50+AirYears

In escence, modulation is the modification of the carrier wave in order to transmit information. There are several ways to do this. Sometime, a signal can be victimized by undesired modulation not part of the circuit design or operators intent.

The first radios, like many of the early single channel radios from the 1930s to the late 60s were very simple. The carrier was held OFF until a control was keyed, at which time the carrier was turned ON, and detected at the very simple reciever. Later, the carriers were ON anytime the transmitter was on, and the carrier was Amplitude Modulated by a tone generator activated by pressing the key, with the resulting signal decoded in a more complex reciever. 400 Hertz was a common modulation frequency.

This eventually developed into transmitters with multiple tone generators to enable multiple controls. The reciever would either use tuned filters, or a resonant reed relay system with multiple reed contacts of different lengths, each responding to a given frequency. Early proportional radios used a single modulating frequency, but varied the frequency with which the transmitter was keyed, and later also the proportion of on to off, to get multiple control functions. In the early 50s, the Good brothers, probably among others, tied this in to their TTPW, Two Tone Pulse Width system, which used two different tones, each with a rate/width control, to give up to 5 or 6 controls. Then along came Frequency modulation, in which keying the transmitter either dropped the carrier frequency or raised it a small amount, which provided the control information. More modern proportional gear modulates in a sequence of events with a reference pulse followed by some number of pulses, with the width of the pulses and their position in the pulse string detemining whioh control is being directed. This can be any form of modulation. The latest scheme, which is probably FM, sends out a stream of digital words which contain an identifier for the particular control channel and the position for the servo to go to and hold. You should see what the waveform looks like on a truck's power line communications system. Looks like a combination of AM and FM.

Regardless of format, any modulated carrier will have not only the desired information, but will also pick up harmonics and sub-harmonics from many sources, including but not limited to poor solder joints, tolerance stack in the component, atmospheric conditions, loose battery connections, loose internal connections, Drift in other transmitters physically close to either the prime transmitter or the plane it's controlling, and sometimes even RF reflections from nearby things like power poles and lines, fences, steel building skeleltons and siding, construction equipment. The reciever is not only getting this mess, but can itself be affected by atmospherics, internal component tolerance stack-up, bad connections, as well as things like metal to metal contact in the engine and control linkages, Ignition pulse in spark plug equipped gassies, shadowing and reflection from metal or carbon fiber pushrods, carbon fiber structural members, even larger engines, affecting the antenna. Hey, sometimes, especially at higher frequencies, even something as simple as a lead-mounted transistor or capacitor slightly out of design position, can cause problems.

I once was using a nice $7000 oscilloscope to look at a high frequency signal to try to find a problem. Through most of the circuit board, the signal was nice and clean when a captured signal was subjected to the scope's Fourier Analysis program. Then at one point in the circuit, there were a number of sub harmonic spikes, a couple of which had a higher amplitude than the original signal. Our circuits are usually built with Automotive grade components. In that one part of the circuit, we ended up having to go with Mil-spec parts, in spite of the crying of the bean counters. Problem solved, just throw money at it. Actually, the components cost a whole $.30 more than the original design parts.

Years spent trying to develop circuits that can function safely in heavy RF fields has convinced me that RF design is almost equal parts Engineering Skill and knowledge, Science, Art, Luck, and some Black Magic. Some circuit designers agree completely with this view. The designers will over time improve interferrence resistance, but probably won't totaly eliminate it for another few hundred years. Maybe! 72 MHz was better than 27, 2.4 GHz is apparently better than 72 MHz. Some investigative work was done on 900 MHz with good results. Seems the higher the frequency, the less risk of interference. But, the trickier the physical circuit layout.