Which humbucker variant for very nearby 'processr clock radiaton' rejection?

[peskywinnets]

I'm faced with making my own small humbucker (I'm very constrained on space) to help reduce 'EMI pickup' relating to a 'PIC bestowed' circuit board (with 4Mhz, 20khz clocks) that's going to be by necessity extremely close to the proposed humbucker.

This humbucker pickup will not be used passively, so I'm not worried about it's output level & therefore I can go much smaller than a traditional humbucker). In fact this proposed humbucker's signal won't even be heard - it's signal will be used into a PIC for deriving frequency/level of the string etc)

The PIC will be to one side of this proposed humbucker - just 2.5cm away , so I'm wondering which humbucker variant (if any) would yield the best EMI cancellation (for all the signal won't be heard, the less crud on the signal the better)....

1. Side by side (ie traditional) - probably not ideal, one coil would be significantly further away (relatively speakin) from the source of EMI (the PIC) & therefore not cancel well?

2. Stacked - possible - this would put the coils equidistant from the EMI source, but I've only about 8mm headroom to play with & it'd likely be tight to squeeze in two coils vertically - also, my initial attempts with stacked humbuckers weren't promising!

3. Split - this is the variant I'm leaning towards as I reckon it'd be the easiest to 'balance' wrt EMI cancellation (I've made myself a CNC winder so wrt to 'balancing coils', I can be accurate to 1/200th of a turn )

I'd be grateful for any thoughts, comments, tips, advice (eg whether the idea is dead in the water etc!)

[Joe Gwinn]

Dual-coil humbuckers reject low-frequency (~100 Hz) fields from distant sources, where distant means many times the distance between the two coils.

What you have here is a high-frequency field (20 KHz and 1 MHz) generated a few cm away. So, no dual-coil humbucker scheme is going to help.

Your best solution is to put the PIC board in a shield can of some kind. If airflow is needed, part of the shield wall can be made of copper insect screen fabric, or be perforated. Don't forget to bypass all wires passing through the shield with capacitors to the shield.

If it matters. Does the 20 KHz noise actually cause a problem? It's a bit higher than most can hear through a guitar amp. It may be sufficient to put a simple RC filter in the lead to the output jack, so the 20 KHz does not make it to the guitar amp.

[peskywinnets]

Just to clarify - I haven't done this yet (but will be soon....for no other reason than to have a dabble with some whacky ideas I'm floating around!).

re the 20khz - not sure what the problems will be yet. It's not the 20khz that worries me (the pickup's impedance at 20khz will be massive ...& as you say, it's likely that only dogs would be able to hear it), but the 20khz clock is a square wave & therefore has heaps of harmonics content - it's these harmonics that might find there way into my pickup signal and scupper my ideas)

Not sure a PIC 'shield' will be practical (no space - risk of shorts etc) - so will likely just have to suck it & see!

[RedHouse]

Since you said it will be active, you just need to implement some 4th order low-pass filtering, start at about 15khz.

I've had some very good results with the circuits in Don Lancaser's "Active Filter Cookbook".

What kind of PIC circuit are you running so close?

[David Schwab]

Just to clarify - I haven't done this yet (but will be soon....for no other reason than to have a dabble with some whacky ideas I'm floating around!).

re the 20khz - not sure what the problems will be yet. It's not the 20khz that worries me (the pickup's impedance at 20khz will be massive ...& as you say, it's likely that only dogs would be able to hear it), but the 20khz clock is a square wave & therefore has heaps of harmonics content - it's these harmonics that might find there way into my pickup signal and scupper my ideas)

Not sure a PIC 'shield' will be practical (no space - risk of shorts etc) - so will likely just have to suck it & see!

Hold a quartz timed watch or a cell phone, etc., by a pickup and see what you hear. You will pick up noise.

You might try sticking a grounded sheet of aluminum between the circuit board and coils. Aluminum seems to work better than other materials for shielding this kind of stuff.

Best bet might be to have the circuit external to the pickup.

[peskywinnets]

The PIC will be running a 4Mhz internal oscillator (no external Xtal involved), this is in turn divided down internally within the PIC to about 20Khz, (for some PWM related stuff).

My circuit has a LPF at the input (spec'ed set to pass circa 7khz & below - like I say this pickup signal is not to be heard, so I'm not too worried if the circuit's pickup signal sounds pants).

There's a few things I want to dabble with wrt guitars and onboard PICS (this is intended for my Variax 300 guitar and therefore has no other mag pickups to worry wrt EMI) ....I guess this is one that either sink or float instantly - just need to get my backside in gear & wind a small pickup to co-locate next to my circuit to see which it's gonna be! (my gut feeling it's gonna be sink....but hey, gotta make that journey yadaa, yadda)

[David Schwab]

(my gut feeling it's gonna be sink....but hey, gotta make that journey yadaa, yadda)

Yes you do! Keep us informed.

In the past I have used 555 timer circuits to flash LEDs in guitars, and if not careful you can hear the ticking sound. Shielding helps.

[Rick Turner]

The smallest humbuckers I know of are the ones in the Roland GK synth pickups. Each string gets a micro humbucker, and you can put them really close to the strings next to the bridge for best isolation and S/N performance.

Or what about using hex piezos? Try a Baggs T bridge. Tons of output, low noise, wide bandwidth, great fundamentals.

[Joe Gwinn]

Actually, there is a simple solution: get a 8 MHz clock. The resulting 40 KHz is way above human hearing, so no shielding needed, except a RC filter to keep from jamming the guitar amplifier.

[RedHouse]

Maybe he's concerned abut hetrodyning.

[Joe Gwinn]

Maybe he's concerned abut hetrodyning.

That's the reason for the simple RC filter. Without specific circuit details, it's hard to be more specific. These effects are very nonlinear, so even a simple filter can have dramatic effects. I'd also worry about intermodulation distortion and intermodulation products.

[RedHouse]

That's the reason for the simple RC filter. Without specific circuit details, it's hard to be more specific. These effects are very nonlinear, so even a simple filter can have dramatic effects. I'd also worry about intermodulation distortion and intermodulation products.

Yep, that's what I suggeted back in post #4

[Joe Gwinn]

Yep, that's what I suggested back in post #4

Yes, but with a twist - I would not use a purely active filter, as the filter istelf can suffer from nonlinear effects if fed too much ultrasonic signal. Better to at least start with a simple passive filter to knock the ultrasonic signal down, so the following active stages (if needed) are able to stay linear.

[peskywinnets]

Maybe he's concerned abut hetrodyning.

This is actually my main worry (though the name for it escaped me).

As David earlier pointed out... hold a mobile phone close to a guitar pickup & you'll almost certainly hear some form of interference - but since it's likely all mobile phone clock frequencies are gonna be way above human hearing ability ....the pickup must therefore be 'picking up' some hetrodyned/sub harmonic frequencies.

I'm away for a few days, but hopefully by next weekend I'll actually be able to physically place a small homemade pickup next to my PIC circuit ...then i'll have some form of closure one way or the other (but I gotta design/cut a custom suitable small bobbin out of acrylic, then make a holder for the small bobbin so I can mount the bobbin on my winder etc...so getting my hands on such a small pickup is not a particularly quick job)

To paraphrase Arnie ... "I'll be back".