I've built a simulator for the analog front end (AFE) of my digital Theremin and thought I'd share it here. I was simulating it in LTspice but changing component values was somewhat cumbersome and calculations of the exact resonance point difficult. For simpler circuits (both analog and digital) I find it's often easy to build a specific simulator in Excel and then play around with it. Anyway, it's posted here and is generally applicable to many LC Theremins:
It's interesting to plug the Etherwave Standard (EWS) values into it and see that the tank resonance is almost completely dominated by the antenna capacitance and linearizing coil inductance. (My AFE requires a clear quadrature signal in order to work and I'm not sure the EWS resonance point provides this.)
The voltage on the business end of the linearization coil (i.e. the antenna) can easily be in the many hundreds of volts, which makes sense I suppose - if you are trying to detect tiny capacitance changes you want a large voltage signal. Another interesting thing is the double resonance that you will see if you play with the values at all. This makes me wonder if RS Theremin's designs are jumping to this second higher resonance? I see the second resonance on the bench with my AFE, but I don't see the peak voltages associated with it.
I wasn't able to download from that site for some reason. If you want to e-mail it to me, I can host it directly on theremin world for you. This is a reminder to me that I need to get working on the file/image galleries.
Thanks for sharing! I'm looking forward to seeing the sheet!
Thanks for sending it! I've uploaded the .zip file to the "Resources for Theremin Builders" list page (under the Tech/DIY section). I'd provide a link directly here, but I still haven't fixed that bug that prevents linking to other pages at the site in the forums (it's on my short list - hang tight folks!).
This simulation is a good starting point. Unfortunately the parallel winding-capacitance of the linearization coil is not yet taken into account although it can have a very strong impact on the tone spacing close to the antenna.
In the Etherwave Pro's pitch arm there is a small variable capacitor in parallel with the first of the four coils so that the position of this additional pole in the curve can be adjusted.
Dewster, are you up for v2 of your simulator?
Btw: in practice jumping to the 2nd resonant frequency (which is the lower one) occurs only when the self resonant frequency of the series circuit by linearization coil(s) and the antenna is greater or equal the self resonant frequency of the oscillators tank circuit (without antenna attached).
In the Etherwave Standard/Plus/Pro, the higher one is used and intended.
dewster said: "This makes me wonder if RS Theremin's designs are jumping to this second higher resonance? I see the second resonance on the bench with my AFE, but I don't see the peak voltages associated with it."
Want a real simulator?
I will send you a 900 kHz oscillator board already constructed and pre-tuned to a gold coil antenna along with an old RS detector/pre-amp board. I have old stuff left over from research. You just need to furnish a good earth ground and small camera tripod 1/4-20 to suspend the base board with pre-mounted antenna. Remember your first construction project when life was simple? Think Legos, I have the pieces. Lev Sergeyevich had a favorite toy department (Detskii Mir) in Moscow where he found his electronic parts in his later years. He often visited when the sun was shining.
"We never know what is possible with the human spirit, Lord forbid it might just be something fun."
There is the possibility the Lev Antenna which is a gift... is not for everyone. )-'
@Thierry: "This simulation is a good starting point. Unfortunately the parallel winding-capacitance of the linearization coil is not yet taken into account although it can have a very strong impact on the tone spacing close to the antenna."
I worked on it some more. I added a second linearization coil, and all three inductors now have parasitic capacitance and DCR:
Also two selectable tank excitation modes. It's nice to have a more accurate simulator, but it is a bit more cumbersome to use with all the parameters. I hope it isn't too off-putting.
@RS Theremin: "...the EWS has a strong EM field reading of the 278 kHz the oscillator freq detectable with accuracy to just over two feet away from the Ant.
If I do the same experiment with the Lev Antenna you can only get a freq. reading if the plastic part of the probe in your hand is pressed against the coiled antenna. The RS Theremin Lev Antenna response is still out to 4' and linear."
What are the LC values you are using in your tank? I'd like to simulate it. The EWS has tons of voltage, I'm kind of surprised it doesn't light up a neon bulb near the antenna! Bob Moog certainly had a plan as I believe this benefits SNR (fights radio, 60Hz, etc.).
My AFE currently runs around 700kHz with the linearization inductor the same value as the one in the tank, but I'm very constrained by the inductors in my junk box (they came from a cannibalized graphic equalizer - perhaps the contents of one's junk box define the limits of one's practical knowledge?). From simulation it's clear I need 4mH or so at the antenna with a high self resonant frequency (i.e. low parallel capacitance) to maximize sensitivity. When my AFE was oscillating I saw pretty linear behavior of four octaves or so watching 16.7 ms out (60Hz period) from the trigger point on a scope.
Jason, thanks for doing that update!
RS Theremin, I'm rather new to LC oscillators so I'm sure you're way ahead of me (and I'm certainly not trying to school you) - it seems the operating frequency isn't necessarily as simple as resonance, which isn't always as simple as the LC equation. Any phase shift can move the operating point, and the transistor or whatever is doing the sensing and driving may be doing so in subtly unobvious ways?
And thank you for your very kind offer! I'll be in touch via your web page? (Trying to get out of town right now for several days - music teacher's conference in NYC for my wife.) I'm currently limited by my coil selection (need to make an order) but it seems everything works better in my AFE somewhere below 1MHz.
Does anyone know the part number for the 10mH linearizing coils in the EWS? I have yet a third version of the spreadsheet that takes Q into account and shows coil current, and I'd like to more accurately simulate the EWS.
[EDIT] After poking around some likely candidates are Bourns/Miller 6306 and Hammond 1535G. The Hammond has higher Q (108 vs 95) otherwise the specs look the same (F0=0.71 MHz, R0=31 Ohms, 100 mA max).
At over $5 a pop, semi-linearity comes with a big price tag.
I thought that the Miller had a SRF of only about 530kHz while the Coilcraft is at 710kHz, but these infos may be outdated....
The Coilcrafts give significantly better results in the EWS than the Millers while it seems to be the inverse in the Etherwave Pro's pitch arm.