# How to observe powerline modulated signal?

I was assigned to observe the modulated signal of a PLC modem, but I have not really started yet. How do I do this? Should I use an oscilloscope or spectrum analyzer? If so, do I just connect it to the power line where the PLC modem is connected?

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What protocol/frequencies does it use? –  stevenvh Apr 8 '12 at 11:22
you will probably need some kind of filter to separate the signal from the mains 'signal' - observing a signal of a few volts riding on top of a 50 or 60Hz, 120 or more volt sinusoid will be nearly impossible –  JustJeff Apr 8 '12 at 11:23
@gervy, did you ever do this successfully? –  larsks May 30 '12 at 19:37

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.

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the modem transmits the signal using wavelet OFDM.operating frequency is at 2-30MHz do i use the same value? and if i plug the oscilloscope at the end of the said filter will i be able to see the signal already? thanks alot –  Gervy Apr 9 '12 at 4:03
@Gervy: Since the filter above was designed to reduce 60 Hz to low levels and pass 100 kHz with little attenuation, it will certainly pass 2 MHz with effectively no change. It will load high frequency signal with 2.5 kOhms, but in my experience the power line only has a few 10s of Ohms impedance at 1 MHz, so that shouldn't be a problem. You could use lower cap values and reduce 60 Hz even more without effecting 2 MHz much. For example, 1 nF sets the individual HPF rolloffs at 16 KHz, 265x above 60 Hz and 126x below 2 MHz. –  Olin Lathrop Apr 9 '12 at 13:20