Need some help with pitch antenna inductors.

Hi everyone. 

I'm sorry my replies are so slow. I'm having a midterm exam session right now, so times are hard. 

So, in case I decide to get rid of the antenna inductors and tune the parallel oscillators to higher frequency, how should I choose the L/C ratio? Is there any significance in this ratio?   

Christopher, thanks for the app. Why do you think that I work on Vancouver Island? I don't. 

So, in case I decide to get rid of the antenna inductors and tune the parallel oscillators to higher frequency, how should I choose the L/C ratio? Is there any significance in this ratio?   

Tune to a higher frequency?  You probably need to stick to the specifications of the original designer though do experiment. In the future using a different theremin design you might try a higher frequency.

Student in training.

Another long time Thereminist is located on the island and I thought that might be you.

Are you getting theremin sound yet, record it as it will tell a lot. At first probably dry and squeaky.

Christopher

"I'm sorry my replies are so slow. I'm having a midterm exam session right now, so times are hard." - Wintermute

You have my sympathy.  I still get nightmares from my college years - the hardest but most rewarding thing I ever did.  Hang in there!

"So, in case I decide to get rid of the antenna inductors and tune the parallel oscillators to higher frequency, how should I choose the L/C ratio? Is there any significance in this ratio?"

For the most sensitivity you go for tiny C because it is in parallel with the antenna and hand capacitance.

Hailing from  the UK I am just starting my build of a Theremin based on the circuits contained in the article "Understanding, Customizing, and Hot-Rodding Your Etherwave Theremin" freely available from the Moog website. Like many before me the inductors are hard to source. For the fixed inductors I have chosen to use SMD devices as these seem readily available. I purchased the following from High Noon Investments Ltd., 9, Stirling Road, St. Leonards On Sea, East Sussex, TN38 9NW via Ebay.

Inductor 10mH 10% 0.18A SMD Part# Bourns SRR 1208-103KL 3off
Inductor 2.2mH 10% 0.4A  SMD Part# Bourns SRR 1208-222KL 2off
Inductor 4.7mH 10% 0.28A SMD Part# Bourns SRR 1208-472LK (Fnl) 1 off
Inductor 1.5mH 20% 0.12A Smd Part# Bourns SRR 6603-152ML (Fnl) 1 off

This gives me the 10mH + 10mH + 10mH for the Pitch Antenna and 4.7mH + 2.2mH +2.2mH + 1.5mH for the Volume Antenna giving nominal values of 30mH and 10.6mH total respectively. I added the 1.5mH to bring me closer to 10mH total on nominal value but given the tolerances might not be needed.

Don't be afraid of the SMD moniker. Six of these items measure roughly 13mm x 13mm x 7mm and the smallest is roughly 3.5mm in diameter. I have mounted these "dead bug style" along each side of my prototype boards with hot glue.

For the variable inductors I ended up making a purchase from USA from seller Semisurplus on ebay the details of these are:

CADDELL BURNS 6740-33 47UH Subminiature Shielded Variable Inductors from CADDELL BURNS 6740-33 47uH Subminiature Shielded Variable Inductors
full specifications can be found at http://www.caddell-burns.com/PDF/A1042.pdf minimum Q is given as 55. I had the last three and it looks as though Caddell Burns have ceased production. However if you can get hold of them they seem on paper at least to be a good substitute for the Toko.

The fixed inductors came in at £9.03 whilst the Caddell's came in at just over £28 with shipping.

Obviously until I've got the circuits fully built and tested I have no idea how these components will fair, but i will keep you posted how the build goes.

Looking through other posts I think I may have been premature in ordering my inductors. It seems that the fixed inductors above are critical and should be 3 pi wound .... oops. If anyone here has succesfully wound there own coils could they please post their construction details. Also having doubts about the Cadells now.

It seems there is a lot of voodoo in getting these circuits to work correctly and as i don't wear a witch doctors mantle i have decided not to pursue this project. I looked at purchasing the etherwave board but apparently these are no longer available and have been discontinued.

"It seems there is a lot of voodoo in getting these circuits to work correctly..."  - dodgyg1

Yes, kind of.  For series EQ inductors you're mainly looking for a self-resonance significantly (>2x?) above the oscillator operating frequency and decent (>50?) Q.  After that you're looking for low temperature dependence, then maybe size, cost, etc.  If you can get your hands on them, you might try inductors made by the German manufacturer NEOSID.  Their part number 00-6122-32 (link) is at something of a sweet spot with 4.7mH and 700kHz self-resonance.  No clue what the drift is, and I don't have any personal experience with them either.

[EDIT] Ha! ILYA beat me to it (I'm in TW twit filter purgatory lately - it probably knows something I don't ;-).

[EDIT2] "Seems your 10 mH inductors have the individual SRF  around 300 kHz, so being connected in serial they will produce mach less total SRF value."  - ILYA

Is that true?  I think of SRF as an actual capacitance across the ends of the individual inductor that you can't remove, so stringing inductors in series wouldn't necessarily increase the SRF of the "lumps"?  I mean, I think the most you can hope for is to hold onto the SRF, not improve it, via series inductances.  Or maybe it's some complex middle ground? If only there were a way to simulate circuits...  ;-)

dogdig1,
your enemy number 1 is the SRF (self resonance freq) of the inductor. For correct operation it must be at least highter than the operation frequency of oscillator (~300 kHz). Two times and more is welcome.

Seems your 10 mH inductors have the individual SRF  around 300 kHz, so being connected in serial they will produce mach less total SRF value. Oops.

... If anyone here has succesfully wound there own coils could they please post their construction details.

look in my design (the coil data is in the table):
link (google auto translation)

"... not improve it, via series inductances..."

dewster,
i meant the resulting SRF of stringing inductors will be always worse than a SRF of its particular member.

Generally speaking, the so-called "self capacitance  due interturn capacitance" is a myth multiplied in technical literature.
There is only a SRF as a primary phenomenon (caused by inductor wire to be a long transmission line).