# How is power line communication (PLC) possible without screwing up the line as a viable power source?

[I am not an EE so bear with me with this] I've been learning about power line communication and see that it is mainly achieved by modulating the carrier signal (for this discussion let's say that's 60Hz AC power). For the AC power to be viable as a power source, I would assume that the frequency and amplitude needs to be steady. Therefore, if you modulate the freq. or amplitude of this signal, then I would think that would basically eliminate it as a viable power source. How is PLC possible without eliminating the line as a viable source?

The data signal is superimposed on the mains voltage. The mains voltage will typically be 120 V, 60 Hz or 230 V, 50 Hz and the data signal might be 1 V, 2 MHz.

Figure 1. Graph of $$\ 100sin t + 10sin(100t) \$$ generated by Wolfram Alpha.

The chart above gives an impression of the resulting waveform of a higher frequency superimposed on a lower one.

The data signal is so small that it will have no effect on mains-powered loads.

• Wow the visualization really helped make it make sense, thanks! Commented Dec 29, 2023 at 4:38

For the AC power to be viable as a power source, I would assume that the frequency and amplitude needs to be steady.

The fundamental frequency is quite steady - much better than 1% in the US grid. But the harmonic content is highly variable. High frequency additions due to data transmission are of no significance in relation to the primary function of power delivery.

I’ve seen the mains waveform anywhere between a visually perfect sine wave to something that looked like it was drawn by a drunk person. In both of those limiting cases, the data transmission signal would be too small to see on an oscilloscope time domain display. Of course it would be easy to see on a log (dB)-scale frequency domain display of a spectrum analyzer.

As an aside, almost anything modern and electronic can run from DC and is slightly more efficient that way. So the exact mains waveform is of no big importance as long as the polarity changes are not too slow. A square wave works from day 40Hz down to 0Hz. Above 40Hz up to about 400Hz, a “sine wave”, even highly distorted one, works fine. That’s because the AC mains gets directly rectified in modern power supplies - it doesn’t feed a transformer at mains frequency. Universal motors also work fine at DC mains (hence their name!). The devices that need mains between 50-60Hz are induction and synchronous motors, and devices with a mains-transformer-based power supplies. The latter - transformer-based supplies - will usually work OK up to 400Hz, with the rectifier getting more efficient as the frequency goes up.

• "I’ve seen the mains waveform anywhere between a visually perfect wine wave to something that looked like it was drawn by a drunk person." Which came first? The wine wave or the drunk? Commented Dec 29, 2023 at 3:16
• @Transistor I'd like to keep that comment for posterity. The hilarity must survive somewhere, after all. Commented Dec 29, 2023 at 4:50
• I wonder what share of induction and synchronous motors are now being run with VFDs which would eliminate the 50 or 60 Hz requirements? Commented Dec 29, 2023 at 21:39
• "Almost anything modern and electronic can run from DC" -- to be clear, this is true in principle, as SMPS are pervasive; but in practice, because AC remains widely available, many designs use it as an assumption, and will fail in various ways if you actually try DC. So just an FYI for readers to be careful. (I think the most common reason is convenience junk like X-cap discharge, which often use a lack of mains pulsation as a trigger to discharge them; putting DC into such a unit blows the discharge circuit.) Commented Dec 30, 2023 at 0:21