The problem you mention is a common and a very complex one. Assuming all the equipment is operating properly we know by definition that the signal is modulated on the power line at 120 KHz at the zero crossing point. This allows a usable signal of a few volts to be impressed on a line that has over 150V peak voltage. These signals are filtered with tuned circuits hence there is the possibility of alignment problems but they get a much stronger signal in the pass band. Good filters etc will give good results; poor filters will give marginal results. Remember that these units are low priced and at best have marginal filtering abilities.
The unit putting the voltage on the power line (transmitter) can only put out a few volts. The receiving unit (module) has a minimum sensitivity level for a valid signal. If the signal level is below the minimum erratic or no function occurs. Conversely if the signal is above the minimum the unit should operate properly.
Noise will swamp the front end of the receiving unit decreasing or eliminating its ability to receive and disseminate a valid signal. Noise is typically generated by an outside source.
The carrying media (power wiring) was never intended to handle low frequency RF (Radio Frequency) signals not was it designed to block them. The solution is to get enough good signal to the module so it can perform properly. Solving this gets complicated.
Attenuation of the signal is the proper way of saying the signal level is reduced. There will always be some attenuation of the signal in any normal system. The trick is to minimize the attenuation or at least keep it low enough so a good signal can get through.
There are three primary things capacitance, inductance and resistance that will attenuate the signal to a non usable level. Expect a combination of two or three of these working in conjunction to be causing the problem.
House wiring is a source of capacitance; the effect is definitely there at 120 KHz hence line length will have an effect. The resistance in typical power wiring will have no effect. The inductance can depend how the wire is routed and against what.
Power transformers are designed to operate at 50 or 60 Hz; consequently they have a high inductance which will stop the 120 KHz signal. You see this in a typical home and add a bridge between the phases to cause the 120 KHz to go around the transformer. The advantage of this is that your signals do not pass through to other systems.
AC (Alternating Current) loads such as heaters, lights etc consume AC and do not normally interfere with 120 KHz signals. Motors may or may not depending on there design. On the other hand electronic equipment most by design attenuates the 120 KHz signals. This is because the FCC (Federal Communications Commission) has placed restrictions on both conducted and radiated emissions allowed on commercial and residential equipment. Radiated emissions go through the air while conducted emissions go through the wiring. We are concerned with conducted emissions when working with X10.
Look at the cord that connects the video from your monitor to your computer. There is a swelling of some type near one or both of the plugs. This normally contains ferrite which attenuates high frequency signals.
Electronics needs DC (Direct Current) to operate internally. This comes from a power supply of some type. In bygone years most power supplies were built with a 60 HZ input transformer which isolated the unit from the power line (120 KHz as well). Depending on the transformer design many of these were not a problem.
Then came transformer less electronics such as TVs, these use higher frequencies which have to be isolated from the power line. The easiest way was to attenuate them. The attenuators were cheep but efficient and would kill a broad range of frequencies including 120 KHz signals. These do not pay any attention to the source of the signal they just attenuated them; this is consistent across just about all power supplies.
The cost of energy has kept increasing to the point energy conservation is important. The best way to save energy is to use less. The original power supplies at best could get an efficiency of 50%. The transformer less designs was approaching the 70% efficiency range. This is not good enough today so the switch mode power supplies have become popular. These are typically better then 95% efficient. These use high frequency designs and power MOSFETs (Metal Oxide Semiconductor Field Effects Transistor). The MOSFETs can switch in the Megahertz range, and generate lots of high frequency noise. Consequently the line attenuators are much better and also attenuate the 120 KHz even more.
This is a short and simple explanation of what may be causing the problem; it is not an explanation of how to solve it, that is your job. The problems can and sometimes do get much more complicated then what I have covered her.
Good Luck
Gil Shultz
