Continue: Building EM Theremin / rebuilding EtherWave circuit

Hi Scotty,

Glad you are getting somewhere.. Although I really dont understand whats going on with your Theremin! ;-)

[i]"I changed the value again to 2.5mH this time. The pressure could be easily hold and varied over a wider range.
So it seems even using mostly the values from the original schematic doesn't prevent from nasty surprise :)"[/i]

Which EQ coil is this? The total antenna-side inductance (L8,L9,L10) should be about 12.5mH, and the circuit side (L7) inductor about 2.5mH .. By "pressure" what do you mean - and to which inductor/s is this applied? - If Finger pressure, particularly on the antenna side inductors, I strongly suspect it is capacitance you are adding - and that the cause of your problem is that the volume oscillator is running at the wrong frequency.


[i]"the volume sensor was made of a bended brass rod. So I changed it to a simple telescopic/retractable antenna from a radio - and now it works! Seems that brass material isn't a good choice for the sensors (I used it because it could be better bended than aluminium).
I cross-checked the new sensor with the old coil value, but it didn't work, so there were two independant problems."[/i]

There is only capacitance and inductance which determines the resonance of L7-L10 combined with the volume antenna.. Brass, aluminium or whatever reasonably conductive material will work - no difference would be seen between an antenna OF THE SAME DIMENSIONS and having THE SAME LENGTH / POSITION of connecting wire, and being placed in the SAME POSITION regardless of whether this antenna was made from brass, copper, aluminium, steel.. So I strongly suspect some other reason causing your problems.. These can only be one of the above, or failure of the galvanic connection.

[i]"Now, I can go for the cabinets - hoping the planned sensor design will work ;)
And maybe I could find a way to get the circuit more resistive against temperature drifts."[/i]

My next document update should help.. Check my last post - I will edit this each time I update, to let you know.

At this stage, forget "temperature drifts." - any drift you are seeing is almost certainly not thermal.. I think you probably have E-fields leaking all over the place, and until everything is correctly tuned and verified as stable, thermal is the last of your worries... My advice now is this:
1.) Disconnect all antennas AND the EQ coils.
2.) Check the 'natural' frequencies of all oscillators.
3.) Ensure that the signals from these oscillators are clean and stable - you will need a good 'scope - and ideally a FET buffer close to the points you are testing - scope loading is significant!

You are making progress - but dont break out the champaign just yet! From bitter expierience, I think your journey probably has some way to go yet ... You did the layout for your PCB.. Check everything - Is the decoupling placed close to active current sinks? Is grounding SOLID and ADEQUATE.. Remember - having thick ground track is what you want - But if this track reduces ANYWHERE then this forms a significant inductance which can completely screw up everything - If in doubt, tack some extra ground connection wires on the board - take a thick piece of wire to every significant ground area, and link these back to a common good ground point close to the supply (regulator) components.. If things improve, you can then remove each link in turn, to find out which is/are really needed.

[i]"
I'd like to thank you very much for your help and your patience with me (and my poor english :) ).
I'd also be glad to support you for your work on E14, if possible.

And also I'd like to thank Thierry for his tipps about the coils, etc. I hope he's reading this thread and maybe he's responding. And I hope he's fine, especially for his leg."

[/i]

Scotty - its real nice to be appreciated..! ;-)

Fred.

Hi Fred,

[i]Glad you are getting somewhere.. [/i]
Me, too :)

[i]Although I really dont understand whats going on with your Theremin! ;-)[/i]
As I'm not as familiar with analog circuits as with digital ones (especially microcontrollers), I assume it's either the test environment (remember, open circuit, many other electrical devices nearby and quick'n'dirty sensor pods) or the layout of the PCB which influences the oscillators and sensitivity (also I used different variable coils -> TOKO 5P series instead of 10P series).

[i]Which EQ coil is this? The total antenna-side inductance (L8,L9,L10) should be about 12.5mH, and the circuit side (L7) inductor about 2.5mH ..[/i]
As by the suggestion of Thierry from the original thread, I rejected one EQ coil for each sensor respectively, and I used different values and a different orientation of the coils.
So now there are three coils for each sensor, regarding the volume sensor there are values of 2.5mH (sensor), 10mH and again 2.5mH, which give a total inductance of 15mH. The original L10 coil is the rejected one. I replaced the 10mH coil with a 2.5mH one, and the volume circuit worked (or, at least gives the expected result :) ).
So the latest total inductance is 7.5mH.

[i]By "pressure" what do you mean - and to which inductor/s is this applied? - If Finger pressure, particularly on the antenna side inductors, I strongly suspect it is capacitance you are adding - and that the cause of your problem is that the volume oscillator is running at the wrong frequency.[/i]
Indeed, "by pressure" means simply finger tipping the coil (but with pressure), and adding parasitic capacitance was the goal to see if it's causing a difference. Yes, it seems that the oscillator was originally running at the wrong frequency, but it was 450kHz. Also with the new coil value the frequency hasn't changed much, but the circuit worked. I assume it's maybe some sort of oscillator impedance?
BTW, which capacitor is responsible for the resonant frequency of the volume oscillator? For the pitch oscillators, the circuit is clear to me, but not for the volume circuit, I couldn't locate the corresponding capacitor :(

[i]There is only capacitance and inductance which determines the resonance of L7-L10 combined with the volume antenna.. Brass, aluminium or whatever reasonably conductive material will work - no difference would be seen between an antenna OF THE SAME DIMENSIONS and having THE SAME LENGTH / POSITION of connecting wire, and being placed in the SAME POSITION regardless of whether this antenna was made from brass, copper, aluminium, steel.. So I strongly suspect some other reason causing your problems.. These can only be one of the above, or failure of the galvanic connection.[/i]
Hm... Okay, of course the test sensor doesn't have exactly the same shape, but as I wrote above, I already checked the brass sensor with the new coil (but I'll do it again to get sure), also today I connected a nearby similar shaped aluminium rod and it worked, too.
The galvanic connection on the original brass sensor was okay, I measured the resistance (>1R).

[i]At this stage, forget "temperature drifts." - any drift you are seeing is almost certainly not thermal.. I think you probably have E-fields leaking all over the place, and until everything is correctly tuned and verified as stable, thermal is the last of your worries... My advice now is this:
1.) Disconnect all antennas AND the EQ coils.
2.) Check the 'natural' frequencies of all oscillators.
3.) Ensure that the signals from these oscillators are clean and stable - you will need a good 'scope - and ideally a FET buffer close to the points you are testing - scope loading is significant![/i]
E-fields are caused by...? External (such as electronic devices -> I'll do a check in "clean" environment) or internal influence (I didn't use a GND plane on the PCB -> couldn't see one on the photos of this d

Hi Scotty -

Will get back eventually on the other issues.. But you ask about the volume oscillator capacitor..

This is achieved indirectly through several routes.. most important being C14 in series with C15, the other route is via the tuning circuit.


"A GND plane doesn't have negative effects on such a design? I think at least the EQ coils shouldn't be touched by the plane, right?"

I like ground planes! There are all sorts of artforms required in groundplane design, but I find that it is far better to overdo the grounding than underdo it!

Sure - lying an inductor over a ground plane is a bad idea (unless you do this deliberately to introduce distributed capacitive coupling ) and care needs to be taken over attenuation of high frequency signal tracks being capacitively coupled to the ground.. But unless one is well above frequencies seen in Theremins, the far bigger problem is likely to be lack of good grounding.

Fred.

Hi Fred,

I'm sorry for the late reply. There was very little time the last weeks to keep me with the theremin :(

Meanwhile, the prototype cabinet has arrived. I'll try to begin work at it this weekend. Then I can also make the test for the frequency drift I actually encounter on my test environment.
I'll also do a try with copper foil connected to GND/PE and mounted under the board to see if it incfluences the drift. If this doesn't help, I'll make the suggested star-topology of GND to each pin.

What about your document on E14 regarding calibration of a theremin? I read the first approach with interest :)

Have you heared anything from Thierry (or his leg)?

Regards,

Scotty

Hi Scotty -

Sorry.. been too busy to do anything anywhere right now.. Got a big, top secret Theremin venture rolling at the mo.. But, if all goes to plan, I should be producing a batch of special Theremin-type products in the next 6 weeks which will be part of a big Theremin event.. And this will be quickly followed by a set of modules to allow OEM's and constructors to easily build Theremins to whatever complexity level they want..

Thats if all goes to plan.. I will get back to on-line activity when I have some time.