Pitch Antenna not Functioning

Hello,

I've read some other posts regarding this topic regarding pitch antennas, and tried some of the suggestions, such as placing inductors inside a plastic bag, moving the antenna away from most objects, etc. Nothing worked.

I was wondering if there is anything wrong with these inductors we are using: http://www.farnell.com/datasheets/24558.pdf (part no. B82722A2102N001).

Also, when I touch the antenna, the frequency shifts very slightly, but other than that it does nothing.  Some other users had this happen but a  lot of the suggestions we tried also didn't work. We are using a 3/8'' copper tube, 50 cm in length. 

The project we based the oscillators on is: http://www.cs.nmsu.edu/~rth/EMTheremin.pdf 

Right now we just want to see the pitch antenna respond, so we have two identical working oscillators connected to the detector part of the schematic, and we want to see the antenna responding to hand movements. Below is what we have so far.

Please let me know if there's anything we can try, or what sort of tests we can make. The main thing I have in mind is taht we do not have a variable inductor (L5), but we have tried many different values and the end-result is the same, so we're very skeptical that this is the problem.

Thanks!!

triplebig, you really don't want to be substituting parts, particularly with the EW circuit, until you have it up and running with the parts the schematic calls out for.  The inductors you gave the link to seem to have low self-resonance (i.e. high parallel capacitance) so if you are using them in a series resonant mode (i.e. for L1, L2, L3, L4, L7, L8) it probably won't work.

The EW is a strange mix of parallel excitation of series resonance that is difficult to tune even under the best of circumstances.  Both L5 and L6 must be variable as they are interacting with a third highly resonant structure (L1 thru L4 and the antenna/hand capacitance).

But I encourage you to keep at it as Theremins are a great way to learn about inductance, capacitance, oscillators, mixing, etc. (things they don't seem to teach very well in school).

Dewster, thanks for the reply. 

Could you kindly explain what you mean by low self-resonance, and why that wouldn't work? Is there a better way that we can use these inductors?? 

Since we live in South America, the amount of inductors we find as high as 10mH are very limited. We tried to wound our own inductors but that would take forever.

Also, we cannot find variable inductors for L5 and L6, which brings us to another problem. Is there any brute-force way we can improvise these? 

Coils are the least ideal of all passive electrical components.  Significant inductance requires gobs of windings on an air core.  If you look at how low self resonant coils are built they do things like use a ferrite core to concentrate and magnify the magnetic field, put a cardboard spacer over the ferrite, place multiple "donut" windings on the rod, utilize scramble winding, etc.  Ferrite brings with it temperature dependence and other issues, the severity of which is based on the formulation of the ferrite compound.  This is all an effort to miniaturize, but the physically smaller it is the more difficult it is to minimize the inherent capacitance between the terminals.  This capacitance is in parallel with the coil and forms a resonant circuit, so the inductor is no good to you as a pure inductance near and above this inherent LC resonance.  In series resonant mode you are up against the antenna capacitance which is generally less than 10pF.  You can calculate the parasitic capacitance of a coil by looking for the resonance frequency peak on the datasheet graph and calculating backwards from the LC resonance formula: F = 1 / [2 * pi * sqrt(L * C)]

If you really can't get components, maybe try butchering an AM radio for IF transformers.  Not sure what you can do for the series inductors though, they are specialty items that I'm a little amazed are still being manufactured as they belong to a bygone era.

The most practical recommendation I can give you is to abandon the EW circuit altogether.  Go for a parallel resonant design like the Theremino or Thierry's circuit where the parasitic capacitance isn't an issue.  Also, higher frequencies greatly relieve inductance requirements.

[EDIT] From the datasheet the Epcos B82722A2102N001 appears to have a self resonance at 500kHz.  Plugging this and 10mH into 1/[L*(2*pi*f)^2] = 10pF parasitic capacitance.  You are either over the line here or dangerously near it because this is on the order of the antenna free space capacitance.  For these EQ series inductors you want to use parts that have a self resonance frequency significantly above the oscillator operating point (maybe 3x or better, though I haven't studied this in any depth whatsoever).

[EDIT2] For comparison, the Bourns 6306-RC (10mH RF choke used in the EW) has an SRF of 710kHz minimum, which works out to a parasitic capacitance of 5pF maximum.  So the Epcos coil might possibly work, but it seems fairly borderline.

"From the datasheet the Epcos B82722A2102N001 appears to have a self resonance at 500kHz.  Plugging this and 10mH into 1/[L*(2*pi*f)^2] = 10pF parasitic capacitance.  " - Dewster

A "if all else fails" trick that can sometimes salvage a situation if you dont have the right inductors to hand, is to use more inductors in series to achieve the required inductance..

The larger the inductance, the higher the || capacitance is likely to be.. Higher SRF inductors (like the Bourns 6300 series) actually consist of three seperate windings spaced apart, sharing a common core - this spacing helps to reduce total parallel capacitance..

So - if you had smaller inductors with higher SRF (lower capacitance) and put more of these in series (for example, say eight 5mH in series to get 10mH rather than four 10mH is series) you will improve the resulting collective inductor.

But really - these antenna inductors are critical.. You can get away with quite a lot on the the tank side, but not much on the antenna side.

Regarding IF transformers - almost any 455kHz IF transformer can be adapted to make the tank inductor.. Some (most) of these IFT's have a capacitor fitted inside the can, a this needs to be removed - there is a tapped winding on the primary, and between this tap and one of the other ends you will usually find a winding with the right order of inductance (~100uH). You dont absolutely need the inductance in series with the variable inductor, particularly if your variable inductor has an adjustable span covering the required tuning.. And you can always fiddle with the tank capacitor value a bit - its not critical.

Search for 42IF106 here for details on IFT's

Fred.

Hello,

I've read some other posts regarding this topic regarding pitch antennas, and tried some of the suggestions, such as placing inductors inside a plastic bag, moving the antenna away from most objects, etc. Nothing worked.

I was wondering if there is anything wrong with these inductors we are using: http://www.farnell.com/datasheets/24558.pdf (part no. B82722A2102N001).

Also, when I touch the antenna, the frequency shifts very slightly, but other than that it does nothing.  Some other users had this happen but a  lot of the suggestions we tried also didn't work. We are using a 3/8'' copper tube, 50 cm in length. 

The project we based the oscillators on is: http://www.cs.nmsu.edu/~rth/EMTheremin.pdf 

Right now we just want to see the pitch antenna respond, so we have two identical working oscillators connected to the detector part of the schematic, and we want to see the antenna responding to hand movements. Below is what we have so far.

Please let me know if there's anything we can try, or what sort of tests we can make. The main thing I have in mind is taht we do not have a variable inductor (L5), but we have tried many different values and the end-result is the same, so we're very skeptical that this is the problem.

https://www.7pcb.com/

Try more inductors (with 40 mH overall inductance) in series for reduce self-capacitance. 

F_res_linearising_coils shall be a little lower, then F_res_tank. 

Insert self-capacitance of  linearising coils, it inductance, also tank parameters into the xls file from http://www.thereminworld.com/Forums/T/28492/lc-tank-linearizing-coil-excel-simulation and check overall state of your theremin...

And see 2nd post in this tread http://www.thereminworld.com/Forums/T/28530/antenna-tuning?Page=0