The Physics of an AM Radio Receiver

The Physics of an AM Radio Receiver

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The Physics of an AM Radio Receiver

The AM Radio has been around for a long time. When the AM radio was first invented, it was not meant to be used to broadcast music. That is why today most music radio stations are on the FM band. Instead, the AM band was used to carry voice frequencies, thus, all the AM talk radio stations. Due to new technology, music is broadcast over the AM band but does not have the same sound quality as the FM band. According to the FCC regulations at www.fcc.gov, the AM broadcasters are only allowed 5 KHz each side of their carrier frequency for their side bands. These side bands will be explained in more detail later on. In order to understand an AM receiver, one must understand each stage and what it does. An AM receiver can be broken down into six stages. These stages will be explained one at a time.

The first stage is the RF amplifier. The AM antenna runs into the RF amp where the desired frequency is selected. As stated by Grob (1997), the typical band for AM is 535 Hz to 1605 KHz. For the duration of the paper, we will assume that we are trying to receive a station that is located on 1290 KHz signal. The antenna used must be able to pick up all the stations on the AM dial. Knowing this, we need to select the one we want to hear. In this case, we want 1290 KHz or The Country KOWW. When the dial on the radio is turned to select 1290 KHz, it does two things. There are two variable capacitors

The Physics of an AM Radio Receiver being adjusted. I will begin by discussing the first capacitor. This capacitor is connected in parallel with an inductor. While the capacitance changes as the dial is turned, the resonant frequency of the LC circuit also changes. This process is called inductor and capacitor in parallel. When the capacitor is in just the right spot, the resonant frequency will be 1290 KHz. This LC circuit is designed so that only the resonant frequency is passed. The 1290 KHz will now be passed on to the next stage and all other frequencies will be filtered out. The RF amp must also have good sensitivity, which according to Scott Rasmussen (2003), is the ability to amplify a very weak signal.

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This is important because the signal coming in from the antenna may be as little as just a few micro volts peak to peak.

The next stage really involves two stages, the mixer and the local oscillator. The second capacitor that was mentioned before, but not discussed, will affect this stage. As the dial is turned, it adjusts this capacitor as well. This capacitor is in the local oscillator. It causes the local oscillator to oscillate at a frequency that is equal to the sum of the desired radio frequency and 455 KHz. So when receiving 1290 KHz, the local oscillator must oscillate at1.745 MHz. The mixer then mixes the 1.745 MHz with the 1290 KHz. This is done with a transistor operating in a nonlinear mode. When this takes place, several frequencies are produced. This brings us to the next stage of the receiver. During this stage several frequencies will be used. These include: 1290 KHz, 1.745 MHz, the sum of the two frequencies, the difference of the two frequencies, and all the harmonics. The frequency that we want is the 455 KHz. The mixer and oscillator will always adjust to produce a frequency of 455 KHz. Now, the rest of the receiver stages can be tuned to 455 KHz. By doing this, all of the AM radio frequencies can be tuned into and mixed down to a set frequency that the rest of the receiver will work with.

The next stage of the receiver is called the IF amp. IF is the intermediate frequency. Depending on the receiver, there may be one to three IF amps. These are also known as tune LC circuits. These circuits must be tuned to resonate at 455 KHz. These stages need to have good selectivity, also. According to Scott Rasmussen (2003), selectivity is the ability to amplify the desired frequencies and reject all unwanted frequencies. As mentioned before, the FCC only allows a total bandwidth of 10 KHz. During the IF stage, only 450 KHz to 460 KHz are allowed to pass through. The purpose

The Physics of an AM Radio Receiver of the IF stage is to prevent all other frequencies besides the 450 KHz to 460 KHz to pass through the stage. In this stage, the reason why music does not sound very good on the AM dial is revealed. When listening to music, one hears frequencies anywhere from 50 to 15 KHz (Grob, 1997). Due to the laws placed by the FCC, only a 10 KHz bandwidth is allowed on the AM dial. This does not provide enough room for all of the audio frequencies to be broadcast on an AM frequency.

The next stage is the AM detector. This detector is basically a diode in series with a capacitor that is in parallel with perhaps another capacitor or a resistor. The modulated envelope that is created by the AM transmitter is now present at the AM detector. The negative cycle of the wave is eliminated and the outer edge of the envelope is turned into a simple sine wave. This sine wave, which can be seen on an oscilloscope, is actually the audio without the AM carrier frequency. This audio signal can now be amplified and heard through a speaker.

The final stage is called the audio amplifier. The sine wave produced by the previous stage must be amplified several times in order for audible sound to project from a speaker. This projection can be achieved in several different ways. Bipolar transistors can be used to make an audio amp, an op amp could be used, and even a mosfet amp could be used. No matter what kind of amp is used, it will take several stages of amplification to produce a large enough signal to power a speaker.

All of these stages make up an AM receiver. When tuned in properly, we will not only hear the Country KOWW, but each different station that we tune into. However, these are just the basics of an AM radio. Most AM radios have something called AVC (automatic volume control) or AGC (automatic gain control). This will regulate the amount that the signal is amplified. In other words, when you are near the radio transmitter and the signal is very strong, the receiver does not amplify the signal as much as it would if you were far from the transmitter and the signal is very weak.

All of these stages can be found in any AM receiver. They may be modified and of a higher quality, but they will still contain each of these stages. When all of the stages are tuned properly, they work together. However, if one stage is out of alignment, it will through off the entire circuit and none of the stages will function properly. Due to the

The Physics of an AM Radio Receiver
creation of an AM radio, someone such as Rush Limbah can have his voice heard by people everywhere.

The Physics of an AM Radio Receiver

REFERENCES

Grob, B. (1997). Basic electronics. New York: Glencoe McGraw-Hill.

S. Rasmussen, personal communication, April 21, 2003.

US Government. (2003, January 7). FCC regulations. Retrieved April 16, 2003, from http://www.fcc.gov.html
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