Need some help with pitch antenna inductors.

Hi everyone!

About the inductors, that are connected in series with the pitch antenna. Would toroidal ones be OK instead of straight coils? Is there a way to theoretically calculate the optimal value for these? Thanks!

Since the series inductor usually resonates only with the fairly small antenna & hand capacitance, you need to pick ones that have a high enough self resonant frequency (maybe double the operating point or better) or identically, low self capacitance.  Ones that have low temperature dependence are the best, but the ferrite used in common chokes and the like aren't usually specified for this (and they are generally quite poor in this department).  Winding your own air-core single layer solenoid can give you a high Q, low self capacitance, low temperature dependence inductor, which is why they turn up in the better Theremins.  But they are big and bulky which makes accommodating them inside the cabinetry somewhat tricky.  I have a spreadsheet that can help you with winding if you decide to go that route.

The value depends on what type of oscillator is stimulating it.  If you're hanging it off of a parallel tank you need to talk to Thierry because that's his department.

Wow, thanks! 

So air core is better for theremin than ferrite core?

What do you mean by hanging it off of a parallel tank?

Hello Wintermute

Some theremin designs do not need inline antenna chokes or coils for a perfectly linear pitch field, experiment and see if you experience a difference.

dewster knows his stuff though, guess we will wait until he gets back from Princeton. (-'

Christopher

"So air core is better for theremin than ferrite core?"  - Wintermute

That's my general EE opinion, but of course it depends on how it is used.  If the tank stimulating it is of a similar ferrite, then it might not matter so much, or it might actually be best to match all the ferrite temperature dependencies so that the variable and local oscillators drift together.  I haven't taken this route so I'm not the best person to ask about it.  But I have temperature tested some Bourns 6300 series, as well as one of my own air cores wound on PVC schedule 20 pipe, and the air core has almost zero temperature dependence in comparison to the Bourns.

"What do you mean by hanging it off of a parallel tank?"

Many (most?) Theremins have an LC tank, with a relatively small (in comparison to the series coil) inductor in parallel with a fairly large (hundreds of pF) capacitance.  This tank interacts in fairly complex ways with the series inductor feeding the antenna, stimulating the series inductor / antenna capacitance resonance and detecting this resonance.  Because of the double tuned nature this also is something I'm avoiding in my designs.  It seems to offer some linearization effect near the antenna, but IMO it's not worth all the trouble when it comes to getting the tuning right, which can be a bear.  Opinions will differ here.

I should also mention that ferrite with an air gap will generally be much better behaved regarding temperature than ferrite without an air gap because the magnetic properties of the gap tend to dominate.  Things like chokes wound on a rod of ferrite are inherently air-gapped.  Chokes and transformers where they are trying their hardest to miniaturize the device are often ungapped, and the worst offenders.  You also want to look at current capacity, inductors tend to go non-linear way before you reach this point, with obvious heating effects.  But large current capacity with a large L value often means non-gapped and highly temperature dependent, particularly when found in a small package.

I've looked and looked (and looked) and haven't found any commercial series inductor offerings to my liking.  I want to just throw money at it and have someone put a solution in my hand, but I haven't been successful in that regard.  It's likely possible as there are some fairly flat ferrite formulations out there, and gapping them would improve them further, but the market - outside of Theremin series coils - is likely non-existent.

You should also know that I'm speaking in a rather ultimate, pie-in-the-sky, kind of way.  Bourns 6300 series is what you might want to try to see if they meet your needs.  I will also say that Theremin was no fool, and his large air core series inductors were quite ingenious.

Thanks a lot!  That's a lot of information, definitely something to think about. 
I'm very roughly following schematics from this page: http://www.strangeapparatus.com/Theremin.html 
But the guy there says that those inductors need some tweaking in each individual case. 
So, as long as I use the ferrite core ones, do you think it matters much if they are toroidal or cylindrical? 
And one more thing: how does the gauge of the wire affect inductors performance? Because those toroidal inductors usually have small amount of turns of very thick wire, while cylindrical ones usually have many many turns of quite thin wire. 

The "strangeapparatus" Theremin circuit is nothing to work with! Several renowned engineers here agree that this circuit seems to be tinkered together by copy and paste from different other circuits without the author exactly understanding what he did. The oscillator's configuration (free-run frequency and L/C ratio) are totally mismatched with the so-called linearization coils.

A "linearization" of 40mH together with a static antenna capacitance of about 10pF gives a resonant frequency of about 251kHz. That means that the oscillator should run around that frequency. Now we have still to set the pitch range: Let's choose it for a maximum pitch of for example 3 kHz. That means that the resonant frequency of the linearization coil with the oscillator's parallel tank capacitor has to be ideally around 9kHz, so that the parallel capacitor has to be 7.8nF. Now the free-run frequency of the oscillator (without the linearization coils and the antenna connected) should be 251 - 4.5 = 246.5kHz, so that the parallel inductance has to be 53.3uH. At the moment when you connect the antenna circuit, the frequency will jump up by these 9kHz = 255.5kHz and go down to 252.5kHz as you approach your hand to the antenna. Thus the fixed pitch oscillator has to be set to 255.5kHz and you'll have a theremin with optimal (for this kind of linearization circuit) linearity.

I think that one should first fully have understood the interaction between the oscillator's parallel resonant circuit and the (linearized) antenna series resonant circuit. I've written down a summary of the theory here. Just scroll down on that page.

Thierry over the years we all have known that the Strange Theremin is a disappointing "flytrap", why not remove it from the TW database so more budding theremin enthusiast don't have their careers devastated right out of the gate? Have you ever heard what it sounded like? OMG

Christopher

That's basically a good idea, Christopher, but I couldn't find it in our "Schematics" section. Am I getting old and blind or do people find this circuit elsewhere?

I am bad, sorry Thierry I was thinking Glasgow not Strange Apperatus

How about removing the Glasgow if you recommend the Strange. (-'

You could even remove the Radio Shack theremin as it has advance way beyond that and is one of the few Theremins besides yours that can still give an authentic theremin sound, not a cheap imitation sound.

I have concluded most seek out the cheap simple imitation because that is all they know.

As that mean and crazy man said: "We are here to protect the theremin!"

Christopher