Netgear ADSL router DG834

[midibob]

Hi,

Interesting problem with this router. The device was emitting a strong hash around 3.68MHz and also at some other frequencies above 4MHz. As this is in the middle of the amateur radio band it was annoying to say the least.
Sniffing around with a small coil attached to my receiver I found the noise was predominantly coming from the wall wart and the rear of the router where the power connector was.
The wall wart feeds 12v AC to the router where it is rectified with a bridge and smoothing cap, so pretty standard.
Putting a ferrite choke on the power lead reduced the symptoms a little but nowhere near enough.
With scope attached to the smoothing capacitor the waveform was horrible. Loads of spikes and all sorts. Looking at the smoothing cap I noticed it was a 2200uF Teapo!! Oh dear!

Changed the cap out and the reduction in noise and the waveform was very noticeable. Also I added a 0.2uF across the smoothing cap and that completely eliminated the noise.
On testing the Teapo the ESR was fine and the value, on my cap tester, showed to be correct too?
Anyone care to try and explain that one?

Midibob

[ipman]

Re: Netgear ADSL router DG834

A high capacity electrolityc capacitor will not filter that high anyway, no matter which ESR have.
If I were you, I would have tried the clasic 0.1 microfarad or even bellow.

[midibob]

Re: Netgear ADSL router DG834

I would tend to agree but the difference in noise output between the two caps was very noticeable. I did originally try the 0.2uF across the Teapo but it was nowhere near as effective.

[PCBONEZ]

Re: Netgear ADSL router DG834

As you didn't mention what series the Teapo was or what you replaced it with I'm assuming a lower ESR helped the High Freq a tiny bit more than the Teapo could.
As you found a 2200uF doesn't do that much at all for higher frequencies.
-
It's probably coming into the wall wart through your wiring.
Might be a POS power supply with no input filter [on about anything] or a local radio station. Could be a lot of things.
.

[midibob]

Re: Netgear ADSL router DG834

The Teapo is marked up as :-

SC105c
08/03
A3

I don't know what the SC or A3 mean as I haven't looked into it.

The interference was generated by the router and obviously used the wiring to the wall wart as an aerial.
The good bit for me was changing the cap (I can't remember what the replacement was, it was just one I happened to have in stock and used to substitute) the interference from the router was greatly reduced and completely eliminated with the addition of the 0.2uF cap. Just don't understand why the Teapo should have been that bad considering the ESR reading?

[PCBONEZ]

Re: Netgear ADSL router DG834

SC is the series.
A3 is a lot number or something.

2200uF Teapo SC come in lots of sizes.
.