Let's Design and Build a (simple) Analog Theremin!

"...i know it's not the real world, nor a alternative for breadboard. but usefull for trying circuits without burning the half house down..."  - xtheremin8

Yeah, some people rag on circuit sims as non-realistic and so poor training aids.  I think they're great, and a give a lot of insight into what is going on.  The physical world of circuits is governed with an iron fist by Murphy's law, so only a small subset of what you can sim will actually work the way you would like it to.  Oscillators are particularly picky.

"are the wound coils, variable conductores out of question?"

Coil winding is OK, but only simple single layer ones on PVC or similar, no special windings on special forms or moving ferrite.

I think I'm going to have to cave on the NO FETs! thing.  FETs are almost ideal when it comes to oscillators, they are higher impedance, are self-biasing with a single resistor, and they have a gentler conduction transition zone which likely causes less switching noise on the rails.  I think they may be more useful in an active mixer than bipolar transistors too.

Getting bipolar transistors to work linearly in a low voltage environment is kind of a nightmare, I've been spicing my ass off and can't get sufficient isolation in the mixer stage.  I never really thought about it before but if the collector gets current starved then the base sees the entire emitter impedance, not beta times it.

Why talking about a pitch range to 16kHz? While that is indeed roughly the upper hearing limit of middle-aged humans, this is not what even the highest acoustical instruments produce as fundamental - something like a piccolo goes to IIRC 5..7kHz or so, and that's nastily high :-) And, give us at least one overtone to hear, dude ;-)

http://www.themusicespionage.co.uk/wp-content/uploads/2012/08/Frequency-Chart.jpg

-> Thread is bookmarked, this will be interesting!

"Why talking about a pitch range to 16kHz?"  - tinkeringdude

Oh, finding the geometric mean of the theoretical limits is just a rough starting point.  Not cast in stone or anything.

I'm kind of giving up on Colpitts as well, a split L Clapp is so much more forgiving in terms of antenna loading &  stalling.

This is a great paper on oscillators:

http://ece2011.p4o.net/admin/vco.pdf

I didn't know it until yesterday, but the oscillator I started out with on the digital Theremin thread is actually a Vackar, where a CLC PI section gives 180 degree phase delay, and feedback is from a capacitive divider.  They can work fine in a Theremin, but when implemented with discrete transistors there isn't a convenient low impedance point to tap off of that has a nice looking waveform.  And they tend to poop out when touching the antenna (though they start back up).

Still playing with oscillators, it would be great to avoid FETs (harder to find and buy, Vgs is ill-defined which can be bad for low voltage circuitry) but nothing is as simple, clean, and stable as a FET.  The BJT transfer function is exponential, and this makes them turn on more abruptly than FETs (which are square law devices).  BJTs are great when you need gain and a well defined turn-on point.  FETs are better when you want better isolation, and when you really don't need much gain, and don't want the oscillator yanking the supply rails around unnecessarily.

Great theme!

Why "no ferrites" in the coils - RM6 from EPCOS may be suitable for it?.. Material N48.

http://www.epcos.com/epcos-en/529410/products/product-catalog/ferrites-and-accessories/rm-cores-and-accessories

sorry for russian comments on the picture...

"Why "no ferrites" in the coils - RM6 from EPCOS may be suitable for it?.. Material N48."  - Alesandro

Too exotic.  I might cave to IF transformers, but even they seem to be going the way of the dinosaur.  It's a terribly constricting position to put one's self in, low voltages probably being the worst, but what do you do when you want to make a design that will last while the sands continuously shift?  Unfortunately no one makes components specifically for Theremins, otherwise it would be a snap.  My digital Theremin stuff will likely last longer (if I can ever get it off the ground).

Okay, now I must also ask ;) Are you really convinced that both of the last audio OTAs, LM13700 and NE5517 are going to be phased out any time soon? There aer some applications for it I guess, and that there are now only those few around, the market may be in balance there?  Whenever I order a bunch once in a while as a side to "any order" (yeah I admit I'm not entirely sure, hahah!), they are on stock in any quantity I'd like & available next day. Unlike LDRs I once ordered :D
What's with these few but existing & flourishing companies that still / again make analog polyphonic synths - would you know whether they use OTAs? (certainly not the special synth chips of old like ssm2040 or so).

I stock my SMD OTAs for roland-ish 4-pole VCFs for when I'll go polyphonic with my synth stuff & want small voice boards (vs. all discrete).
But VCA wise you have made me curious with that 2-JFET thing you dug out somewhere - although, JFETs for this kind of stuff also seem quite hard to get? And the few times I looked only the SMD versions available (which people say are not "beginner friendly", I'm personally not sure I agree unless it's some fine pitched stuff)

"Are you really convinced that both of the last audio OTAs, LM13700 and NE5517 are going to be phased out any time soon?"  - tinkeringdude

I'm just trying to come up with a design that can be built from generic / junk box parts - no matter what era that junk box is being accessed from (though I assume somewhat recent past to somewhat distant future).

just popped by, only to say i'm still on the topic.(spicing up my life etc.) quite difficult things, oscillators. and spicing anyway. found livios library, try to use that because of the already defined transistors, fets..

smd...the joy of every hobbyist. i would like to stick on a transistor only topography. just for simplicity. even smd, still 3 legged. so quite solderable with traditional irons. 

these, any coils,transformers would need a step by step, how to guide, me thinks. but that russian draw above looks cool. thats more my era of how things were illustrated.