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Posted: 4/4/2014 2:00:49 PM
dewster
From: Northern NJ, USA
Joined: 2/17/2012

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"Even if not galvanically connected to ground, everything conductive (or at least everything not shielded by some other potential) is at least capacitively coupled to ground if not galvanically."  - FredM

Aye, there's the rub.  I keep thinking there is a balanced way out of this, but with the player effectively grounded it always seems to come back to a ground reference of some sort.

"Do we have a situation at times, where inductance in the ground wiring combined with capacitive coupling to ground could actually form a seperate resonant circuit?

And could this perhaps give some explanation for some weird behaviors, and perhaps even occasional "improvements" in range / linearity or whatever - things which are seen but are rarely repeatable?"

I just did a quick experiment with my battery powered Clapp:

  1.875 MHz - ungrounded

  1.741 MHz - grounded (scope lead)

This is a difference of 134 kHz, or 7%.  An operating point shift this dramatic could easily mess up the fine balance between the tank and EQ leg resonances required by the EWS to linearize pitch response.  Maybe it's that simple?

Posted: 4/5/2014 1:23:43 AM
FredM
From: Eastleigh, Hampshire, U.K.
Joined: 12/7/2007

threads posts

Hi Dewster,

Alas, I dont think you can derive any kind of "real" numbers from your above experiment, because by connecting the ground all you are effectively doing is increasing the bulk capacitance "loading" the oscillator - But yes, I fully agree that the deviation is highly likely to move the operating point of the VFO and therefore impact linearity - This is why I have always felt that tuning by adjusting the reference oscillator was wrong - Best linearity IMO can be obtained by having the REF fixed, and adjusting antenna length for tuning (tuning the antenna resonator, not either the VFO or Ref). IMO, changing antenna length is better than adding a tuning capacitor to the antenna resonator, because sensitivity is compensated, wheras a 'wasteful' variable capacitance affects sensitivity.

(I have been playing with an EW board and my screw adjustable antenna, and tuning the null-point with the antenna allows me to use the tuning control to adjust linearity! ;-) - Leaving the "tuning control" (now used as a linearity control) in a set position and tuning using the antenna will, I believe, give consistant linearity regardless of normal changes in capacitive environment - but I wont say this until I have confirmed it ;-)

 I have sketched how I see the configurations:

Normal GROUNDED OPERATION:

Player is coupled to ground capacitively (C6 - ideally 100pF or more). L1 (the ground wire) is low Z, ( low R, Low L) and C1 is effectively shorted. C5 is small (direct capacitive coupling of player to circuit boards 0V, which probably only becomes noticable when the player is close to the theremin), C3 is small (direct capacitive coupling between antenna and circuit boards 0V)..

Main capacitive path is from antenna to ground via player (C4<->C6) and to ground via background (C2) giving a total capacitance of    (C4<->C6) || C2

UNGROUNDED OPERATION:

Theremins 0V is only coupled to ground via C1 and C5 which are both small - Antenna couples to ground via C2, but as ground is only coupled to the circuits return path (back to 0V) through C1 and C5, the antenna sees a much smaller capacitance. The same is true for the player, whose coupling to ground must go in series through C1, and is therefore greatly reduced.

INDUCTIVELY GROUNDED:

IF this is significant inductance in the ground lead (long cable to ground for example) it will behave in the frequency dependant way inductors do - higher impedence as the frequency increases.. If / When this happens, C1 will no longer be "shorted" and the possibility of C1 || L1 forming an important resonant entity perhaps becomes real.

---------------------------------------------------------

I wrote the following before I wrote the above - I think the above is more simply explained.. the following repeats the above less clearly I think..

----------------------------------------------------------

With an ungrounded theremin, "0V" coupling (the boards "ground") to the player is mainly determined by C1 and C5 which are  small capacitances - assuming C6 is comparatively so large (~100pF) as one could regard it as a "short". C2's effect will be greatly reduced because it is effectively in series with C1. IF player coupling to ground is poor, C6 becomes significant (as in, too small) and needs to be taken into account, both when the theremin is grounded or ungrounded

If (as with my H1 setup) one cannot connect to ground, then one needs to increase C5 (and also probably C1) by increasing the 'plate' area at the junction of C1+C5 (the 0V connection to the board)

C1 is the critical component - with a grounded theremin, this is "bypassed" by the galvanic ground connection (shown as L1) IF L1 is low and resistance is low - As soon as C1's "blocking" function is removed, C2 is no longer in series with C1 and its effect is greatly increased, as is the effect of the player, as their 0V coupling is no longer "blocked" by C1.

But now we get to L1 - IF this is significant (long cable to ground for example) it will behave in the frequency dependant way inductors do - higher impedence as the frequency increases.. If / When this happens, C1 will no longer be "shorted" and the possibility of C1 || L1 forming an important resonant entity perhaps becomes real.

Fred.

ps - C3 is shown for completeness, its coupling from the antenna to the board.. Also, these capacitors and inductors are not "physical" components - they are an equivalent circuit showing "components" "formed" by the environment.

Posted: 4/5/2014 11:46:26 AM
dewster
From: Northern NJ, USA
Joined: 2/17/2012

threads posts

Hi Fred,

Regarding the EW, I agree entirely, and it is interesting that with your variable length antenna the pitch knob will likely become more of a linearity adjustment.  I still think it is perhaps easiest and best to just forgo linearization of the near field, particularly with the difficulty in tuning, and the recently discussed complex EW Pro resonant modes giving trouble at start up.  Make the Theremin pitch field wide enough so people don't need to spend significant time there during normal play.  I think audiences like to see more distance between hand and antenna as well, as it looks more impressive.

I wasn't thinking about the stray capacitances C3 and C5, very nice that you have included them and summarized the configurations.  Your text and figures would make a nice addition to a book or paper on the subject!

The oscillator needs some kind of foothold to wave the antenna voltage around, and I keep thinking there must be a way to somehow make this symmetrical rather than ground referenced.  A second identical antenna waving out of phase doesn't seem to be the answer as it would cut sensitivity in half and be susceptible to disturbance and noise.  Ah well.

==========

I'm kind of stuck studying VFO circuits and trying to figure out how best to buffer the output of the crazy JFET to my logic.  A single NPN (ala livio's circuit) works, but any time you use a BJT or JFET with serious gain you run into Miller capacitance, which would then couple the output into my resonating capacitances with temperature dependence.  I could use a JFET or BJT as a voltage follower, and then run this to a gain transistor, but that seems like it's getting overly complicated and draws significant current so there is more ohmic heating going on.  What to do.

==========

[EDIT] Was starting to get insecure about my latest JFET oscillator so I thought I'd do another comparison to livio's Colpitts both in Spice and on the bench.

I see now why he uses a larger lower capacitance in the divider - this tends to boost the output voltage swing without increasing transistor bias current, though it also makes it stall easier.  The tiny capacitor driving the tank reduces sensitivity ratiometrically with the antenna capacitance, but going below about 10pF keeps it from starting on my breadboard so you're limited to roughly 1/2 the sensitivity right off the bat.

My Clapp with the split tank inductor and 10pF to ground is a compromise in sensitivity giving >2/3 of the theoretical max, but it doesn't stall and can work down to quite low average drive currents (<1mA) while providing around the same voltage swings as livio's Colpitts.  The same trick of increasing the lower capacitor similarly increases output swing, but at the expense of more amplitude reduction when touching the antenna.  The Clapp doesn't work so hot without the split & tapped inductor, if I hadn't stumbled across that I'd probably be considering the Colpitts instead.

I've read that the Colpitts tends to keep the tank voltage constant, Clapp tends to keep the output voltage constant.  I can't say I see much difference between the two in this respect when they are adapted for high voltage Theremin use.  I've also read that bias currents should be kept minimal to reduce noise, but I can't say I've seen any direct evidence of this either.

Posted: 4/8/2014 5:01:27 PM
dewster
From: Northern NJ, USA
Joined: 2/17/2012

threads posts

Been spending way too long looking at oscillator buffering.  With a 3.3V supply there isn't much opportunity for gain.  But gain isn't really necessary, while isolation is.  A FET follower is the simplest and gives the best isolation so I've been concentrating on that. 

The above is what I may settle on.  It draws 2.4mA on the bench (with regulator) and gives a somewhat sine-like 2.5V p-p output that doesn't jump towards + or ground when the signal is attenuated by touching the antenna, and will drive significant capacitance.  It's only going to drive an FPGA pin a short distance away, but I don't want digital noise creeping back into the sensitive oscillator.

Spent some more time this morning with livio's oscillator investigating 60Hz interference.  The inherent bandpass nature of the Colpitts (grounded tank inductor) had me worried that it might be better at rejecting 60Hz than the Clapp above.  Luckily (?) I have an extremely noisy LED lamp on my desk that emits 60Hz like crazy via the AC adapter.  I measured ~20ns of 60Hz FM with livio's oscillator, ~50ns with the Clapp above.  Considering livio's Colpitts has ~1/3 max sensitivity and my Clapp has ~2/3, it's pretty much a wash in the 60Hz department.  So I believe I can stop worrying about this issue for now and deal with it later as necessary.

It can sometimes take a person months to find a bit of wheat amongst all the chaff in this field!

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