First trials of RF injection into cables

At Alpha Audio, we always try to go that extra mile. Or a step. Last year we tested 64 cables for numerous elements: capacitance, inductance, resistance, timing, et cetera. Literally hundreds of measurements were made. Perhaps more than 1,000. But still, we haven’t really examined one element yet: shielding.

When you lie awake, you start thinking. And your author then often thinks about how we can improve measurements and what kind of research we can start.

The other day we were working on some power supplies when we saw an antenna lying around. It had once been purchased for another project that we never got around to. But that antenna and the RF box gave an idea which became more concrete over night.

Soldering

Now we are not going to deploy an ordinary antenna in this test. In fact, what we are curious about is the influence of wifi. Our generator goes up to 3 GHz, so we can emulate 2.4 GHz wifi at most. Which is fine for a first test.

We took a wifi antenna off an obsolete router and soldered a bnc plug to it. This way we can plug it into the RF box and loop it through to the generator.

Furthermore, inside the RF box we connect some speaker cables. Initially, we connected an 8 Ohm resistor outside the box, but it picked up huge amounts of RF from other sources. Logical; it was outside the box…. Stupid. Eventually we shut off the inputs not in use and connected the 8 Ohm resistor internally.

Emulating Wi-Fi

The Aeroflex generator can generate a 2.4 GHz signal with numerous modulations. One of them is QAM modulation. A 2.4 GHz Wifi signal also uses QAM modulation, so in a sense we are emulating a Wifi signal in this way.

We initially measure with 1V, which is on the high side, but this way we can see if we pick up anything at all. Later we will start to see what we see with lower signal intensity.

Subtle but visible

We do a so-called persistence measurement through the Tektronix. The bandwidth of our Tektronix is 1 GHz. So in fact we don’t see the 2.4 GHz signal, but because of the QAM modulation we do see effects of modulated signals on the carrier-wave.

Each box is 1mv. That is very little, but it does show that rf is being picked up. Just look at the differences between no rf enabled and rf enabled. There is consistently less blue ‘haze’ in measurements where rf is disabled. We also see thicker, red spots in measurements where rf is enabled. In short: we see an effect… although it is very little and we dare not draw any conclusions from this.

Bonus: phase noise

Where we do dare to draw firm conclusions is this bonus-measurement. Just for fun we put the Volumio in the RF box and then measure with and without RF switched on… Damn! You really don’t want to put a wifi access point next to your digital equipment!

We see an increase from 3.55 ps to 10.58 ps! That is extremely significant and a much bigger difference than mounting a good switch or cable. Bottom line: really keep those Wi-Fi APs away from your digital devices!