7. "What do AM and FM stand for? How come in old radios it's easier to tune AM frequencies than FM ones? What do new radios have that old ones don't"

I know the answer to the first question but I'm a bit hazy about answers to the second and third questions, but I'll make a suggestion or two at the end. But first, AM and FM stand for Amplitude Modulation and Frequency Modulation, repectively. They are two different types of signals that are transmitted from radio and TV stations. Let's look at AM first.

An audio signal, for example, music or voice, is taken from a microphone, say, and it is mixed with another wave called a carrier wave. The frequency of the audio signal is in the range 20 Hz (very deep bass) to 15,000 Hz (very high treble); with AM radio the frequency range of the carrier wave is from 530kHZ to 1600kHz, which is much higher than the audio range. Each, authorized AM radio station has its own carrier frequency, so, for example, "1040 on your dial" means a station whose carrier frequency is 1040kHz. With AM mixing, the amplitude of the carrier wave is modulated, i.e., made to vary, with the amplitude of the audio signal, as shown above. It is this AM signal that is amplified and sent from the transmitting antenna of the station as an electromagnetic (radio) wave. For technical reasons - mainly so as to maximize the number of available AM stations! - the audio frequency range used is limited to a maximum of 5kHz (called low fidelity). While this is fine for talk radio or sports broadcasting, it's not so good for music reproduction as all of the higher frequencies are not used. On the other hand it means that there can be 106 stations in the AM range.

In the case of FM, below,

there are two major differences. Firstly, the carrier wave frequencies of FM radio stations are much higher than AM, i.e., from 88MHz to 108MHz. Secondly, it is the frequency of the carrier wave that is modulated, so that the frequency of the carrier wave changes in proportion to the amplitude of the audio signal. The FM signal is amplified and sent from the transmitting antenna of the station. The higher carrier wave frequency allows the FM stations to use essentially the complete audio range of frequencies (called high fidelity, or hi-fi), which is why FM reproduces music so well. Over the FM range, a total of about 100 stations can be accomodated.

So, you may ask, how are the signals unravelled by your radio? Well, the first thing is to detect the signal from the station you want! That is achieved by using a tuned circuit like that shown below.

By varying the value of the capacitance in the L-C circuit one can select a resonant frequency where the current (and power) in the circuit is a maximum ... so you "tune" the capacitor to give you the carrier wave frequency of the desired station. The inductor (L) can be a mutual inductance so the "tuned" signal can be sent to be further processing.

After the tuner, the signal has to be de-modulated, i.e., unravelled from the modulation. In the case of AM this is done rather simply by subtracting a signal that is equal in frequency and amplitude to the part of the AM wave that is due to the carrier wave. What is left is the audio signal that can be further amplified and sent to the loudspeakers. With FM it's a little more complicated, but basically the carrier wave is removed and the remaining signal re-configured back into an audio signal that is amplified and sent to the speakers.

But what about stereo, you ask. No problem! In that case the FM station sends two signals. There are produced in a really, smart way.

There are two microphones that produce two audio signals; let's call them the left channel (L) and the right channel (R). These two signals are mixed in such a way that two output signals are produced, one is (L + R) and the other is (L - R). In turn, these two signals are processed to produce FM signals, one at the carrier frequency and the other at the carrier frequency plus 19kHz, that are transmitted from the station. When you tune to the station, de-modulation in your radio receiver recovers the two signals, (L + R) and (L - R). To get the individual L and R channels these two signals are added to produce:

(L + R) + (L - R) => 2L

(L + R) - (L - R) => 2R

A comparison of AM and FM signals from the same audio and carrier waves shows that the AM signal varies in amplitude whereas the FM signal has constant amplitude.

This means that the power of the AM signal varies but the power of the FM signal is constant, so the latter can operate at full power all the time. So, a change of power (amplitude) from an AM station is interpreted by the radio as a change in volume. That's why when you go into a tunnel and you are tuned to an AM station the radio volume decreases; the reduction in signal strength received by your antenna is interpreted as a reduction in volume! With FM, changes in the power (amplitude) makes very little difference; the receiver only responds to changes in frequency not amplitude.

The main advantages of FM over AM are then:

1. Improved signal to noise ratio with respect to man made interference (sparks, etc., often called static).
2. Much improved reproduction of audio signals because a wider audio frequency range is used.
3. Smaller geographical interference between neighboring stations.
4. Less radiated power.
5. Well defined service areas for given transmitter power.

1. Much more bandwidth is required, i.e., a much greater frequency difference (in Hz) is required between stations (as much as 20 times as much).
2. More complicated receiver and transmitter systems.
3. Because high frequency FM signals travel in straight lines FM reception only operates effectively when in-line-of-sight. On the other hand, AM signals can be reflected off the ionosphere and so have a longer range, maybe up to 1,000's of miles ... remember what Marconi did?)

Finally, you might be interested to know that the carrier freuqencies for TV stations in the U.S. lie between 54MHz and 88MHz for channels 2 to 6, and between 174MHz and 216MHz for channels 7 to 13. The UHF (ultra high frequency) stations have even higher carrier frequencies, between 470MHz and 890MHz. A TV transmitter works in a similar way to FM radio except that both the audio and video signals are mixed with carrier frequencies.

Modern radio receivers have many more processing stages than I showed above. Various means - some of which I know about and others, which I don't! - are used to increase sensitivity and selectivity, i.e., the ability to detect and discriminate weak signals and to distiguish them from other, stronger stations, and to minimize any distortion of the original signals.

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