Parts clarification for Etherwave Theremin build

Thank you all for your kind answers and advice!

dewster: I see what you mean by the poor inductor industry in relation to Theremin builders. Technology shifts with no mercy hehe, but best of all is that there are always diy methods of recreating practically anything with enough wit and perseverance. As I started to mess with inductors and testing the ones I've scored from old equipment, I tried to build my own, variable and fixed, using a bit of paper rolled around screws, old ferrite cores and what not, loose enough to be able to adjust the inductance. The very first intent actually worked, I used an old radio antenna coil ferrite bar (around 6mm in diameter), broke it in 3 sections of around 2.5cm long and on top of a bit of paper wound a couple of layers of some magnet wire from an old transformer. The wire is 0.01", or around 0.25mm in diameter (measured with digital caliper the outside diameter). I was after the original 100uH and 68uH variable coils. By measuring the inductance, I had to unwind all of the second layer and with several cuts and measures got it to 100uH. They worked quite well in the oscillator giving a clean and strong sine on the scope. The drawback is the coarse value shift when moving the ferrite inside of the coil with the lack of an adjustment screw - but oh well, it gave the starting incentive to keep going!

It definitely makes sense to use the most appropriate and convenient parts for the application. I will look into getting the best possible low TCO components.

About hand wound air core coils, what should be the optimal limits of their DC resistance? As I understand, the lower the resistance, the higher the Q? What would be a reasonable maximum of resistance those 47u and 22u fixed coils should have to stay within the operating range of the oscillators with the surrounding components values as suggested without having to adjust them?

Thierry: I appreciate your comments. I understand how there are different approaches to implementation of technologies and how it all comes together in a functioning device. So many factors, engineering and soul-wise, in the end it is all art in one form of another. Its incredible what it is possible to make using combinations of both, sometimes going out of rules, even if messing things up, one ends up learning from the process and maybe even innovating something in the meanwhile. I myself often cling on "reinventing the wheel" having one in front of me ready to use hehe.

Once more, appreciate all the responses, as well as for indirect, yet very useful help and ideas I have gotten from yours and others older posts in the forums - some very concise and tested/experimented information, always useful to know from first hand experienced people.

Best regards,

Konstantin 

"About hand wound air core coils, what should be the optimal limits of their DC resistance? As I understand, the lower the resistance, the higher the Q? What would be a reasonable maximum of resistance those 47u and 22u fixed coils should have to stay within the operating range of the oscillators with the surrounding components values as suggested without having to adjust them?"  -- kostix

I don't believe resistance (Q) will be a factor in the uH range.

My personal solution to the variable inductance conundrum is to sidestep the issue altogether.  It's not worth thinking about unless you're into vintage everything (not me, there's too much future ahead).

Ok I will keep that in mind bout the fixed coils, thank you!

I know what you mean about sidestepping the outdated for more innovative approach. I imagined several digital/analog hybrids with an array of earcandy features like different tones, digitally generated waveforms and all kinds of FX, I realize the possibilities are endless in that regard. But as of being the first theremin build for me, a more classic variant resulted more attractive to learn about all the theory behind which always felt peculiarly interesting for me.

Anyone ever tried implementing a variable inductor with an opamp based floating gyrator circuit? Would that work instead of a physical coil in the EW?

Cheers!

"I know what you mean about sidestepping the outdated for more innovative approach. I imagined several digital/analog hybrids with an array of earcandy features like different tones, digitally generated waveforms and all kinds of FX, I realize the possibilities are endless in that regard. But as of being the first theremin build for me, a more classic variant resulted more attractive to learn about all the theory behind which always felt peculiarly interesting for me."  - kostix

For a first Theremin you could sidestep variable coils by changing the length of the antenna for tuning.  FredM had threaded antennas on some of his to do this.  Maybe a combination of telescoping antenna and threaded mount.  It makes you touch the antenna a lot so you would want to protect it against ESD somehow.  And you couldn't use an EQ coil I don't think.

"Anyone ever tried implementing a variable inductor with an opamp based floating gyrator circuit? Would that work instead of a physical coil in the EW?"

I brought it up in the "crazy ideas" technical thread and thought it was a new idea but it wasn't (the forum search works pretty good here):

http://www.thereminworld.com/Forums/T/29205/crazy-theoretical-technical-ideas?Page=6

It didn't look too good in simulation so I didn't proceed further: it was kind of touchy, you can't get higher voltage swings out of a gyrator, and variable opamp delay due to temperature would likely be a huge issue (there are reasons the most stable oscillators have so few transistors). There are few things as stable as an air core inductor. 

OK I see the idea of tuning with the antenna length adjustment. Will look into that, thanks!

For now I finished a design of a PCB, would love if anyone can give any comments. I procured to put attention to signals first and then route the power. Its mostly based on the component layout proposed in the hot-rodding document with minor arrangement changes.

Here is a high resolution export from Eagle:

EW Theremin PCB

Thanks in advance!

"if anyone can give any comments" - Konstantin

1. Unreasonably labirinted (the viewpoint of PCB designer with 5+ experience in airspace industry).

2. No prototyping pads and traces in blank area of PCB. No alternative pads for some components  (the viewpoint of hobbyist) 

and, again (from http://www.thereminworld.com/Forums/T/29278/my-etherwave-pcb):

3.  No ESD protection circuits, no buffers like the ESPE01/YAEWSBM modules. 

dewster: about the air wound coils, can the coil have some sort of paper or plastic base on which to wind it, or more optimal would be to wind it and dip it in some laquer or glue for it to maintain the windings glued and then remove the base on which it was wound? What if i leave a thin roll of paper under the coil on which it would be wound, will that play a role in its temperature "rating"?

Also, what about making the already wound coil some sort "dress" around it some sort of thermal isolation, or maybe it would make more sense to isolate each of the oscillators blocks of components?

"... about the air wound coils, can the coil have some sort of paper or plastic base on which to wind it, or more optimal would be to wind it and dip it in some laquer or glue for it to maintain the windings glued and then remove the base on which it was wound? What if i leave a thin roll of paper under the coil on which it would be wound, will that play a role in its temperature "rating"?"  -- kostix

First you want a stiff, inert former.  Second you want a former with low relative permittivity, but that's a somewhat distant second (for self-capacitance reasons).  PVC pipe should be fine, phenolic pipe is harder to find but model rocketry provides if you want to go that route.  Wood would be fine too.  But PVC is the easiest and I really don't see a compelling reason to go crazy trying anything else.  Use clear nail polish to secure the ends, cover it in the "battery" type PVC heat shrink tubing if you want a more polished, more durable final product.

Don't take my word for anything, it's really not all that difficult to evaluate coils from a temperature standpoint given a home freezer, a sensitive LC meter, and some kind of temperature sensor taped to the coil.

OK perfect, appreciate the tips, Ill do some tests and see what works best! I could also dip the whole thing in epoxy and  leave that as is/ encapuslated in the glue, lets see how that works. 

I could also dip the whole thing in epoxy and  leave that as is/ encapuslated in the glue, lets see how that works.  -- kostix

Maybe if you're building a Tesla coil and are worried about dielectric breakdown, otherwise epoxy is not necessary and probably super messy.  The wire wound on PVC pipe with ends affixed via nail polish is really stable.   PVC heat-shrink takes maybe a minute to do and looks quite professional.  I use wire-wrap wire to go from the winding ends to the circuitry (threaded through small holes in the pipe - do this before heat-shrink and affix it also with nail polish).

ILYA makes some very good points above!