Noise

Is it periodic? Harmonic of 50 Hz line?

Lowpass filter it.

If it is a supply ripple, add a bit of the reverse ripple to the output.

I did that successfully in an intercom with nasty supply noise.

It's not supply ripple. I've scoped the rails and they're dead flat.

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If it is then it can only arise through induction from the domestic supply wiring, in which case I'd expect it to be 50Hz and not some harmonic of that.

Tricky bit is, 200Hz is already pretty low. :(

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Not sampling frequency is it? A tuned filter oughta help.

NT

Activated twin-T notch, one for 200 and one for 600, then lowpass.

Cheers

Phil Hobbs

Cursitor Doom wrote in news:q54d13$cq1$ snipped-for-privacy@dont-email.me:

How many probes does your kit have? for five quid.

Not 12 I can gaurantee.

Isn't that less than 10 bucks?

I have serious doubts about any analysis a 7 dollar toy makes. Oh... did I misspell 'tool'?

It likely generates the noise you are seeing.

Have you placed it into an anechoic box?

Or is it that your body is an antenna, and all the 'cancellation' attempts in the world will not attenuate the noise you are injecting into that probe contact?

If you have an oscilloscope, like trigger the scope and look at the signal. If it doesn't move horizontally, it's line sync'd.

Capacitive coupling from the AC line could have big harmonics. It's been a long time since the AC line was sinusoidal.

Or 150 Hz brickwall.

An elliptic lowpass with a notch at 200 would be good too.

The unit probably notch filters mains fundamental and the first couple of harmonics in order to work at all. You are seeing artefacts from the incomplete cancellation of higher frequency mains harmonics.

You could probably get away with a steep elliptic low pass filter or chebyshev type 2 with the null sat on 200Hz or just post processing of the data offline in Excel if superficial noise bothers you that much.

Chebyshev type 2 has the advantage of a flat pass band response although it is somewhat more tedious to implement well.

I found the answer to the problem. It was simply the lack of electrode lube on the skin contacts pads. Given a smear of this conductive lube on the sensors and the noise just vanished. I mean, it may still be there, although now invisible, but is *tiny* compared to the desired signal which is now as sharp as a razor. I'll try to post some pictures of the trace if time permits over the next couple of days. So 10 out of 10 for those industrious Chinese people. :)