So this is more of a continuation from
here. Basically my neighbour has a solar inverter which is located close to her phone line and as a result, our ADSL bandwidth suffers speed loss. This has been diagnosed by Telstra in writing. My hypothesis was the PWM frequency of the inverter had something to do with not being filtered correctly and is being induced into the telecommunications line. Because our ADSL uses frequencies under 2.2MHz, any sort of interference with a similar frequency would cause a lower signal to noise ratio.
I went over to her place today and plugged my software defined radio into her phone line to check out what the frequency spectrum looks like. A software defined radio is sort of like a spectrum analyser but it's USB and plugs into the computer and you can buy them pretty cheap. I used a Hamitup 100MHz upconverter as my one normally can't go below 25mhz or something like that. I also used a voltage divider to reduce the signal going into the SDR, I used a 10k and 1k resistor and had it in a 10:1 ratio. It seemed to work fine surprisingly.
So this is the control, where i'm looking at the telecommunications line WITHOUT the inverter operational. I turned it off at the power box. There seems to be some peaks around 1.6Mhz, no idea what they are.
This is when the inverter was turned
on with a frequency range of 0 - 2MHz. There is a very clear centre frequency around about 1.55MHz, with harmonics extending either side of it. As ADSL uses frequencies below 2.2MHz, I believe these harmonics on the telecommunications line to be the cause of the slower internet. If some frequencies are unable to be used for adsl bandwidth, the modem will have to negotiate a connection with the DSLAM equipment at a slower speed.
This is identical as above, but the frequency has been shifted to be between 2MHz - 4MHz. As can be seen in this example, it appears the harmonics are dissipating frequency after about 3MHz.
So in theory, anything above 3MHz should still be able to be used without problem. Because the national broadband network's FTTN uses frequencies up to 25MHz (VDSL2), this means that while 3MHz might be unusable, the rest of the spectrum should be fine. I'll try to post an update on Monday.