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AM or FM |
R. W. Stuart I have been asked to discuss the properties and advantages of the AM system or the FM system as they relate to our RC flying controls. The AM/FM controversy has existed with both the amateur and commercial radio people for the last fifty years; and still is not resolved. The challenge for RC is to transmit to our airplanes information about control surface positions (ail, elv, rud) and about throttle position (mtr), as well as aux controls. The feedback function is what you see and hear- if you do not see it, you are not going to control it. So far our control can be either AM or FM. The transmitter is designed to convert stick position into an intelligence, called coding, which is useful to the whole system. The transmitter coding is a series of 7 or 8 pulses one each of which is assigned to a function ( ail. elv, rud, etc). The stick selected position ( elv neutral, up, or down) then varies the length of the pulse to encode the desired elv position. The encoder then moves to the next function (rud neutral, rt, or left) and varies that pulse length per stick position. The logic rolls through the series of pulses many times per second and up dates the function assigned pulses with new or the same (you did not move the stick) pulse length information. Try TV Chan 2 with your transmitter on and the antenna close to the TV set; you will not get a picture, but you will hear the rate at which the pulse train goes through its updates. Note that the "raster" changes as you tweak a stick- pulse lengths are changing. So far may be either AM or FM. Modulation is the process of impressing on a radio frequency, for instance 72.910 mc- Chan 56 (note this is a busy freq at the LCRC field) the intelligence generated by the encoding. AM is amplitude modulation and FM is frequency modulation. For AM the intelligence varies the amplitude of the radio frequency- in the most straight forward case the rf (radio freq) is turned on for any given pulse and turned off during the space between pulses, so that the amplitude of the rf varies with the pulse as an on-off rf- amplitude high and then amplitude zero. Crude, but all of the rf power is intelligence. It is the way we used to work around the world using only 4 watts of input and Morse code. If you have RC coding, speech, or music to modulate into rf the amplitude of the rf is varied at the code, speech, or music rate, but only 1/4 of the rf power contains intelligence; an rf "carrier" with no intelligence is transmitted as well as an upper and lower "sideband" with intelligence. Still amplitude modulated, but subject to "static" from lightening and other sources which produce all frequencies at very high amplitudes, resulting in troublesome noise at the receiver. Another system provides a tone for pulse and a different tone for space and these two keep a constant (not on-off) rf signal going to the receiver, which helps to solve some of the weird behavior of receivers. Tone modulation, still AM; however, is not rf power efficient and hears static. Frequency modulation, FM, swings (varies) the rf frequency three kilocycles or so on both sides of the channel frequency at the intelligence rate. Prior to transmission and at the receiver the rf signal is "clipped" so that no amplitude variations are in the system for both transmission and reception- no noise and no wasted rf power. Thus my preference for FM and double conversion receivers. Double conversion was discussed briefly in a prior paper and may be discussed further in a later paper. FM and AM require the same rf bandwidth. Phase modulation (PM) will not be examined here, but may show up in some of the PCM technology. The typical conservative inexpensive minimal RC equipment is FM or AM. The trend is toward FM. |
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