Implementation factors
The wiring in a house isn’t a perfect transmission system.Worst of the transmission impairments the signal sees is the power transformer that feeds your house, as shown in Figure 6, because it splits the single-phase power into two sides, with a neutral tap between the two. Appliances see 120 V between a power rail and neutral, or 240 V, between the two power rails.
FIGURE 6: Home power system schematic
Suppose there’s an X-10 transmitter connected to the upper power outlet in the figure and an X-10 receiver on the lower outlet. Signals output from the upper power outlet transmitter are referenced between the one power rail and the neutral, and are carried directly on those two wires to any receiver on the same circuit or with some loss to any circuit connected to the upper power rail.To get to the lower rail, however, the signal has to be coupled through the power transformer (or some 240-volt appliance) onto the lower rail. Power transformers are tuned to be efficient at 60 or 50 Hz, however, and their relatively high impedance at the 120 kHz signal frequency does not couple energy well — it’s entirely possible that only a tenth of the original signal passes to the other side of the breaker panel. Even ignoring noise and other problems, should the coupled signal drop below the 100 mV (millivolt) reception threshold, receivers on the other side of the transformer won’t work reliably, or won’t work at all. You fix this problem using a coupler, a device that provides a low impedance path for the 120 kHz signal from one side of the breaker panel to the other to reduce the loss, or a repeater, a device that actually amplifies the signal before retransmitting it onto the other side.
Signal attenuation comes from other sources, too, because many types of devices on the power line will themselves attenuate the X-10 signal. Relatively small capacitances in power supplies and other electronic circuits look like high impedances to the normal 60 Hz power waveform, but may look like a low impedance to the 120 kHz signal and therefore reduce its strength considerably. The effects of each such small loss add together, too, so while no one computer, television, or other device may be enough to make your system unreliable, a large enough number of them may. You need a filter to fix this problem. Filters work as shown in Figure 1-7. Coils are the dual of capacitors, in that their impedance goes up with higher frequency, while a capacitor’s impedance goes down. Putting a small inductance coil in series with each wire in the power line lets the power flow without hindrance, but blocks noise and the 120 kHz signal. The filter blocks any X-10 signal present on the power line on the left in Figure 7, keeping that signal from being attenuated by the low impedance capacitance in the device on the right.
FIGURE 7: X-10 filtering
Noise also corrupts X-10 signals. The theoretical waveform of Figure 3 is what you think about, but what you get may be the mess in Figure 8 that results from even a moderate amount of noise on the power line.We’ve left the tones in, but no X-10 receiver is likely to figure out where they are and whether a zero or one is being sent. The figure is the same one as Figure 3, with random 10-volt peak-to-peak noise added on to the 120 volt power line signal.
FIGURE 8: Power line noise
Devices that switch the power line on and off generate noise. Devices that generate noise that can affect your X-10 system include:
Power supplies — Decades ago, power supplies used transformers to change voltage levels, then rectified and filtered the output to produce direct current. Such linear power supplies aren’t very efficient, though, so the newer switching power supply design has almost completely replaced the older design. A switching power supply turns the power feed to a filter on and off very quickly, keeping just the right amount of charge in the input capacitor of the filter to maintain the precise output voltage. The switching operation generates noise back into the power line, however, noise that can affect X-10 systems if the power supply or an external filter doesn’t block it. Everything electronic that doesn’t run on batteries has a power supply.
Fluorescent lamps — A fluorescent lamp contains an arc between two electrodes that dies and re-strikes at every voltage zero crossing of the power line. Each time the arc restrikes, it generates wideband, high-frequency noise. Compact fluorescent lights areworse, because they contain switching power supplies, too.
Motors — Many motors include a commutator and brushes to rapidly switch the incoming power among the different coils in the armature. Every time the commutator and brushes disconnect one coil and connect another, there’s a surge of current that generates power line noise.
Your neighbor’s X-10 system — What’s signal to your neighbor is noise to you, and if X-10 signals from adjacent houses couple in to your system, you’ll see strange results as your equipment interprets and acts on commands leaking in from next door. The coupling is through the main power transformer and power lines feeding your house, so a filter on the mains is all you need to solve the problem should it occur.