Keep in Touch Series

AM vs. FM

Did you ever wonder at what was the difference between AM and FM? Did you every wonder at why FM is preferred over AM? And what about PPM and PCM? Did you ever wonder?

Glossary:
AM -- Amplitude Modulation
FM -- Frequency Modulation
Modulation -- The method by which information is attached to a radio signal for transmission

In "The Superheterodyne" we saw that radio signals are transmitted at different frequencies, called channels. This is to keep the information I'm sending to my plane separate from the information you are sending to your plane. It's a bit like a multi-lane highway where each lane is a channel and the cars are the information which are separated by the lane markings. The road itself is the medium -- the air. OK, no worries so far. But let us look at what we mean by "Sending information".

Let's take a simple radio transmitter (TX) and receiver (RX). Let's also assume that in this example the receiver is connected to a light bulb so that when the TX is transmitting, the light bulb comes on. Let's take it one step further and say that the stronger the signal being received by the RX, the brighter the bulb will glow. OK so far. In the system just described we get some knowledge on how strong the TX signal is by looking at the brightness of the bulb -- right. Well we have just transmitted "Information" over a radio link.

Let us now add a control to the TX (like a volume control on a TV) that allows us to change the output power of the TX up and down. As we move the control, the light bulb at the receiver will brighten and dim corresponding to the control movements. If we replaced the control with a microphone, and the bulb with a loudspeaker, we could transmit voice, music, etc. Just like the Broadcast boys do.

So, what have we done? We've developed a system that allows us to transmit information by varying the output power of a transmitter. We "Modulate" the output power -- we MODULATE the AMPLITUDE of the power -- thus we have an AM system.

Notice, in the diagram above, how the signal from the microphone (Modulation) alters the height (Amplitude) of the radio signal. Notice, also, that the radio frequency (The number of ups and downs per inch) stays the same.

OK, let's go back to the bulb. Just suppose that we change the receiver so the bulb doesn't respond to signal strength, but does respond to how close the transmitted signal's frequency is to the frequency set at the receiver. That means, if the RX receives a signal from a TX that is exactly the same frequency, the bulb will glow brightly. But if the TX frequency is off, the bulb will glow dimmer.

To repeat what we did to the AM system, let's now consider controlling (Altering) the TX frequency with the electrical impulses from a microphone and replacing the RX bulb with a loudspeaker. Now we have a FREQUENCY MODULATED (FM) broadcast system.

Notice here, that the signal from the microphone (Modulation) changes the frequency of the radio signal, not its amplitude. Of course the amount of change in the radio signal's frequency is substantial smaller that the frequency difference between adjacent radio channels.

Well, of course, in R/C we don't transmit voice or music from a microphone. What we do want to do is to transmit some information that represents the position of a control stick -- the elevator stick, or the rudder stick (the sticks on an R/C TX are essentially the same as the 'volume control' we used in the above examples). And, of course we don't use loudspeakers on our receivers either. Rather, we have servos that change their position relative to the position of the TX control stick. More about this in part 3.

So, I guess the next question is what are the other differences between AM and FM, and why is FM more desirable than AM. Good question. I'll try to answer it for you.

If you have a radio in your car that can select AM and FM stations I'm going to ask you to try our a little experiment. This is how it goes. Next time you're in your car, tune to a local FM station. What you will here is a clear, noise free transmission. Now tune to an AM station. What you will hear now is a lot more "Static or background" noise. Some or most of this noise will alter as you rev. your engine.

What's happening is interference - Electro-magnetic interference. The noise that alters with your engine's RPM is the easiest to explain. In a four cylinder auto engine, there will be two high voltage sparks generated. So, at 3,000 RPM there will be 6,000 sparks generated every minute, or 100 sparks every second. Each spark actually transmits, into the air, a very short, but high intensity, electro-magnetic power spike. This power spike, because of its nature, contains millions of different radio frequencies. Any receiver close by, like the one in your car, will accept this power spike as a sudden increase in its received energy level. So to the RX, it looks like the transmitter has momentarily increased its power.

You've probably guessed what comes next. If the RX is AM, this sudden apparent increase in energy/power is considered a valid amplitude modulation. The RX's output device responds. In your car it's the loudspeaker that "Pops" with a background "Buzz" that varies with the engine speed. In R/C, it's the servo that moves to an erroneous position that could cause the plane to crash.

In an FM system, the RX still receives this spike as increased energy/power, but, if you remember, it only responds to frequency changes, not amplitude changes. The FM system is much less interference/noise prone than AM.

What else causes this electro-magnetic interference? Metal to metal joints that move, mechanical engine parts moving fast, movement in the servo itself. All these can and do cause amplitude related interference.

So, the rule is: In most cases, an AM system is OK, but it's well worth the extra $20 - $30 it will cost for a comparable FM system. Your plane is worth much more than the difference in cost. (Actually, I'm not sure if you can buy new AM radios anymore. But all this is worth knowing if you plan to buy a used radio anytime)

Don't forget when buying a new radio, what ever type it is. Steer clear of channels 20 and 21. On these channels you'll get interference from a different source, namely the local TV transmitter station.


Your homework assignment!

Picture this. You are 10 years old and so is your buddy who lives across the street from you. You can see each other from you second floor bedroom windows, but the distance is too far for you to hear each other. Neither of your houses has a telephone. You see each other at school every day and during the days at weekends. Your parents are very strict and you both have a curfew of 8:30 P.M.

You both decide to set up a communications system so that you "talk" to each other after curfew. The materials that each of you has includes a light bulb, switch and battery. You know how to wire it up so that you can pulse the bulb on and off with the switch.

What plan might you and your buddy devise to use this equipment as communications devices. The answer in the next section - "PPM vs. PCM"


Remember, none of this matters if you haven't charged your batteries.

Tune in again soon to find out the difference between PPM and PCM