Radio Facts By James Goss     Most  of us in the RC hobby has seen how the radio equipment has changed over the years.  Some may even remember the vacuum tube  systems  used in the fifties. Since the solid state RC radio equipment came out using analog modulation, not much has changed until the mid  80 `s.  Up until then we had standard AM , amplitude modulation, and FM, frequency modulation, as a means of modulating the RF  cw signal.  It is said that FM receivers will pick up less noise than AM receivers.  RF noise can come from anything that produces an electromagnetic  or electrostatic field,  such as lightning, high voltage power lines, or the metal to metal contact  of two metals rubbing each other on your airplane.  Noise of this type has amplitude, which is another way of saying it has a voltage value, and will react with an AM receiver more than  an FM receiver.  As an example, if you were flying your airplane with an AM receiver and there was lightning nearby  you may get a glitch, which means your airplane does some aerobatics on its own. For the purpose of this article a glitch will be defined is a sudden loss of your normal signal. Another example of noise can be heard on your regular AM radio in your car or home. During a lightning storm you will hear all kinds of popping on AM, but minimum noise on  the FM radio.     AM  rc systems are still on the market and a lot of them are still being used for one reason, they cost less. I still have several in use and have never had any problems with noise, or did I?   Perhaps that great knife edge snap roll was only a glitch.     Modulation of the transmitted signal can be broken down into two main types, analog and digital.  With analog  pulse modulation , some characteristic of the pulse such as height or width will be changed in proportion to the original signal. AM uses height change and FM uses width change, also known as pulse width modulation.       In digital pulse modulation, a binary code is generated .  The way this works is as follows:  An analog voltage signal is generated when you move the transmitter sticks. The sticks are connected to pots, short for potentiometers, which are variable resistors. When the control sticks are moved, the center tap on the pots will have a new dc voltage. This voltage is sampled and its binary equivalent is generated. The rf carrier is then changed according to this voltage signal. All modulation systems will sample the information voltage to be transmitted, but they all have different ways of indicating the sampled amplitude.     The two new forms of modulation  in use today for radio control systems are the Pulse-Coded  Modulation, PCM, and Pulse-Position Modulation, PPM.  This gives us a total of four systems, AM, FM, PPM, and PCM. This could be confusing to a new comer to our hobby.     In pulse position modulation the original signal is first pulse width modulated, then differentiated, and clipped. All this means is that you end up with a very narrow pulse width or spike that is either negative or positive going in polarity. These pulses vary there position on a reference line and is in reference to the zero signal pulse position. The pulse will either lead or lag the reference pulse. The amount of lead or lag is in proportion to the original signal above or below its reference line. Thus the name pulse position modulation. This is still analog modulation because some form of the pulse is changed.     Pulse coded modulation is a form of digital pulse modulation and is the major type of digital modulation in use today.  PCM  converts the amplitude of the original signal into its binary equivalent. Binary is a simple base two number system in which a one represents a high and a zero represents a low. Either on or off is the whole format for digital electronics. This binary equivalent represents the approximate amplitude of the original signal generated by the control sticks. When you move the control sticks through their complete range you will generate many different unique binary codes, one for each input sampling point of the original signal.       Time sharing of the carrier signal is used to place all your radio's channels in operation, be it a three, four, six, or eight channel radio. The carrier signal is the signal that is generated by the transmitters  master oscillator. If you have channel 22  for example, the oscillator will be running at 72.230 mhz. This is determined by the crystal you plug into your transmitter. It would have 72.3 mhz marked on the crystal. By the way,  crystals have no polarity, you can plug them in either way.  Even if you are not moving a control stick the carrier is still being transmitted. The 50 channels we have to select from is spaced 20,000 hz, or 20khz, from each other. They start at channel 11, which is 72.01 khz, and end at channel 60, which is 72.99 khz. That's not much distance and all your three, four, six, or eight channels you have on your radio must be within that bandwidth. We are lucky because Canada only has10 khz between their channels.     You can't interchange the receiver and transmitter crystals. This is because the receiver oscillator must run at the IF, Intermediate Frequency, above the incoming signal frequency. For example, on ch. 22 the transmitter frequency is 72.23 mhz so the receiver will be running at the IF above this frequency or at 83.03 mhz.  When these two frequencies beat together, their difference, which is 10.7 mhz is passed on to the IF amps. If your receiver uses double conversion, the 10.7 mhz will be further reduced to 455 khz. This signal will now be demodulated, which takes the coded signal off the carrier and it is then sent to the servo where it is compared to a reference signal generated in the servo. All good rc systems today will be double converted which helps reject unwanted signals from getting to the receiver. They do cost a little more  for the extra circuits.     Another question asked by new comers to the rc hobby is how much power does the radio produce and how far away will it work.  Most radios today have a maximum rating assigned to them of one watt of radiated power.  Due to efficiency most will produce around .75 watts.  This is plenty of power for our airplanes because you must keep them near the field to see which side is up anyway.  With most systems you can collapse your transmitter antenna and still have control of your plane. Try this experiment: Get about 300 ft high and have someone to start lowering your antenna until your plane starts to act up, at which time raise the antenna and regain control. This will give you some idea about distance.   Antenna theory could be a complete topic by itself.  I hope this information will be of some use to the new members of our club. The End

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