Continue: Building EM Theremin / rebuilding EtherWave circuit

Hi,

this is the continue from this thread. (http://www.thereminworld.com/forum.asp?cmd=p&T=4238&F=3)

I don't know why the thread has been closed. Since no admin has written anything about it, I assume it has been closed by accident or by forum rule?

First, I want to thank Thierry and FredM for their help in the original thread. I apologize that I have not continued the thread directly after the original thread has been closed.

But I was not passive in this time :) And I want to share the actual status, asking some questions and I hope it might be helpful for other selfmade theremin builder to read this.

So this week I assembled the first board. PCB was made by a PCB manufacturer, the part orientation is based on the one mentioned in the hot rodding manual from Moog with few changes (again, thanks to Thierry and FredM). I tried to get as less as possible differences between the basic circuit and my design to ensure that I understand the function principle as good as possible. But some of the bigger changes mentioned to improve the function will get into the next version :)

Unfortunately, I was not able to locate any information about the PCB layout, but I think it could be probably one of the best Etherwave imitations.

At this time, I'm on the tuning phase of the pitch circuit, the volume antenna is not connected yet. Since I don't have the cabinet finished yet, I mounted the board on a base pertinax plate, using plastic screws. I know that this will not give the same results as a fully assembled theremin device, it's only intended as a quick test environment. So I've done the suggested initial check list mentioned in the EMTheremin article. Current checkpoint is pitch tuning procedure.

The pitch antenna (~50cm aluminium rod, not tube -> will be changed to copper tube with 10mm outer diameter when the cabinet is available) is mounted approximately 7-12cm away from the board. A grounded copper foil is mounted between the board and the antenna.
First thing I've learned is that it's really hard to tune if you do it on the workbench next to PC, oscilloscope, etc. :) This environment has given me only 2-3cm for the distance of the pitch antenna, mainly independant of the oscillator settings. Also the pitch range is poor.

So I did it again on the clean living room desk without any near active electrical device or something like that. It improved the range to approximately 15cm and even the oscillator tuning has now a better effect on the pitch range. But this is not enough. If I'm right, I can expect about 50-60cm distance on a good tuned circuit, right? Also, I assumed that the pitch range is at least three octaves, but for my unexercised ears it's somewhat of a half octave in best case (also it's hard for me to check the 3kHz mentioned in the EM article checklist with my ears, and if I measure audio output frequency with the scope the signal is very noisy so I cannot read the exact frequency).
Now, I wonder if I can expect pitch distance and range of a Etherwave on my quick test assembly.

For now, my current checklist contains the following steps:
- on the basic voltage measurements based on the checklist of the EM article, all voltages except the voltage over R6 (emitters of Q3 and Q4) are okay. The measured voltage is somewhat about 0.02V instead of the expected 0.6V. I don't know if this is a true error since I verified all oscillator frequencies with the oscilloscope.
- I didn't think of the winding direction for the antenna coils, so I have to check it (it's mentioned in the EM article)

At this time, I'm just happy that it works even at this very first try :)

Regards,

Scotty

Hi again Scotty,

First – you say:

[i]”I don't know why the thread has been closed. Since no admin has written anything about it, I assume it has been closed by accident or by forum rule?”[/i]

Due to some technical difficulties with multiple postings on any one topic, we have been restricted to 26 posts per topic.. This is automatic, and can be a bit of a pain.. What we tend to do (as you have just done) is to create a continuation, with a link back to the preceding.. Also, it is worth adding a link in your last posting on the old thread to point to this continuation.

[i]”First thing I've learned is that it's really hard to tune if you do it on the workbench next to PC, oscilloscope, etc. :)”[/i]

Tell me about it! – This is a real pain.. One needs the damn equipment, but it can make ones measurements meaningless just by being there! – I now have a couple of super HP signal generators to simulate reference and variable oscillators, and move to the kitchen table when I need to play with the real oscillators / antennas!

[i]” It improved the range to approximately 15cm and even the oscillator tuning has now a better effect on the pitch range. But this is not enough. If I'm right, I can expect about 50-60cm distance on a good tuned circuit, right?”[/i]

Yes – you have some problem.. you should get about 60cm.. And you should get -0.6V on Q3E/Q4E.. Check the voltage on the base of Q3, it should be extremely close to 0V, and (if functioning correctly) its Emitter MUST be close to 0.6V below the voltage on its base.. -0.2V is too high – even -0.5V would be too high.. Check that R5 is actually 1k (not 1M!).. and check that the 0V R5 connects to is REALLY at 0V

>>Edit: Remember - base to emitter is effectively a diode.. If the diode is functioning, there will be its forward voltage (in the order of 0.6V) between base (anode) and Emitter (Cathode) when forward biased.. If one forces E to some voltage, and forces B to a voltage > Vf with respect to that voltage, large currents flow and the B-E junction heats and is usually destroyed.. This is (one reason) why there is almost always a resistor at the base to limit maximum base current. If the B-E voltage is
[i]”Also, I assumed that the pitch range is at least three octaves, but for my unexercised ears it's somewhat of a half octave in best case (also it's hard for me to check the 3kHz mentioned in the EM article** checklist with my ears, and if I measure audio output frequency with the scope the signal is very noisy so I cannot read the exact frequency).
Now, I wonder if I can expect pitch distance and range of a Etherwave on my quick test assembly.”[/i]

** Play a 3kHz tone from something, at the same time as the EW.. If you are lucky, they will beat!

You probably cant expect perfect operation, but you can expect to get better than you are getting now! .. 6 semitones is a tiny deviation in frequency on the variable oscillator.. I advise you to put a small trimming capacitor between the antenna and ground (as small as you can get – or perhaps a 10pF trimmer in series with a 3.3 pF capacitor) and adjust this.. see how much the frequency changes, and report back.
Also.. Check your EQ coil.. A short on this will dramatically reduce the frequency range.

[i]” - I didn't think of the winding direction for the antenna coils, so I have to check it (it's mentioned in the EM article)”[/i]

It is ??? – As long as the windings are going in the same direction (as in, half are not wound in one direction, and the other half in the other direction) I cannot see what difference it would make.. If you have 4 coils, and they are alternately fitted in opposite orientations, then (if they are close together) you could get field cancellations which could effectively reduce both the inductance and

You might find the following useful:

Etherwave Oscillator Simulation – Ltspice ( http://www.element-14.com/community/docs/DOC-16876)

Etherwave Front-End Simulation (Antenna,Osc's + Mixer) – Ltspice (http://www.element-14.com/community/docs/DOC-16875)

And there are other items at the Theremin General / Resources Group – Element-14.com ( http://www.element-14.com/community/groups/theremin-general-resources?view=overview) .. I would love you to contribute anything you can to this site.. So far, I am the only person placing any documents here.. its getting lonely! ;-(

LT_Spice can be useful to probe a circuit simulation to see what, theoretically, you should be getting from your prototype.. (Unlike Proteus and some other simulators, you do not need to re-run the simulation when you change the point you are probing, or add more probes - this is one big advantage of LT_Spice.. The other advantage is that its free) .. see here (http://www.element-14.com/community/message/2945#2945)

Hi Fred,

[i]... we have been restricted to 26 posts per topic ...[/i]
Ah, okay, good to know :) Means to put as much information as possible in each post to get best usage ;)

[i]Tell me about it! – This is a real pain.. One needs the damn equipment, but it can make ones measurements meaningless just by being there![/i]
Indeed. As I'm mostly with digital circuits and microcontrollers (and additionaly RFID in business), which are mostly resistant to such things, I didn't thought about it - until I read the EM article again with eyes as open as possible :)

[i]Yes – you have some problem.. you should get about 60cm.. And you should get -0.6V on Q3E/Q4E[/i]
I checked it again with voltmeter, this time the voltage was okay. Maybe a measurement error on the first time. To get safe, I'll resolder the transistor/resistor pads.

[i]** Play a 3kHz tone from something, at the same time as the EW.. If you are lucky, they will beat![/i]
Hey, good idea! I'll try to get a 1kHz and 3kHz sine mp3/wav file on my mobile phone :)

[i]I advise you to put a small trimming capacitor between the antenna and ground (as small as you can get – or perhaps a 10pF trimmer in series with a 3.3 pF capacitor) and adjust this.. see how much the frequency changes, and report back.[/i]
Ah, this will influence the resonance frequency of the variable pitch oscillator, right? In fact, it will simulate the influence the human body, right? I could get trimmable 4-25pF caps on office tomorrow.
The antenna has still to be attached to the circuit when I do the frequency variaton measurement?

[i]Also.. Check your EQ coil.. A short on this will dramatically reduce the frequency range.[/i]
Sorry, please explain 'EQ'. Do you mean the fixed 47µH coil in the oscillator circuit?

[i]It is ??? – As long as the windings are going in the same direction (as in, half are not wound in one direction, and the other half in the other direction) I cannot see what difference it would make.. If you have 4 coils, and they are alternately fitted in opposite orientations, then (if they are close together) you could get field cancellations which could effectively reduce both the inductance and the Q.[/i]
Yes, the EM article mentioned it on page 5, article "Antenna circuit boards". I used Hammond PI coils, series 1535 (Datasheet (http://www.hammondmfg.com/pdf/5c0037-38.pdf))as suggested by Thierry, and I also used his suggestion for the orientation, each coil is mounted 90° degree to next one. I checked the direction on my board now, all of them now point in the same "direction" (only one coil of the volume antenna has to be switched). For the 90° orientation, even if a coil direction is wrong, I assume the effect of field cancellation is minimized, but I did it to get sure :)

[i]Perhaps try taking the coil out of circuit, and just wiring the antenna directly to the board… If you still get 6 semitone change (as in, performance does not change at all) then something about the coils is likely to be the problem! The sensitivity (particularly on the EW) before the EQ coil is a tiny fraction of the sensitivity after the coil.[/i]
Ah, okay, meaning the antenna coils are responsible for increasing the sensitivity?
Just for my understanding. For now, I understand the principles of the oscillators and mainly the most of the VCA part, but didn't really understand the function of the antenna coils and the resistor/diode combination on the volume coil pointing into the circuit.

[i]Also – Check the frequencies your oscillators are producing.[/i]
Okay, I will check the frequencies before C2, C6 and C12 and report the values.

[i]You might find the following useful:...[/i]
Indeed, thank you. I'll check the documents and will also give LTSPice a try (worked a little bit with it on business).

[i]I would love you to contribute anything you can to this site.. So far, I am the only person placing any docume

Scotty -
I did a long(winded) post covering all the issues.. and it didnt appear here 8-( .. And rather than doing in a seperate WP and pasting it here as I usually do, I had done it only here - so lost the damn lot! 8-((

I will put it all together again - but it may be a day or so..

Jason or Omhoge or anyone able to locate / recover my posting and make it appear.. nah - its been lost in the internet aether forever... :-(

I think ill just do it as a .pdf on E-14, and just post a link here..

Hi Fred,

[i]I did a long(winded) post covering all the issues.. and it didnt appear here 8-( .. And rather than doing in a seperate WP and pasting it here as I usually do, I had done it only here - so lost the damn lot! 8-(([/i]
Oh, that's a pity, I know such behaviour from other forums where I post. Seems they sometimes follow the rules of Murphy - short post, all okay, but longer postings...

[i]I will put it all together again - but it may be a day or so..[/i]
Okay, in the meantime I'll try to do the measurements you suggested.

I've played around with the pitch circuit, and have a setting where I thought I can give the volume antenna a try. But it didn't work, too. Seems that that voltage on the VCA processor output (IC3/pin 12) doesn't react to the volume antenna, regardless of the value of the L11 coil.
I'll investigate it :) I've read somewhere that the value of R14 could be too high, but reducing it didn't help. I can see only a very small voltage change if I move the volume potentiometer.

Regards,

Scotty

Hi Scotty -

Can you tell me what test equipment you have access to.. email me if you wish.. Also, you have the full Proteus EDA if I rememer correctly?

I am putting a document together detailing MY methods of setting up a Theremins oscillators and antenna Eq.. The easiest way I have found uses some equipment most constructors wont own (like HP 3325A Synthesizing Function Generators, which allows one to output sine waves from 1Hz to 20MHz with 1Hz resolution.. £150 each on Ebay..) But prior to owning these, I made some simple test kit (by simple, I mean simple.. Diode, resistor,capacitor and meter for example) which simplifies tuning greatly.

With regard to the EQ (antenna) coils - Check Oldmelicula website (Search for member Christopher.. RSTheremin.. He host this site) Christopher and Don Parrish-Bell have an excellent article on the EQ coil's function - It not described as I would, but it is (at least) an equally valid explanation to what I would give.

I had loads of links on my lost post - but this loss is pushing me to do a comprehensive description of the Theremin Front-End.. There is little out there which fully covers this in a non-mathematical way that constructors can easily refer to.

Hi Fred,

[i]Can you tell me what test equipment you have access to.. email me if you wish..[/i]
It depends on where I am :)
On the home workbench, there are just a "hobby" laboratory power supply, a voltmeter and a poorly working old 20Mhz oscilloscope (it wants to go to pension and would be replaced by digital oscilloscope soon if could find a good one).
On office, I can get access to frequency generators, digital counters, and even a network analyzer, etc.

[i]Also, you have the full Proteus EDA if I rememer correctly?[/i]
Huh? No, that's not me. Don't know what Proteus is? Ah, I've looked on the web, it's a PCB design software. I use EAGLE and AltiumDesigner.

[i]I am putting a document together detailing MY methods of setting up a Theremins oscillators and antenna Eq...[/i]
Sounds good :)

[i]The easiest way I have found uses some equipment most constructors wont own (like HP 3325A Synthesizing Function Generators, which allows one to output sine waves from 1Hz to 20MHz with 1Hz resolution.. £150 each on Ebay..)[/i]
Okay, no problem, as I wrote above, I could access a function generator by business. And as I stated above, I'll try to update my home equipment, since I planned the update anyway. But it needs time until a good offer comes along the way :)

[i]But prior to owning these, I made some simple test kit (by simple, I mean simple.. Diode, resistor,capacitor and meter for example) which simplifies tuning greatly.[/i]
Yeah, usually I try to get things working with easiest methods. This approach increases experience level much more (thinking of our trainees - they like to use the autoset button on the Tektronix oscilloscopes, but I don't give them the Tek's, but some older oscilloscopes -> "Hey boss, it doesn't show the signal" -> Hey guy, you have to set it up first!") I think you know what I mean...

[i]With regard to the EQ (antenna) coils - Check Oldmelicula website (Search for member Christopher.. RSTheremin.. He host this site) Christopher and Don Parrish-Bell have an excellent article on the EQ coil's function - It not described as I would, but it is (at least) an equally valid explanation to what I would give.[/i]
Okay, I'll take a look on it as soon as possible.

[i]I had loads of links on my lost post - but this loss is pushing me to do a comprehensive description of the Theremin Front-End.. There is little out there which fully covers this in a non-mathematical way that constructors can easily refer to.[/i]
Great idea. It enables real hobby electronics to understand the principles.
For me, I prefer the mid-way, meaning not all by theory and formulas etc., but also your mentioned non-mathematical approach.
I'm looking forward to read your description.

Now I'll go investigating the problem with the volume antenna :)

Regards,

Scotty

First few pages done Understanding and calibrating the Theremin Front-End (http://www.element-14.com/community/docs/DOC-20512)

Any comments welcome from anyone, and please - if you see errors, think something should be added, can explain something I dont understand, or can contribute in any way - Please email me!

Hi Fred,

I've taken a look into your document, it read's fine. If possible, I'll take it at the weekend again, and could make some suggestion for the "non-formula" approach for better reading and understand for non-electronics (could be hard to make suggestions from such a point of view since I am a electronic) :)

Regarding my volume antenna problem:

Woooooohooooo!

Good news, I got the volume circuit to work!!!
I finger-tipped some parts to get parasitic capacitance into circuit (sometimes this is a good approach for finding the error -> learned that approach the hard way on business). As I pressed the middle EQ coil L8 of the volume circuit, I heared a very short sound - and I saw a peak on the volume voltage (U3 / Pin12)!
I pressed again the finger on the coil. Depending on the pressure level the sound could be heared continuously, but too much pressure and it's gone away again.
So I thought it might be the sensitivity of the circuit and I changed the value from 10mH to 25mH. This made it more worse because the correct pressure level was very hard to find.
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 :)

But the volume antenna (exactly: the sensor, according to your document :) ) didn't react even with the new coil values.
For my test environment I used a simple aluminium rod as pitch sensor. But 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.

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'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.

Regards,

Scotty