I have some powerline networking adapters which can transfer at a peak rate of 200 Mbps (I get around 60 Mbps typical, but I am sharing the network with many other devices and many pieces of equipment are running on the line.) How do they do it? I always thought that it was during the zero crossing, but packing so many bits into such a space must be very difficult, nigh impossible. HowStuffWorks has an article on it, but it's only relevant to adapters which go at 14 Mbps.
Electronic – How do these powerline networking adapters work
power line communication
You didn't give the all-important information about what frequencies you are using for the communication. You can't know if a filter is right if you don't know what frequencies you want to separate.
However, the main point is to attenuate the power line frequency by a large factor. Here is one simple passive way to do that:
This is a simplistic filter which is basically 4 high pass filters strung together. I'm assuming the communication frequency is high compared to the power frequency, like 100 kHz or more. Each high pass filter stage has a rolloff of 1.6 kHz. 60 Hz is far enough below that so you can make the approximation it will be attenuated by 1.6 kHz / 60 Hz = 27. These filter stages will interact some to give more overall attenuation, but even ignoring that you get at least the single filter stage attenuation to the power of the number of filter stages, or about 495 k in this case. That means the 120 V 60 Hz power signal will be down to less than 240 µV out of this filter. 100 kHz, which we're assuming is your lowest frequency of interest, is 63 times the high pass filter rollof frequency, so won't be effected much by this filter.
The signal directly out of this filter is not ready for running into a chip like a opamp or microcontroller A/D yet. The power line has all kinds of nasty noise on it, with some transients that can be a few 100 V. This filter will pass such short term spikes, which would blow up a opamp or microcontroller. There needs to be some sort of clamping function, keeping in mind that those component have to be able to handle 100 V or more for a short time.
The voltage rating of the filter components needs to be carefully considered too. C1 will take most of the 60 Hz voltage, and certainly needs to be rated for power line operation. Even after just the first stage, the 60 Hz voltage will be low enough to not be a problem for ordinary resistors. However, the resistors need to be able to handle the temporary high voltage spikes that won't be attenuated by the filter. Either get resistors rated for a few 100 Volts, or implement the resistors shown in the schematic by 3 or 4 ordinary equal resistors in series.
What interface you need to line depends on the specific power line modem.
Some will have the interface built in so that you can connect phase and neutral directly, while others may require an isolation interface.
The manufacturer of your modem should provide very very very clear documentation of what is required.
You will usually only be connecting to one phase so have to deal with Phase to Neutral voltage.
If a coupling capacitor was needed I would use one with full "X1" certification.
X2 is of lower spec than X1 but still tested.
X3 is lower spec than X2 and is untested.
It's likely that the value of the users lives, your life and the equipment used is far higher than the cost of a top rated capacitor.
This EPCOS document provides valuable information about X rated capacitors
Some useful related information here Line filter capacitors
Class X is for applications where failure could not lead to electric shock (hot to neutral).
Class X1 capacitors are intended to operate safely even in the presence of spikes on the mains supply of up to 4 kV (installation category 3 or overvoltage category 3 according to IEC60664), which are normally industrial supplies, but some standards call up class X1 capacitors if they are connected directly to the mains supply upstream of the equipment fuse, irrespective of the type of mains supply.
Class X2 capacitors are intended to operate safely even in the presence of spikes on the mains supply of up to 2.5 kV (installation category 2 or overvoltage category 2 according to IEC60060), which are normally residential, commercial and light industrial supplies.
X capacitors can be found from 0.001 uF to at least 10 uF and are only made in film.
Classes X1, X2, and Y were originally defined by the IEC in IEC 60384-14. CENELEC has adopted EN 132400 (technically equivalent to, but structurally different from IEC 384-14 2nd edition), which now defines seven classes of line-filter capacitors.
Class X1 capacitors are impulse tested to 4 kV (higher for capacitors over 1.0 uF).
Class X2 capacitors are impulse tested to 2.5 kV (higher for capacitors over 1.0 uF).
Other - not suitable:
Class Y1 capacitors are impulse tested to 8 kV, and Class Y2 are impulse tested to 5 kV.
Classes X3, Y3, and Y4 are for lower-voltage capacitors, none of which are presently called up in safety standards.
Other impulse tests also apply. These include a 1000 hour endurance test during which the capacitor is subjected to a continuous overvoltage condition, plus periodic 1000 VAC spikes, and a flammability test during which the capacitor is hit with a series of transients while under rated voltage. Capacitors conforming to IEC60384-14 normally also conform to EN132400, and vice versa, and should be accepted in all European countries.
If you want a simplistic answer. Here it is
You have the power source. It gives you 220 Volts at rather low frequency (as noted: 50-60 Hz). You connect a capacitor with a low value and an inductance of a low value to this line this way: low inductance doesn't allow 50-60 Hz to pass in - it shorts such currents to the ground (being placed after capacitor! not to short everything at all), on the other hand capacitor in it's place again doesn't allow low frequency in and passes through high frequency, which we send or receive. The other party does the same - we have a working transfer line.
But, ofcourse we connect different devices to our power socket. What happens in such a situation is simple too: mostly they are - ac/dc convertors, which use either transformer, or impulse scheme. This schematic doesn't allow high frequencies, eg they provide huge resistance to it. But we remember the capacitor in both our devices - it provides low resistance, so our-generated high frequency signal takes the easiest way: our receiver. Ofcource there will be noise and leakage, our coding system and filters will deal with it.
That's the picture in it's simplest form.
Hope this is what you were asking ;)