Let's Design and Build a (mostly) Digital Theremin!

"Looking at 330 uH the 20 to 60 degree change is about 0.65%. Then 0.065/60=108ppm/C"  - livio

I think you mean: 0.0065/(60-20)=162.5ppm/C

Yes it was my error... and yes 0.0065/40 = 163 ppm!

And yes, the self resonance and the Q are very low compared with an air coil but I do not think this will cause problems, at least in our circuit. Could it be that there are differences in your circuit, I do not know exaclty your last configuration.

Values of the first temperature cycle are strange. Maybe I have exaggerated a little (more than 100 degree) with the hair drier and cooked the inductor...

Cycle       uH at 20 degree       uH at 50 degree     difference    percentual     ppm/degree

1                  326.0                          330.5                      4.5             1.4%            460

2                  355.2                          357.1                      1.9             0.53%          178              

3                  357.0                          358.5                      1.5             0.42%          140                    

4                  358.1                          358.6                      0.5             0.14%            47                   

5                  358.3                          358.9                      0.6             0.17%            56

It seems that after some cycle the inductor is affected by a conditioning process and improves its performance. This seems to me quite logical knowing how it is constructed. (or maybe I have cooked it too much in the first cycle) There is also an evident increase of the impedance after the first cycle... also my LC meter is not so precise... so maybe it is better to refer to the data sheet than of my data. Substantially the data sheet data are confirmed.

What I can say for sure is that these inductors in a circuit colpitts like ours behave very well, I would recommend you to try if they are good enough for your FPGA.

"Since I've never owned more than one Theremin, I have no experience with Theremins interfering with each other.  What are the symptoms?" - Dewster

Ok, first - my theremins were circuit boards not full theremins, and were just the pitch sections.. So it may well be that behaviour varies if volume oscillator interaction is also occurring...

If you think "ring modulator" then this is the main kind of effect I found with my boards - what I saw on the scope (which is what I mainly used - listening to the kind of noises prototype theremins can produce can only be done for short times without driving one mad - particularly when they are interacting! - I probably listened for too long... ;-) was amplitude modulation of the audio signals - this is what I was mainly looking for, as it gave quick identification of interaction.

On my bench I never got any noticable locking - but I certainly got this in the Royal Festival Hall before I cut the common ground leads.. Which is strange, and I cannot explain it - those theremins were farther apart than the ones on my bench, but when interfering one could play one, and another would leave its auto-muted state (which occured when the variable oscillator frequency was equal or greater than its reference oscillator frequency) and start playing at some non-musical interval related to the other - and the other's pitch altered and was impossible to control accurately, and both produced horrible 'ghosting' tones reminiscent of early Radiophonic Workshop ring modulation experiments.

Most often though the "slaved" theremin would 'blip' eratically out of its muted state, and the played theremin would be 'jerky' and have unpleasant audio effects....

But as I had a live "show" and about half an hour before people (including the director who was to sign off payment for these instruments) started wandering in, I never spent a lot of time on studying the phenomena! - All I was trying to do (and did, 5 minutes before the doors opened) was to reduce or eliminate the problem - My first action was to detune the instruments as far from each other as I could, but with 16, eliminating interference between some caused interference with others - so I did it based on distance, detuning the closest so that interacting theremins were the furthest from each other..... I got to a nearly acceptable setup this way, but then a voice in my head said "cut the ground connections" - I had run a solid seperate ground lead  between the instruments deliberately to improve their operation, so went and cut about 30cm off the wire between each theremin, leaving the remaining cable 'floating' as independant "ground antennas"... And all the theremins stopped interacting and behaved beautifully (until an electrician discovered the cut floating leads and tidied them up by removing them one night after I had gone home)

(This electrician was a really sarky git - he had ignored all my wiring specifications, and I had to almost fight him to get wire for the ground lead - then when I was flapping about because the theremins werent working, he came up to me with a big smile and said "I see you didnt need that ground wire, so I tidied it up and have reclaimed the wire" - Thats when I walked down to Maplin, bought some wire, bought some alu foil, and put grounded sheet "antennas" into the plinths - the improvement to performance was fantastic after that..)

One other cautionary aspect of this .. I had tested the instruments for interaction long before the show - it had worried the hell out of me - but was able to run them (4 at a time) in my kitchen at closer proximity than they were at the show, and had not been able to get them to interact! ... !?

That was 2010 - I then had heart failure - Why did I ignore the signals ??? It was so clear - I should have walked away from theremins then and never looked at the bloody things again!!!

Fred.

 ... Hell - Perhaps livio and Christopher are right ... Perhaps EM does play a bigger role - or perhaps PP is right and theres littl mystical people inside the theremin and if you piss them off accidentally they cause trouble!  ;-)

"...I do not know exaclty your last configuration." - livio

I'm kind of losing track myself! ;-)  The counter wound split air core idea (to reduce far field RF) is making me think my original series tank needs some review as it was super easy to deal with (had a nice sine wave output with square wave input).

"What I can say for sure is that these inductors in a circuit colpitts like ours behave very well, I would recommend you to try if they are good enough for your FPGA."

Thanks, I'll keep them in mind.  One of the few commercial inductors I've seen with actual temperature data.

"If you think "ring modulator" then this is the main kind of effect I found with my boards..."

"On my bench I never got any noticable locking - but I certainly got this in the Royal Festival Hall before I cut the common ground leads.."- FredM

Thanks for that info Fred!  What were the audio connections like - routed to a common mixer board?  Or common grounded amps?  Seems like there could be RF on the common connections, or as you say, who knows, maybe RF in the air (tonight, oh lord </phil collins>).

"What were the audio connections like - routed to a common mixer board?  Or common grounded amps?  " - Dewster

Each instrument was entirely isolated galvanically - they were built into a pair of active loudspeakers stacked on top of each other, the lower speaker had the power supply and amplifier, and power + audio leads were run to the upper speaker where the theremin circuit was fitted, with its antenna (my screw adjustable build) sticking out the top. The speakers were bolted together with a plastic plumbing connector and cables passed through this, speakers were oriented so they faced oposite directions and phased so that their ports were additive (the bass was wonderful!)

Mains came into the lower box (L+N only) directly to a transformer primary - and I tapped the DC from the amplifiers PSU, this DC was then fed to a linear regulator via a RF blocking inductor.

The only "real" ground "connection" was capacitive via the transformer windings (Coupling to N and L capacitively) . The 0V/internal "ground" was taken directly from the PSU and independently from the theremin board to a star point terminal under the theremin - it was this point that was originally taken to the common ground wire, but got connected to the floating "ground antennas" and later to the alu foil.

There were no audio or other connections to anything, loads of sockets had been fitted for connection to mixers etc, but these were never used (I am pleased that no-one wanted to use them, even though they had been a big additional expense to fit filters to so that no RF escaped via them) - the only socket used was the headphone output (driving headphones).

Fred.

livio -

I too owe you an apology.

The series inductor connected to an antenna does look like a "loaded" antenna you were speaking about.

I do not have the required radio knowledge to know how significant this is in terms of EM radiation, or whether it changes any of the "arguments", and my expierience with theremins still convinces me that, from an operational perspective, capacitance explains everything..

But in terms of radiated EM, I could have been wrong, and certainly was REALLY WRONG to have spoken to you the way I did.

I hope you can forgive my rudeness.

Fred.

Wow Fred, what you describe seems to be full-on paranoia isolation, wiring wise anyway!  I don't think I'd want to be in the position of providing and setting up a bunch of Theremins for simultaneous play.  Unless you're talking an audience full of those toy battery powered doll things!

http://www.youtube.com/watch?v=BzEHItrTMHM

(Talk about cramped pitch fields...)

Completely OT

"Wow Fred, what you describe seems to be full-on paranoia isolation, wiring wise anyway! "

LOL ;-)

I was paranoid about the event! ... I saw it as my one big chance to get known in the theremin "circle" - The South Bank Art Centre is I think the largest establisment of its kind in the world, having some of the most prestigious venues in London..

What follows is a pathetic sob story ;-)

I wanted these instruments to be as good as possible in the belief that by presenting them people would like what they heard .. But I had only a couple of months, and built 2 prototypes prior to the final - the first used active PC speakers, and I took this to one of Lydias workshops where I was to meet the client - he never showed up, which was just as well because it proved to be rubbish. The next was one which worked well, but when I took it to the Festival Hall it didnt work at all (it used a EPE-2008 Front-end which got into its latch-up mode, the first time I had encountered this..) - I had been trying to keep within the tiny budget and make some money from the show, but decided instead to borrow enough to just build impressive instruments and lose on the deal, but gain the exposure - I thought (after my first two failures) "What would I need to pay to have my theremins on display in the RFH or purchase equivalent advertising?" And decided that the real value of the show must be up in the 10's of £k, so I just went for it - spent 3 times more on each than I was getting paid (without taking my time or other expenses into account) and built absurdly OTT instruments..

And I did get some interest - every thereminist who played them liked their feel and sound (I was not allowed to have user adjustable controls, so had each theremin set up with slightly different waveshaping - I could get to the controls under a hidden panel) , and I got some commissions from the event which could have escalated and perhaps given a return that covered my expenses - except that I had pushed myself too hard, and had actually been suffering from heart disease while doing these damn theremins, and a few months later I nearly died.. was unable to do the commissions, and was confined to running simulations and posting on TW for several years.

I am at the point of thinking that if there is anything mystical about the theremin, its that theyr jinxed! - Oh, those involved with them may have exciting wild lives (perhaps ;-) but risk ending up like Lev - Impoverished, alone, passing away in some grotty rented dump.

I am not the first fool to fall victim to the theremin curse, and I probably wont be the last. One aspect really pushes me to irrationally thinking theres a curse - I took loads of audio and video recordings of these instruments - and others also did - Even had some pro recordings taken by some on-line music sites... When I got home, all my videos were silent - the audio was lost.. Other recordings by other participants were lost, and I have not been able to locate a single audio recording of the final instruments. (they were only really finalised at the show - I would take them apart in the evenings and do modifications on them one at a time - a few days before the show ended they were really good.. I wanted to buy some back - but was absolutely broke)

Fred. 

Blasphemously OT and OTT !

LOL ;-)

Dewster, those dolls sum it all up for me..

At least the Japanese arent ashamed to be playing with toys - they dont need to pretend that their toys are anything other than toys .. its a social get together with their musical barbies... ;-)

And in truth, if I had made 16 musical barbies with limited pitch fields and stuck them on top of narrow plinths, most (99%) of the punters who visited the show would have been just as entertained

In fact, if I had made bollards with capacitive or optical switch sensors ehich triggered some inane sample when people got close, probably 90% would have been equally or more entertained..

Perhaps 1% of those "interested" in theremins are really interested in theremins.. and perhaps 10% of these would notice any difference in sound or playability.

How many people are interested enough to pay real money for a good theremin? Well - Ruslan has had a highly praised theremin up for grabs for a few hundred $/eu and there hasnt been any takers yet..

Lets see - I had 10 days at the RFH with my 16 theremins, of this I only had real thereminists briefly playing them on the first day because they were there for the main event - after this only two thereminists visited the show, spent any real time on my theremins, and I had one confirmed commission from this..

Out of the other thousands of "players" many expressed the wish to buy them - but when I even quoted a price equal to the minimum cost to me, they balked.. I gave out about 1000 business cards, and from these got about 20 people contacting me - but these people didnt want to buy my theremins, they wanted free advice on everything and anything electronic " our son played with one of your theremins and loved it, and we want to get him a keyboard for his birthday - which model under £150 do you reccomend?" was the typical "quality" of the contacts...

Oh, and I had quite a few people contacting me with comments like "Your business cards are crap - I design business cards, and will do you a design for free.. Also.. Would you be interested in us designing and hosting your web site as we will do your business cards for free..."

And TW is a strange thing isnt it.. Is this REALLY a theremin interest site, or is it perhaps more a place that technical and musical misfits can hang out together.. (?) ... In my mind I see this pub with a vacuum tube sign "The Theremin" - And it attracts a particular kind of person.. LOL ;-)

"Tales from the White Hart" - Yeah, thats what TW is! Its the White Hart Pub! ;-)