# First Theremin Build

Posted: 2/21/2020 3:35:27 AM

Joined: 2/9/2020

"The antenna capacitance equations are straightforward. A handful of places quote the cylindrical capacitance formula from which you can estimate the change in capacitance due to the hand and its cross-sectional area. (JPascal and Skeldon)"  - innominataI think you are referring to this paper?hysics of the ThereminKenneth D. Skeldon, Lindsay M. Reid, Viviene McInally, Brendan Dougan,and Craig Fulton. 1998- If so, the formula for the antenna intrinsic capacitance (C in free space) is OK.  - The formula for mutual capacitance (between antenna and hand) is total crap.  I used it for a long time before I collected real data and good sim data, and it really led me astray.  The authors clearly just hand waved their way through that derivation and left it to the rest of the world to test."For this I'm using a common emitter npn Colpitts oscillator followed by a buffer stage. The topology seems simple enough that I'm comfortable with it at the time. The values here are based on Skeldon, initially I simulated the one on electronics tutorials with the radio frequency choke but it didn't seem to have an effect so I left it out."I really wouldn't trust anything in that paper.  Just glancing at it, the capacitance looks rather high and the impedance rather low at the pick-off point for the antenna.  I don't do this for a living, but my main criterion for good Theremin oscillators is large voltage swing at the antenna.[EDIT] Just noticed your reference to "radio frequency choke" above.  Oscillators that are designed to operate with a series choke going to the antenna are really different animals that those that don't use a choke.  You can't take one with a choke and just remove it, you can't take one without a choke and just add it.  For choke oscillators the choke itself is doing a lot of resonating and voltage multiplying (via Q), and when its resonance "fights" with the oscillator LC resonance you can sometimes increase pitch field linearity, but you have to carefully design it to do so.  For your first Theremin I would recommend you use a non series choke type oscillator as they are much easier to tune.

Oodles of useful information. I didn't know what the RFC could practically be used for because this is somehow my first exposure to oscillators.

That's the Skeldon paper I was looking at. Thank you for that heads up! This is the kind of information that can save weeks of time.

Posted: 2/29/2020 7:32:15 PM

Joined: 2/9/2020

Update:

Mixer circuit and VCA work perfectly according to spice simulations with minor changes in values and removing unnecessary decoupling capacitors.

Problem:

Can't seem to understand what the Band Pass filter is used for. Having difficulty creating one that can block the range of frequencies my volume antenna would be producing - eg 900kHz to 910kHz...

I could heterodyne and use a LPF but then I wouldn't understand this part of the design.

Is it really Volume Antenna -> Bandpass Filter -> Envelope detector? Am I just bad at designing filters?

Next:

The hard part, the pitch variable oscillator.

Posted: 2/29/2020 8:00:14 PM

From: Northern NJ, USA

Joined: 2/17/2012

"Volume Antenna -> Bandpass Filter -> Envelope detector?"  - innominata

Yes, the oscillator feeds a relatively high Q filter.  The sharp, narrow filter resonance is used to significantly vary the amplitude over a small input frequency range.  It's a bit like heterodyning in a way.  You need a second order filter for it to have a Q parameter, hence the LC used here.

Posted: 3/1/2020 1:48:01 AM

Joined: 2/9/2020

Update:

Oscillator works and pitch varies within the range that I want from 746kHz to 735kHz.

Major issue is that the voltage swing isn't high enough at the antenna. It's supposed to be 4Vpp but it's only 800mVp. Not sure why.

This may not necessarily be a problem but loading effects bring it to zero when connected to the mixer. Then there is the issue that the mixer also attenuates signals and won't produce much of an output for a 1Vpp input.

The only thing left to worry about hopefully will be the bandpass filter and harmonics.

Posted: 3/1/2020 1:54:38 AM

Joined: 2/9/2020

For antennas I literally just sawed a 1/2" hollow copper tube with 40mill diam into ruler lengths.

Then I wrapped a wire on the bottom end, taped the wire down and stuck its opposite side in the breadboard.

I'm a little shocked how smoothly things are going. Especially that my waveforms almost look identical to simulations in shape.

For the VCA I used LM324 and a pJfet because I didn't want to have to make a negative signal. Seems to work fine.

Posted: 3/1/2020 9:34:02 AM

From: Berlin Germany

Joined: 4/27/2016

I have some remarks and asks. Nice to see the progress and the good match between practice and simulations!

The vca is as linear as the pjf or njf  gate-voltage to drain-source-resistance is too. Of course can a vca inclusive limiter be build discret. But fine tuning the overall behavior is sure time consuming, that your project, as you ment, do not has.  How do you split the 9V battery voltage in +/- 4.5V?

Can you explain "Major issue is that the voltage swing isn't high enough at the antenna. It's supposed to be 4Vpp but it's only 800mVp. Not sure why." a bit more? Where do you measure the 800mVp? Alone the high-resistance buffer breaks it down?

Posted: 3/2/2020 1:04:46 AM

Joined: 2/9/2020

I have some remarks and asks. Nice to see the progress and the good match between practice and simulations! The vca is as linear as the pjf or njf  gate-voltage to drain-source-resistance is too. Of course can a vca inclusive limiter be build discret. But fine tuning the overall behavior is sure time consuming, that your project, as you ment, do not has.  How do you split the 9V battery voltage in +/- 4.5V?Can you explain "Major issue is that the voltage swing isn't high enough at the antenna. It's supposed to be 4Vpp but it's only 800mVp. Not sure why." a bit more? Where do you measure the 800mVp? Alone the high-resistance buffer breaks it down?

Okay so in my schematic there is a buffer that follows the oscillator but I didn't have time to breadboard it after testing the oscillator. I connected the output of the colpitts oscillator directly to the mixer without the use of a buffer and that caused loading effect problems.

Where am I measuring the 800mVpp? At the oscillator collector output which is connected to the antenna. I may have been confused about it being 800mVpp, but it was close to 1.2-0.8Vpp. Why is this a problem? Either I modeled this system incorrectly or I've introduced a cumulative 3V drop in my oscillator somewhere, simulations say 4Vpp.

This is only currently a problem because the mixer doesn't have unity gain but drops the signal amplitude, a 1Vpp input drops almost to zero.

I have not implemented battery power just yet. I have access to a power supply so they are being used to generate the +-4.5V rails. I initially intended to use a rail splitter circuit which you can see in the first post. In case that doesn't work I have a TLE2426 rail splitter IC.

Posted: 3/2/2020 1:13:41 AM

Joined: 2/9/2020

Codeoes anyone have a guide on frequency sweeping on spice?Sample of hand position stepping (not sweeping) in LTSpice simulation.E.g. you want to simulate theremin oscillator with different values for hand capacity (capacity introduced to LC tank by hand).Replace value in C with parameter name - e.g. {Chand} instead of 0.1pFThen add STEP command for this parameter listing, e.g.Code:.step param Chand list 0pF 0.1pF 0.2pF 0.3pF 0.4pF 0.5pF 0.6pF 0.7pF 0.8pF 0.9pF 1.0pF 1.1pF 1.2pF 1.3pF 1.4pF 1.5pF
Like here

Thanks for this.

Posted: 3/13/2020 5:14:28 AM

Joined: 2/9/2020

Update:

Wow, designing this part was much harder than I could have expected. A high Q filter at large frequencies is so hard to design with active filters and requires so many stages. Doing it with passive components is not hard but requires being able to find nH inductors...

Thanks to the amazing program Qucs I can at least outsource the design portion of this and quickly search for workable values if I can get my hands on these inductors or wind them myself.

To help with the inductor values I'll have to change my oscillation frequency to be the lowest needed for a meaningful volume control. It's possible that this won't be an issue so long as I can get the smallest inductors to be 1uH.

Also I don't exactly get the need for a bandpass filter... a high roll off low pass filter could work just as well for generating this control voltage.

I've also decided to make use of those IF transformers as adjustable inductors for my oscillators. They aren't available from mouser, element14 or rs-online but Jaycar is still stocking them for some reason.

Posted: 3/14/2020 8:52:38 AM

Joined: 2/9/2020

Update:

The problem related to the voltage at the output of the oscillator was due to bad circuit construction. This is fixed now and the output is a consistent 3.6Vpp.

The oscillator isn't permitting an emitter follower though. The output simply dies if you try to connect an emitter follower, why would this be the case? I've tested to see whether it's a loading problem with different load resistors. Anything above a 10k ohm load is fine which should be the input impedance of my resistor since the output impedance is 400 ohms and Zin - Beta*Zout with Beta being at least 40.