LC Tank / Linearizing Coil Excel Simulation

" I'm not sure how accurate its predictive power is (e.g. changing the antenna diameter). " - Dewster.

Good enough, I would say..

I generally use 15mm diameter antennas, and the results I am getting with this conform really well with my measurements. My prior  simple plate calculations were better than the Glasgow calculations, but these new results really seem to really fit my measured results - I was always getting lower capacitance levels close to the antenna than what I measured - The Glasgow results were much lower, and my own fiddle-factored calculations were closer to what I measures, but still lower.

A magnificent effort by you! Reading that paper you linked - phew! To derive something like you have from that! I recon you are wasted on digital ;-) The analogue electronics world needs more people like you!

I have extracted the capacitance stuff from your spreadsheet and extended the resolution - it is already proving to be useful.. I wish I had something you could use, but all I have right now is analogue stuff ;-) ... Interested in a new mixer? - I have just implemented one which is just wonderful - uses synchronous demodulation and only has one comparator and a CMOS switch driving a SAH.. And removes all the sum components without any filtering (as in, no audio top-end roll-off or RF in the audio - only the difference frequency + its harmonics).

Fred.

"To derive something like you have from that!"  - FredM

That paper is a bit over my head, but it does contain the mutual capacitance solution for two spheres.  The formula I used came from a book Google scanned (BTW, how do they get away with that?  If I scanned part of a book and stuck it on the web I'd probably be in jail within the week!):  "Capacitive Sensors: Design and Applications" By Larry K. Baxter (page 22).  It was the closest thing I could find that didn't include infinite anything in it and the dimensions were similar.  My only contribution was in using a geometric mean to include the second length.  It should track in a directionally correct manner if you change the antenna diameter, just not sure how high fidelity that tracking will be to reality.  Real answers would come from FEA, but how you gonna do that in a spreadsheet?  (It's a shame Excel is so business rather than engineering oriented - complex number handling should be overloaded with the scalar operators, and VBA is a pig).

"Interested in a new mixer? - I have just implemented one which is just wonderful - uses synchronous demodulation and only has one comparator and a CMOS switch driving a SAH.. And removes all the sum components without any filtering (as in, no audio top-end roll-off or RF in the audio - only the difference frequency + its harmonics)."

Wow, great idea!  I used S&H synchronous demodulation in a CO2 detector but never thought of using it for RF demodulation.

Baxters book looks good - I have reffered to it before .. Would buy it, but at £80 I can do without the missing pages in Google for now ;-)

"Wow, great idea!  I used S&H synchronous demodulation in a CO2 detector but never thought of using it for RF demodulation."- Dewster

Nor did I until a couple of weeks ago ;-).. Just quickly give you a bit more data - but I dont really want to discuss it yet - When its built I may publish it, or keep it for my book ;-)

MOVED THE FOLLOWING MIXER DISCUSSION TO HERE

Fred, it seems you've got all of the aspects covered to build a top-flight next-gen analog Theremin!  Your cross modulation scheme is beautiful!

Until I listened to her "Lost" album the other day through speakers (I normally listen through headphones while my wife is teaching piano) I can't say that (close your ears!) I was all that enamored of Clara's sound (a little too "buzzy" on "Art" IMO).  Perhaps this is due to what seems to be better recording and mixing on "Lost" - the piano is played with a lighter touch and sounds a little compressed so the rich resonances are more pronounced, and the Theremin slightly less forward in the mix.  Still acclimating to "Lost" but I think I like the selection of pieces on it better as well.

My oblique and long winded way of getting to (and agreeing with) your point that the ability to generate rich harmonics is very desirable.  I wonder if waveform selection could be reduced to a single knob (e.g. odd harmonics CCW, all harmonics center, +1 octave all harmonics CW) or 2 knobs / joystick (ala the Electronic Sackbut).

" I wonder if waveform selection could be reduced to a single knob (e.g. odd harmonics CCW, all harmonics center, +1 octave all harmonics CW) or 2 knobs / joystick (ala the Electronic Sackbut)." - Dewster

How to give the musician access to the harmonic possibilities is one of my greatest challenges.. Its not just about selecting a timbre.. its also about selecting how this timbre changes dynamically as a function of pitch and perhaps volume.

My musical background is primarily with analogue synthesisers - A keyboard and a panel full of knobs - The instrument is often monophonic, giving one a free hand to twiddle while playing, and the modules are voltage controlled allowing timbre to track what is played, and the player to alter this relationship as they wish, and during performance..

Psychologically, coming to terms with the fact that one has no free hands, and that one cannot even adjust anything in real-time without affecting the pitch, has been really difficult.. It has led me to explore alternative controllers which would give more freedom.

The answer is, I suppose, quite simple.. One needs adjustable parameters to be assignable to preset "timbres", and perhaps the ability to select two timbres with a single control to morph from one to the other (as in, one knob). At present, I have about 8 parameters which I feel are needed to recreate any timbre applicable to theremins - I really dont want to lose any of these, and would probably have 16 more if I was building "my" instrument (I want to be able to control sounds which allow me to create a MiniMoog Model D with all its possibilities, or to play a sax, and to be able to assign a 3d "joystick" volume antenna to vocal formants etc..oopps I just added another 32 parameters, LOL... I dont just want a theremin! ;-)..

The only sensible way to implement the above is through a digital user interface which drives a pack of digitally controlled analogue potentiometers.. Create the preset "timbres" each complete with parameters which define the waveshape and modification to these under control of pitch and volume voltages.

I absolutely hate digital UI's - But I think this may be because they are absolutely unsuitable for real-time adjustment while playing.. As one cannot do these adjustments real-time with the theremin, a digital UI is probably fine.

- Or perhaps I just ship theremins with a selection of presets in flash, and provide a MIDI input with which the presets could be edited using some free on-line patch editor / librarian .. If I got the original patches "right" and / or had some "pro" thereminists check them first, many people would probably need nothing more - Particularly if one could select any two presets and morph between them with a single knob to get the sound they want.

"Your cross modulation scheme is beautiful!"

Thanks ;-) .. It was one of those "eurika" moments I live for! ;-)

Fred.

"Or perhaps I just ship theremins with a selection of presets in flash, and provide a MIDI input with which the presets could be edited using some free on-line patch editor / librarian .. If I got the original patches "right" and / or had some "pro" thereminists check them first, many people would probably need nothing more - Particularly if one could select any two presets and morph between them with a single knob to get the sound they want."  - FredM

It seems to me that 5 pin DIN MIDI is edging towards the door unfortunately.  And personally I have a thing about instruments that don't allow full editing (however cumbersome) via the front panel.  But like you say, a rotary encoder, one or two line LCD, a bunch of menus, and several preset buttons would probably work fine for a keep-your-hands-away instrument (this is the way I intend to go).  This RGB backlit LCD looks like fun:

http://www.adafruit.com/products/399

Change the color depending on pitch or something (with a menu option to make it a fixed color).

"Change the color depending on pitch or something (with a menu option to make it a fixed color)" - Dewster

That is nice! - To me, the colors could correspond to menu levels - Top level (preset selection and tuning for example) being say green, sub level perhaps blue, and red for editing presets..

"personally I have a thing about instruments that don't allow full editing (however cumbersome) via the front panel."

So do (or did) I ! - But looking at how rarely I went into my Roland D110's or DX modules UI editing, I wonder if the UI was actually worth the cost - when one has complex parameters to adjust, and lots of instruments with their own quirks, I feel that those who want to "get inside" are (like me) likely to want an easier interface than is possible on a small digital UI..

I bought a Behringer BCR2000 controller with 32 illuminated rotary encoders and 20 buttons + foot switch inputs etc and preset storage for about £100 .. Link this to a fairly basic bit of software (but this isnt required) and I can fully edit all my (MIDI Equipped) synths, store the patches etc.. One only needs to get into the murky depths of the synth once, develop a common "structure" for them all, and never need to wast creative time messing with incomprehensible multi-level menus on a tiny LCD.

I think there are 3 kinds of users of electronic musical kit..

(1) Those who just want to "plug and play" and select some preset.

(2) those who think they want to "get into" the kit and think they need a UI to enable this - but who actually dont "really" nead a UI (or at least, even if they "need to" are unable to because the interface is too complex - given a panel of knobs they may well have what they really need), and actually find the UI overwhelming and not worth the effort.

(3) Those who really want / need to get into the kit - and most of these that I have met (a few exceptions who are wizards even on the most awful UI - I am not in this group, I find TV remotes and mobile phones a real challenge! ;-) go the route I have, and buy seperate software and/or controller.

From a designers / marketing perspective, the question is how many potential purchasers of theremins fall into which group.. If most are in group 1, then reducing the cost by leaving the UI out (and it represents a fairly substantial percentage of the cost) could be the best route..

Group 2 are the problem - If they represent a substantial number of the (tiny number of) potential purchasers, it may be worth fitting a digital UI and passing on this cost.

The cost of adding MIDI editing is trivial, so Group 3 will be catered for. (possibly because I am in this "group" I am biased - but even from a development perspective, the ability to take my BCR2000 to some Pro Thereminists for consultations, and to create presets they want, makes the whole job a lot easier)

Perhaps the best way is a preset but  MIDI editable unit to start with - A digital UI which is effectively a menu based MIDI editor could then be produced as a drop-in module at a later date if there is demand for it - there would be no conflict as all hardware would be done for MIDI editing anyway, all that would need to be developed would be a simple editor streaming MIDI data...

I love the BCR2000 - its the sort of product one could never make at the same price unless one made thousands.. couldnt even buy the components for the price (if you could buy them) - you would pay at least £5 for each encoder with its LEDS in quantities of 1k, probably more like £10 each.. One of these units with my theremin would achieve what I want... The only thing I would lack would be an extra pair of arms or a telepathically remote controlled torso with arms.. ;-)

The only real problem with having MIDI sockets is that people may think they can use this for recording / playback of music / performances - So perhaps USB and some other connector which can go to a MIDI "adaptor" would be best.

Fred.

"To me, the colors could correspond to menu levels - Top level (preset selection and tuning for example) being say green, sub level perhaps blue, and red for editing presets.."  - FredM

Ooh, that's good!  Can I steal that?

"I think there are 3 kinds of users of electronic musical kit.."

I'm a #1 most of the time, but I like tweaking the presets if the UI isn't too horribly cryptic (and if the underlying algorithm isn't crazy like FM synthesis) so I really want / need all the parameters available to me via the front panel.  This helps future proof it as well, since it is a 100% stand-alone product.

If one has field upgradable firmware the UI could be extended later on, but that might create unreasonable expectations from users I suppose.  Though software is usually so complex it's dangerous not to have some kind of field fix option.

"Ooh, that's good!  Can I steal that?" - Dewster

LOL ;-) .. Please do, but dont blame me when color blind musicians have a go at you! ;-)

It could be even more funky though.. at the top level, it could behave like a colour organ, and when muted, it could flash between red and green.. You might even be able to encode a hypnotic sequence which is enabled if a certain button isnt held when the unit is powered up - this could hypnotise anyone who has stolen the theremin into giving it back..... ;-)

I can see this multi-color LCD being a big hit..

;-)

Fred.

New version up:
http://www.mediafire.com/?vzb0w79tq5xe651 -- dewster

Examining the "Inductor Design" table of your spreadsheet I have found some trouble.

With the initial values below I obtained the absurd results (negative "AC resistance" and negative "Q").
Your thoughts?