Mains hum

At what frequency does it ring?

** Ringing (as seen on a scope) is at the same frequency as the notch minimum. But you will not find it in a spectrum analysis.

..... Phil

The reason I ask, is because if the ringing is high enough frequency, it is out of band and can be filtered out. I'm willing to bet there is an inverse relationship between the half bandwidth of the notch and the ringing frequency. If it is narrow enough (which is what is desired) it will produce ringing that is far out of band and easily filtered. I believe I read the ECG bandwidth is 150 Hz. I expect it would not be too hard to push the ringing frequency up to a range that is easily filtered.

On doing a bit of research on the matter, it seems to be a much discussed topic, but with no definitive solution. However, here is a paper that seems to say the solution is really in the way the electrodes are attached.

Let's construct a mind experiment to analyze that idea. If you stimulate a notch filter with an impulse, a signal containing all frequencies in equal amplitude, what will emerge from the notch filter is a ring at the frequency at which the filter is supposed to have a null?

Is that logically consistent with the frequency response of the filter?

Phil Allison snipped-for-privacy@gmail.com wrote in news: snipped-for-privacy@googlegroups.com:

But if it gets rectified, the ripple on a badly designed supply exhibits noise at f*2.

snipped-for-privacy@decadence.org wrote: ================================

** Well yes,

the hum you hear from an amp with bad filter electros is 100 or 120Hz.

...... Phil

Ricky wrote: ============

** But will not show up on a spectrum analysis. That will look as expected, ie missing a chunk. ** The " ringing" seen on a scope is an *artefact* - created by missing and phase shifted components of the input signal. Same goes for square wave inputs. All components must be there in the right amounts and phase relationships to get a square looking result. Brilliant example for " cognitive dissonance" for most people.

...... Phil

So, you're approaching the beef now.

The kilo-ohm impedances of electrodes are there *after* cleaning the skin and removing the dear chest hair. There was also electrode paste between the siver plate of the electorode and the skin.

You did not respond to my question about a reference electrode.

You can tell a guitarist it's a good idea to shield the cavity, 7/10 times he won't listen though and if you start talking about "capacitive coupled injection" he's even less likely to.

Some years ago, I was the unofficial chief (and only) engineer of a local guitar maker (the owner is a friend of mine).

You don't try to convince the guitarist (who just wants low noise), you talk to the guitar maker (who just wants to know exactly how to do that). And few guitar makers are electronic engineers.

The remedy for capacitive injection was to electro statically shield the routed cavity, originally by carefully fitted brass foil shielding tubs, then (after I arrived) by nickel-dust based shield paint sprayed thickly on the walls of the cavity. The DC resistance between any two points on the cavity wall was about 20 ohms. The underside of the pick guard had an aluminum foil layer glued on. These shields were all grounded to the shield of the coax wire from guitar to amp. The strings were also grounded.

The remedy for magnetic interference, chiefly at power frequencies and harmonics, is various kinds of humbucking construction.

Joe Gwinn

PS: What I also did was to invent various kinds of jigs and fixtures, and introduce them to various kinds of tool not found in luthier tool catalogs. This collectively had a large impact on their manufacturing costs. JMG

That should be addressed to the OP. It's not my setup.

I've already mentioned that current practice either uses a single reference electrode, or combines the signals from all electrodes to use as a common reference for all electrodes. This gives N signals for N leads rather than N-1 signals.

What are you looking for with the question?

When you mention kilo-ohm impedances, that is low relative to the amplifier input. It's also low enough for stray coupling to only produce a relatively low voltage in the signal. That is what the OP is seeing, a low voltage "hum" on his signal. It actually would not take much filtering to make that noise virtually invisible.

You didn't answer my question about the frequency of the notch filter ringing.

I believe the required characteristics of the notch filter in this application would not produce noticeable ringing.

Joe Gwinn wrote: ==============

** All these things were well known and adopted by makers like Gibson in the 1950s. But they were a luxury brand sold to professionals. A radio repair man called Leo Fender began making similar things, very cheaply. Working at a time when triac dimmers were unknown and likewise safety earthing - he gave almost no attention to ES shielding. His famous Stratocaster guitar has only a token effort at such and is a first class receiver of hum and buzzing from the AC supply. Nearby fluoro lighting drives one crazy. Note the use of plastic covered pickups, unshielded wiring and no cavity shielding at all. Fender relied on the player earthing themselves via the steel strings to proved any measure of shielding. AFAIK even recent examples are little better. The instrument is considered to be is '"sacred" by players and must not be changed.

....... Phil

Might I suggest a variation of Jan's idea?

1. Digitize the waveform
2. Feed a 50Hz signal of equal amplitude into a mixer together with the digitized waveform.
3. Adjust the phase of the 50Hz signal until the interference is cancelled out.

I believe this method will have the least detrimental effect on the desired trace. In fact it might be possible to use an app like Audacity to do all the above with just a few mouse clicks, as I'm pretty sure there's a spot frequency filter somewhere in the software.

====================

** Huh ???

Makes no sense at all.

** FYI Mr Dope

The AC mains wave drifts in frequency and phase and changes amplitude in the ECG trace with a host of external factors.

....... Phil

Sure, that will work for an amateur setup like what is discussed here... as long as nothing moves during the recording. That will only work for a stationary signal.

That's my understanding too.

All true.

Also true.

One thing that people do is to put a larger coil in the bottom of the routed-out tub, and use that to humbuck the old-school singlecoil. With good ES shielding and grounding of everything, this can help a lot. There is a patent on this, but I don't recall the number offhand.

Joe Gwinn

Joe Gwinn wrote: ===============

** There is no huge problem with players standing a few metres away from any AC transformers in the vicinity. The mag field from an AC transformer is purely short range. The need for humbucking PUs arose from those who sit next to or even on top of their amps.

As it happens, nearly all humbucking PUs have full ES shielding too, so are dead quiet.

....... Phil

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rUSc9rxOsuD0

You don't speak for everyone here. I personally find JD's posts very relevant, thought-provoking and entertaining. Keep it up, JD!