of an AM Radio

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The Physics of an AM Radio

It’s was a beautiful sunny morning and Bill just kissed his wife goodbye and headed out the door on his way to work. As Bill got into his car he realized that he missed the Utah Jazz game the night before because of a late business meeting and fell asleep before the news came on so he couldn’t catch the highlights or even the score. He remembered that he could catch the rundown on AM frequency 930 (Sports News Radio).

Many people enjoy listening to AM radio because of how convenient it is to listen and catch up on the sports, weather or just enjoy the entertainment of listen to Rush or Dr. Laura, but are much like Bill and don’t have any idea how the signal that carries these programs is sent or even received. In an attempt to understand this process, Bill confronts a part time employee who is currently studying electronics and has a basic knowledge of how the AM radio found his car or even his home works.

Almost all AM radios work under the same basic design. There are two very simple yet very important things that make AM radio possible. What are these things? A transmitter(the station) and a receiver(the radio).


Each radio station that desires to have a frequency(signal) on AM radio must notify the FCC and are assigned a frequency that they can use to send their information out on. The very first part of the transmitter is a quartz crystal. This is used because it is very stable and efficient which is important because there are certain laws and guidelines that the stations must abide by. They cannot go over or below the assigned frequency by more than 5K hertz, making a bandwidth of 10K hertz. After the quartz crystal, is an oscillator where the actual physics of the transmitter comes in. The oscillator is made up of a variety of electronics components including an operational amplifier and a combination of resistors and capacitors.

Resistors are defined as-The impedance to the flow of electric current. The resistance is equal to the voltage across the object divided by the current through it. Measured in volts per ampere, or ohms.(Physics, A World View 513)

Current is defined as-A flow of electric charge. Measured in amperes. (Physics, A World View 513)

A combination of a resistor and capacitor in series or parallel is called a filter.

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MLA Citation:
"The Physics of an AM Radio." 20 Sep 2017
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This is key in an oscillator because the frequencies that are not desired must be filtered out by this combination of components. Inside the operational amplifier is a series of three amplifiers a differential amplifier, a voltage amplifier, and a power amplifier

Capacitors have an internal resistance called an impedance. This impedance is found by the formula = XC(impedance) = The inverse of 2 X pi X freq X the capacitance of the capacitor. The combination of the resistor and capacitor forms an impedance together and this is defined by R(resistance)+j(cap impedance) in rectangle form.(Practical Circuit Analysis Mcdonald 25) The purpose of this oscillator is to filter out the unwanted frequency so only the frequency that the particular radio station operates on will be amplified and transmitted.

The next step in the transmitter is what’s called a frequency multiplier. Generally the frequency that is spoken on the air is only between 100 hertz and 5K hertz and the assigned frequency will be between 540K hertz and 1600K hertz. These are the frequencies found on the dial of an AM radio. The function of the frequency multiplier is to multiply this 100 hertz to 5K hertz signal to that of the transmission frequency.

The next step that must be taken is to build a voltage driver. This driver is what drives or forces the signal into the air. Much has been said about resistance and capacitance but very little about voltage. The voltage in a circuit is defined
by the resistance in a particular circuit multiplied by the current. This idea or principle is known as Ohm’s Law. The Ohm’s Law formulas are V(voltage)=I(current)R(resistance) and likewise I(current)=V(voltage)\R(resistance) and R(resistance)=V(voltage)\I(current). Using this simple principal the voltage, resistance and current of a particular circuit can be found. (Physics A World View 504)

Last of all the intelligence (signal) that is desired to be sent goes through a power amplifier so the signal will have enough strength to be broadcast out to the receivers.
(your radio) The power of a circuit is defined by the voltage of a particular circuit multiplied by the current. The formula looks like this-P(power)=V(voltage)I(current) and P(power)=I(current)squared X R(resistance) and P(power) = V(voltage)squared\R (resistance) (Physics A World View 510). So the amplifier is designed so that the power the transmitter expends is amplified so it will with the voltage driver adequately force the signal to the receivers.

THE RECEIVER:The receiver of an AM radio is made of three basic parts, the antenna, the mixer and the oscillator. The antenna is obviously a very important part of the receiver. This is where the signal enters the radio. The frequency passing through the antenna creates a small voltage (V=IR) which drives the circuit. Within the receiver the radio wave at the antenna is amplified and then mixed with the local oscillator to produce the Intermediate Frequency which is the next stage. The mixer\oscillator stage contains a transistor which is a key component in amplification. This transistor not only amplifies the signal but simultaneously oscillates at a frequency 455K Hz above the desired radio station frequency. So to calculate the frequency the oscillator operates at the frequency of 455K Hz can be added to the frequency of radio station seen on the dial of the radio. The following formula defines this: Fo(Freq. of oscillator) + Fi(Freq. of Intelligence). The frequency of the intelligence is the frequency that is transmitted by the radio station. The antenna and the oscillator coils have resonant circuits that change when the radio is tuned for different stations. Again these resonant circuits act as filters to filter out and pass the desired frequency and reject the frequencies of the other stations.

The next stage of the receiver is the first of two If(Intermediate frequency) amplifiers. The gain of the first If amplifier decreases after the Automatic Gain Control threshold is passed to keep the audio output constant at the detector and prevent overload of the second If amplifier. This is made possible by making the base voltage of the transistor in this stage lower as the signal strength increases. Transistors have three pins or leads that make this amplification possible a base pin, collector pin, and emitter pin. So this drop in voltage on the base pin causes a drop in voltage on the emitter pin. Since the output of this emitter pin is coupled to the next stage of the receiver, this is what causes a decrease in gain.

The stage following the first If amplifier is the second If amplifier. The purpose of this second If amplifier is to increase the amplitude of the intermediate frequency (If) and at the same time provide Selectivity. Selectivity is the ability to “pick out” one radio station while rejecting others.(Omnitron Electronics 19) This second If amplifier at the same time acts as a bandpass filter, passing the desired frequency while rejecting the unwanted signals.

The next to last stage consists of an AM Detector and an automatic gain control stage. The purpose of the detector is to change the amplitude modulated If signal back to an audio signal. This is obviously important because this is what enables the
intelligence frequency to be heard. A process called detection is what makes this possible. The amplitude modulated If signal is applied to a diode in such a way as to leave only the negative portion of that signal. This diode serves a purpose of something called rectification. Rectification is when the negative half of the sine wave the Intelligence frequency rides on is cut off. This acts like an electronic check valve that only lets current pass in the same direction as the anode on the diode points. A diode has a positive lead and a negative lead. The positive lead is called the anode and the negative lead is called the cathode. In this situation, the positive lead or anode is pointing toward the direction the current is passing. When the diode is on (in conduction) it will cause the capacitors in the circuit to charge to approximately the same voltage at the negative peak of the If signal. When the diode is off the capacitors discharge through the resistors in this stage of the receiver. The automatic gain control circuit is simply to maintain a constant audio level at the detector regardless of the strength of the incoming signal. This makes it possible to listen to all signals at the same volume. Without the ACG the volume would have to be adjusted for each station (Omnitron Electronics 16).

The last and final stage is the audio amplifier. The audio amplifiers purpose is to increase the audio power to a level sufficient to drive an 8 ohm speaker.(all speakers have an internal resistance) To do this, the DC(direct current) from the battery if in a car or a battery driven radio into an AC(alternating current) in the speaker. This is partially what the diode is responsible for. In this circuit there is a couple of transistors that work together amplify the small voltage produced by the antenna to large enough signal that the speaker will turn on and operate so the signal transmitted by the station is amplified to an actual audible audio signal heard through the speaker.

In conclusion, there is an outstanding amount of physics that goes into the process of transmitting a signal carrying the desired intelligence to receiving the AM signal with an AM radio. A basic AM radio combines basic electronics knowledge (Ohm’s Law) and some more in depth electronics such as amplification and rectification. So hopefully after a brief explanation of how an AM radio works, Bill will take more thought as to how the signal he is listening to is received and transmitted. The physics behind how an AM radio works is very interesting to me and something I understand fairly well that’s why I chose this topic.

Works Cited:

1.Physics A World View; Kirkpatrick and Wheeler

2.Practical Circuit Analysis; Lorne MacDonald

3.Omnitron Electronics Superhet AM-550TK

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