Earth/Human Ground Problem?

The Theremax I am building changes tone when I touch anything in the house that is grounded including the copper pipes even if I am on the other side of the house.  This happens if I use the audio cable connected to a three prong amp or with a dedicated grounding cable to a receptacle plate screw as a ground source. 

I was thinking that the problem wasn't that the theremin was not well grounded but that I was not well grounded but I have verified < 1ohm of resistance between my wiring ground and all the theremin ground points.  A simple neon receptacle tester shows that the wiring is OK (ground not open).  It appears as though my body has an effect on the entire house ground when I touch it.  I would think that a good earth ground would "trump" my body ground.

Now I'm wondering if my house wiring is not well grounded to earth.  The ground rod outside by the service entrance is rusty and I can move the top back and forth - doesn't feel like it is solidly in the ground.  Our water supply from the street is PVC.

If anyone has had this problem or knows of why what I am thinking just can't be please let me know before I start driving an additional ground rod.

Thanks in advance,

Greg

The answer is simple:

It's you who is badly grounded. A Theremin player is seen by the pitch antenna as a 5kOhm resistor in series with a 100pF capacitor. As soon as you touch a grounded element, you shorten the 100pF capacitor which increases the capacitance seen by the antenna and the pitch will go up.

This effect is normal and can be observed with every theremin.

"It's you who is badly grounded. A Theremin player is seen by the pitch antenna as a 5kOhm resistor in series with a 100pF capacitor. As soon as you touch a grounded element, you shorten the 100pF capacitor which increases the capacitance seen by the antenna and the pitch will go up.

This effect is normal and can be observed with every theremin." - Thierry

Maybe I need to post a video or at least a picture since as they say a word is only worth a milipicture - not sure how many milivideos a picture is worth - I think that quote needs an update.

Anyway, to better describe the situation, I have an electrically grounded (house wiring ground) object hanging from a horizontal string 10 3/8" from the pitch antenna and it turns out that the way I have set the pitch control the theremin tone is a C4 if I am 6 feet or more away from the theremin (the tuner stops moving).  If I slide the can closer the pitch goes up,  if I slide it back to 22 inches I reach zero beat and if I slide it even further away the pitch goes up again because now the oscillator connected to the antenna is running faster than the reference oscillator.  That all makes sense. 

Now, if I put the grounded object back to the 10 3/8" distance to get a C4 when I am not close and I walk up to within arms reach of the object the pitch goes up more and when I touch the can it goes up a lot more as expected because if the can is well grounded, I just grounded myself as well.   Now if I walk back about 8 feet from the pitch antenna to where the amplifier is and I touch the amplifier ground (which is connected to electrical ground through a 3 prong plug and is the source of the ground for the wire connected to the grounded object   hanging close to the pitch antenna and the theremin ground) the tone goes down in frequency so since the antenna oscillator is still at a lower frequency than the reference oscillator when the pitch went down that meant the effective "hand capacitance" went down.  If I am 25 feet from the theremin and I touch a grounded object the pitch also falls.  If I plug the wire to the grounded object into one of the unused front panel jacks instead of an unused amplifier jack exactly the same thing happens as expected.

Given that the capacitive loading on the antenna is proportional to 1/log10(4x) I understand that you can be far away from the antenna and change the capacitance enough for the tone change to be noticed but I would think the tone would still go up, not down when I touch ground.  Given that the antenna oscillator is at a lower frequency than the reference oscillator.

I guess I just don't understand how I can effectively reduce the apparent capacitive loading to the antenna by touching ground 20 feet or more from the theremin.

I also need to get more sleep...

"I guess I just don't understand how I can effectively reduce the apparent capacitive loading to the antenna by touching ground 20 feet or more from the theremin."- Greg.

No, I cannot see a reasonable (or even unreasonable - LOL ;-) expanation for what you are describing..

"I also need to get more sleep" - One can make silly mistakes when tired - this would not explain it, but, lets say you had the null wrongly tuned, and a decrease in difference frequency actually was the result of decrease in VFO frequency, well then at least the most bizzarre aspect of what you describe would vanish..

Fred.

Earth/Human ground problem..

Reminds me of a cartoon I saw back in 1969 I think..

Astronought on the moon unpacking equipment - 3 leaded wire, one wire labelled "Earth"..

So: a single high frequency transistor with a coil, capacitor, and antenna is doing something weird in a complex RF environment.  This could be any number of things, most of them not at all germane to what you are attempting to investigate (perceived logarithmic / linear response of a highly specific instance of delta capacitance).  When this happens to me I tend to go back to basics.

Fred and dewster,

Thanks for the comments and suggestions.  One other piece of informaiton that might help determine what is going on is if I hold the VCA on with a jumper and I touch the volume antenna the pitch also goes down.  When I touch the volume antenna I stall the oscillator and the path back to Vcc (AC ground) is through a 1000mH inductor connected in series close to the antenna, the IF transformer primary and a 680 ohm resistor.  The path to ground is through the collector of the oscillator BJT which is biased to be in the small signal region with a 56K resisotr on the emitter to ground and a 470pF cap (also connected to the emitter) back to the IF transformer primary tap point.  The base is connected to ground through a 100pF cap and a voltage divider to Vcc and ground with 1500 ohm resistor to ground and a 3300 ohm resistor to Vcc.  That part of the standard Theremax circuit is unchanged.  This is real even after some sleep!

Greg

Oh,

And one other thing, if I take the volume antenna off the Theremax the amount that the tone drops when I touch the keyboard amp or the grounded object 20 feet away - which is my soldering iron (cold) is less but still there.  It doesn't seem to matter what the volume oscillator frequency is or if the pair are close to one another in frequency.

On another topic are the LEV results I put on the linearization coil thread at all surprising?

Greg

"On another topic are the LEV results I put on the linearization coil thread at all surprising?" - Greg

Greg, I dont want to sound negetive or anything, but while you are having the weird behaviour you are describing on this thread, I have serious reservations about the "validity" of any results you are measuring..

I am assuming you have no series (equalizing) inductor between tank and antenna - and I think Dewsters "back to basics" comment is spot-on..with these conditions lets examine what should be going on..

(please excuse the gross over-simplification in the following.. I try usually try to make my postings understandable to as many as possible, and really go to basics - I realize that you are well "above" this)

Your tank has parrallel L and C which determine its frequency, lets first forget "antenna" and "ground" labels -  across the tank inductor there will be a fixed C and and a variable C (the player).. There are two "terminals" across which this variable capacitance is normally connected.

One of these "terminals" should be connected to "ground" (this is usually achieved capacitively - it may well be that this "terminal" is physically connected to the +V supply, and this is coupled to groung through a capacitor connecting HF AC signals from +V to "ground" [0V]) - When this "ground" terminal is then connected to physical ground, all wiring / equipment etc which is also connected to ground becomes a large 'plate' of the capacitor in parrallel with the tank inductor.

One potential problem with the above can occur if there is a large inductance between the "ground" terminal and physical ground - This inductance can be created simply by the wire connecting the "ground" terminal and "physical" ground. The higher the frequency, the more significant this inductance will be.

The other "terminal" is connected to the "antenna" - This is the connection for the other "plate" of the capacitor across the tank inductor, and in itself forms part of this plate - as in, it will form a capacitor with the "ground" plate.

Then there is the player - he/she acts as a 'floating' plate - the largest coupling will be the body to the physical ground 'plate', which should usually be in the order of 100pF or so, then the physical position of the player / hand will determine the additional coupling of "ground" to the antenna "plate":

Ground ---||---------/\/\/\--------||-------Antenna------||-------Ground

Player:   100pF       5k           <2p    Background:   10p

The above capacitances should be in parrallel with the tank inductance -

Problems occur when, for whatever reason, these capacitances are not in parrallel with the tank inductance, are only partially in parrallel (have other significant coupling to other circuits) or have other components in series with any element in the above schematic (particularly inductance, high resistance, or small capacitances)

From the above, it can be seen that (usually) the "actual" potential of the "ground" wiring of your house WRT the "physical" ground outside should not really cause problems (other than for your safety) if the theremin is connected to the 'bulk' of ground wiring in the vicinity where you are playing -- Ground in just a "plate" on one side of the tank inductor, the antenna is just a "plate" on the other side of the tank inductor - It is a two-plate capacitive sensor, and provided you are in the field of these two plates, it should work as expected.

Which is why I cannot explain your 20' sensitivity or why the oscillator frequency increases when you do things that should increase capacitance and therebye reduce oscillator frequency.

Fred.

"if I take the volume antenna off the Theremax the amount that the tone drops when I touch the keyboard amp or the grounded object 20 feet away" - Greg

I would remove power from the volume oscillator if I was debugging this circuit - its confusing enough without adding anything which might cause complications!

- in fact, I would also remove power from the reference oscillator, and just look directly at the VFO frequency while debugging the problem (taking care not to load the VFO when doing so).

Fred.