Crazy (?) theoretical / technical ideas

Posted: 4/8/2016 11:02:49 PM

From: 60 Miles North of San Diego, CA

Joined: 10/1/2014

emarii, you have great imagination, it would be simpler and cheaper to have the DJ's sidekick do the effects in real time to match the mood of the room.

In drought stricken So. California we have to explain to our grandchildren what is going on in this video.

Posted: 2/17/2017 3:42:05 PM

From: Northern NJ, USA

Joined: 2/17/2012

Very interesting paper on wireless power transfer:

I tend to ignore this kind of stuff, but this is technically quite fascinating.  Basically they make a room electrically resonant at ~1.3MHz via conductive walls and a split conductive center column bridged by a physical capacitor.  Then they flood the room with RF which creates a magnetic standing wave, and the electric field gets confined to the capacitors in the split.

So effectively any coil in the room is a tightly coupled element in an enormous transformer, and the energy doesn't fall off as the square of the distance like it normally does.  They tune the room with the capacitor in the split, but I wonder if / how they tune the receivers?

Found it on Hacker News this morning:

Video for the tl;dr crowd:

Posted: 12/17/2017 3:28:34 PM

From: Northern NJ, USA

Joined: 2/17/2012

Resonant RF FM Transmission

I've been pondering this for a couple of weeks now and it really doesn't seem all that crazy.  Surely someone has thought of it before me?  Here goes:

A Theremin loses some of the energy of each antenna voltage "swing" to RF (radio frequency) emission.  Since the "antenna" (actually a true antenna in the case of RF emission, though clearly used more as a capacitive plate for the function of the Theremin) dimensions are quite small compared to the wavelength of oscillation, the RF emission is low.  For Theremin use this is great, because low loss means the Q (quality of resonance) - and therefore selectivity, voltage swing, etc. - can be quite high.

Now, say we are instead designing a low frequency FM (frequency modulated) transmitter with a way-too-small antenna.  One way to make the antenna more efficient at transmitting and receiving RF is to put a "loading coil" at the base, which makes the antenna resonant over a somewhat narrow band.  Highly resonant antennas like this often employ a "capacitive cap" on the top of the rod.  Does this all sound familiar?  What you end up with is pretty much a Theremin with an EQ coil, but driven by an almost fixed frequency.

The mechanism for better RF transmission with a short antenna is to compensate by making the voltage swing at the antenna really high.  In fact, Q is a multiplier here, so if we use a high quality inductor and drive it at exactly resonance, we'll multiply the drive voltage by the Q.  If the Q of the inductor combined with the antenna C is, say, 50, and we drive it with a 5V oscillator, we'll get a 250V swing at the antenna.  In other words, the short antenna will be 50x better at transmitting RF.

The trick then is to maintain resonance for different environmental conditions.  If we are designing a walkie-talkie then the C of the antenna will vary greatly depending on the orientation of the antenna, height above ground, proximity of other conductors, etc.  

On my digital Theremin I drive a series coil / antenna and adjust the drive frequency for quadrature phase between it and the antenna.  But an RF transmitter needs to transmit at a single given frequency, so we do so and place a varactor diode on the antenna side of the coil, and adjust it for quadrature phase, thus forming a feedback loop independent of the drive frequency.  I believe one could frequency modulate the drive and have an efficient RF transmitter, given the constraint of a very short antenna (for the wavelength).


Receiving RF is almost the same as transmitting RF, so here we could compare the phase at the antenna to the phase on the other side of the coil, adjusting the antenna varactor for quadrature.  It would likely need some amplification and filtering, and an additional feedback loop of some sort, rather like AFC (automatic frequency control).  Not sure how practical it would be.


And, yes, one could certainly build an "upside-down" Theremin this way.  Drive the tank with a fixed frequency, adjust the antenna C to maintain resonance, and use the correction voltage as the pitch / volume value.  Trouble with this is varactors are non-linear, and it's an AD (analog to digital) step.  Beauty of this is you could perhaps use one fixed frequency to drive both antennas (though maybe not, I've tried something rather like this in the past and it didn't work when the antennas were quite close).  You could dither the drive frequency to spread the spectrum, but probably not enough to mean much.

Posted: 12/17/2017 4:50:58 PM

From: 60 Miles North of San Diego, CA

Joined: 10/1/2014

Fred use to get upset when I mentioned RF and theremin’s in the same sentence.

With my thinking, my guess to explain why my special antenna had perfect pitch field linearity without needing inline coils was due to the “RF” loading effect of the hand as it gets closer to the spiral pitch antenna. The hand capacitance shifts the minute RF current flow within the antenna which changes the loading of the pitch oscillator a bit, shifting the Xc of the oscillator, this counters the normal hand capacitance. The freq used was 922 kHz which is also M. Faraday's birthday, a gift from the other side?

* Even if my explanation is incorrect the linearity is still there and perfect, needs study. The pitch field will be linear or it is not, there is no almost linear. Some how notch tuning?

* Why it matches a musical scale on keyboard note for note in distance I never figured out, that is a man made creation. 

* One last fascinating observation, using the correct connecting point on the tightly coiled antenna will cancel any pitch drift caused by the antenna relative to the surroundings and temperature change.

There is still a lot to be studied how this simple pitch linearity works but that is for someone else with an open mind.

There were engineers that injected my tightly coiled antenna approach with a signal generator looking for an effect, this is like the driver of a bulldozer pushing a feather and constantly saying I do not detect a feather so there must be no feather.

 I developed an attitude from this period that Modeling is good for studying circuits once the basic fundamentals are understood but Modeling might limit original discovery. Nothing will surpass thorough bench testing, a slide-rule and curiosity.

From my experience most all theremin thermal drift comes from transistor PN junctions as my tube theremins did not drift. Coil drift is minimised in theremins by using the same coil in both oscillators, which will offset each other.

Always get a sound sample of any theremin you want to build, if none exists then there is a reason, it is all show and no go.

dewster revealing how to buffer the two pitch oscillator was a breakout moment for me. In france everything is a secret.

Any analog theremin, solid state or tube, can have that classic sound of Clara if you know what you are looking for and how to get there. This is another thing you will never find in Modeling. The vacuum tube has a characteristic that makes it slightly better for a classic voice but tubes are less practical. There is something very beautiful in analog theremin design, almost a connection to nature, maybe organic, not experienced with digital approaches.

The use of cellphones ended my explaining on a webpage how I got my results as communication became short cryptic sentences rather than an in depth email exchange of ideas. In theremins everyone is an expert even if they have never demonstrated anything that works.

From my first crystal radio in the 50's to analog theremin design today, it was a total climax as a hobbyist to hear her voice.

The theremin can be more than a cheap digital whistle.


 PS: I loved Fred as he was one of the three Amigo's in my mind, brothers till the end - Fred, dew and me.


Posted: 12/18/2017 9:02:09 PM

From: earth

Joined: 5/8/2017

A spiral pitch antenna seems likely to be a virtual plate antenna.

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