VDSL versus HF Radio

I’m putting up a 40M dipole. When I Tx on 40m (50W peak) my Internet drops out. Sometimes it comes back, other times the modem loses sync. The dipole has a balun, and is nicely tuned.

I tried some ferrites with several turns on the modem VDSL and power cables which improved the situation a little. But I still get a momentary drop out of Internet on PTT, and if I try hard enough I can still lose sync on the modem.

Now I have NBN (Australian National Broadband Network) with a VDSL link over traditional copper phone lines to a “node” several hundred metres away. Turns out VDSL uses bandwidth up to 30 MHz … so I guess I’m getting right into it’s pass band. Old school ADSL only used a few MHz. The phone line used for this service is 50 years old and has significant differential to common mode conversion. It’s not much of a transmission line. But probably a pretty good antenna!

I build a little jig with a transformer to couple the differential signal to my spectrum analyser and take a look:

Lotsa turns on the primary, one turn on the secondary, some core I found in the junk box. I adjusted the coupling capacitors in line with both arms of the primary so that the modem didn’t lose sync when I plugged it in (about 5pF). Also in this photo is the series LC circuit, but disconnected (open) at this stage.

Sure enough, I could see Rx energy from the node to my modem at around 7MHz, and other energy out to 12MHz. In the 7MHz region, I could see the Rx signal from the “node” at -60dBm. When I Tx SSB on 7.18 MHz my SSB signal was -30dBm. No wonder the modem is choking.

After some experimentation, I came up with a 7MHz LC series resonant circuit connected across the phone line. When the modem does it’s training thing, it sees a short circuit around 7MHz and ignores that region as no good. So when I transmit in that region, there is no modem signal to interfere with.

I started with a 800nH/600pF filter. Xc and Xl is a rather low 37 ohms reactance at resonance, and just a bit higher than that above resonance (e.g. at 8-12 MHz), attenuating a lot of the HF energy. So it was basically a LPF, killing anything above 6 MHz. This stopped the drop outs, but my Internet downstream bandwidth dropped from 55 to 24 Mbps.

Final Design

After some fiddling with a spreadsheet I came up with a 5uH/100pF series LC notch filter. This is simply a 5uH inductor in series with a 100pF capacitor, connected across the VDSL phone line.

This has a few hundred ohms of Xc above resonance, which results in just a few dB attenuation at 8-12 MHz. This obtained 38 Mbps downstream. Upstream was the same (24 Mbps) as with no filter. Good enough.

The inductor is 9 turns on a F37-61 core. Make sure you use a material suitable for high Q inductors. I initially used the wrong core material and couldn’t get a decent notch. I recommend you check the tuning – it should have a deep notch at around your operating frequency.

Here is a sweep of my filter:

I put 560 ohm resistors in series with the tracking generator output and spectrum analyser input to approximate the line impedance using this jig:

Here is a plot of the system in action:

The yellow plot is the original, unfiltered VDSL signal. At the same time I’m transmitting SSB. You can see my SSB signal on 7.18 MHz (yellow peak above the “1”).

Purple is with the series LC notch filter installed. You can see the notch left of the “1” at 7MHz. The “node” has worked out 7-8MHz is a dud band so isn’t sending any information. So nothing to interfere with when I PTT SSB. I’m not sending a SSB signal in this plot.

Note also the 8-12 MHz purple (filtered) is just a few dB lower than the yellow (unfiltered). So the notch filter isn’t wiping out the HF signals.

These plots show a mixture of Tx (-10dBm) from my modem, and Rx (underlying gentle downwards slope) – the signal from the “node”. I assume it’s full duplex, we just can’t see the Rx signal most of the time. I am sampling the combined signal next to the modem, so Tx dominates. You can see the Rx signal better when the modem is training.

For some reason my modem doesn’t Tx in the 6-8MHz band. Probably a good thing for RFI.

Results

Without the filter I get immediate interruptions pings and loss of modem sync after 20 seconds. With the filter I’ve hammered it for the last few weeks with SSB and FreeDV signals but no interruptions in pings or the received audio and waterfall from a local KiwiSDR.

There is a hit on my downstream bandwidth, but it’s not significant for me. Much nicer to be able to transmit on 40m and not have the Internet break!

Here is the finished filter, installed near the modem in some heat shrink:

I’d be interested to see if this idea will work at other sites. Due to the random nature of the phone lines no two VDSL installations are the same.

The design is simply a 5uH inductor in series with a 100pF capacitor, connected across the VDSL phone lines. You need a high Q inductor, I used 9 turns on a F37-61 core. If possible, carefully check the tuning of the notch filter.

I’ve also seen suggestions of using a quarter wave stub (about 10m of phone cable) to get the same effect. This is a neat idea, as you could just buy a 10m phone extension lead, and plug it in parallel with your VDSL line. However once again – carefully check the tuning of the stub – phone cable is messy, uncalibrated stuff!

This was an interesting little project, with a satisfying result. I quite like learning about RF, and (re) learnt about the trade-offs around reactance at resonance, transmission lines, and inductor core material.

Thanks for help and useful comments from AREG members on their mailing list. Several other AREG members are also suffering from the same problem, so I imagine it’s wide spread in other countries that use VDSL.