Dewster, my simulations demonstrate that a 330 pF can suppress 50 Hz by 160 dB, without reducing oscillator sensitivity.
But about the TDK inductor you are right, I have simulated it with serial resistence and parallel capacitance and (with your circuit DewsterV3 with 100pF input cap) the sensitivity decreases from 4% F/pF to 2.32% F/pF
It is interesting to notice that in the CapSensors original circuit (with parallel inductor) this inductor decreases the sensitivity only from an ideal 2.66 F/pF to 2.20% F/pF
"So I set the trigger delay to 2.5/60Hz = 41.66ms and am seeing about 50ns of variation with a 5pF capacitor in place of the antenna. The coil, and possibly everything else connected to the oscillator, is picking up 60Hz. First order of the day then is to construct a counter-wound coil and give that a go." - dewster
IMO it is better to include all in a metallic box grounded.
I do not understand the 50ns, you mean an instability (noise)? If this is the case it is exaggerated, with 2.5 MHz the total cycle is 400ns and 50ns is more than 10% of the base frequency. With a scope the wave must appear completely steady.
"Looking at some VFO circuits, many include a diode from ground to gate in order to control amplitude. This might be able to do double duty in helping with ESD." - dewster
Yes some people adds this diode, I was preferring do not add it, because it worsens the characteristics of stability with respect to temperature. But I believe, that with the serial inductor circuit, we will have to add it. Tomorrow I will make the simulations. This diode is expected to much change the operation point, because it it greatly reduces the voltage swing on the gate.
please, let us design these oscillators without burning them, every day, with your lightnings.
When it will be finished, every one of us will be able to add any additional device, discharge tubes, temperature and atmospheric pressure corrections, heterodyning... But before we must try to make a heart with good features." - Livio
That isnt the way I do things - IMO there is no point in designing a "heart" which has a potentially fatal flaw in it.. You could get a heart that works beautifully when a person is resting, but claps out as soon as they start moving..
By not looking at protection devices as part of the initial design, and by rejecting every comprehensive analysis which includes reality and isnt just in an ESD-free simulation, you risk getting a perfect oscillator that is so sensitive it dies.. Thinking about these issues right at the start is as essential as selecting realistic and obtainable components.
But the reason I mentioned ESD wasnt because I wanted to "pound my drum" - it was because you made claims about the intrinsic "protection" in your design due to the FETs diode effect - This claim was an error, just like your claim about protection afforded by your 18pF capacitor was an error... Making errors is FINE, we all do it - And having our errors pointed out helps us to improve our designs... But ONLY if we learn from our mistakes. You, on the other hand, argue about everything and keep going in the same direction making the same claims as if oblivious.
If you dont want me to speak about ESD, dont make erroneous declarations about your designs immunity to ESD! - And the same is true about other declarations on matters such as sensitivity etc - Dont make claims that are or might be untrue unless you are willing to have these claims examined and errors exposed if they are found. In all science and engineering, one can make hypotheses and float ideas freely to be validated or refuted - but when one makes declarations as "facts" you need to be able to justify your claims.
The other aspect of design is up a level - There is absolutely no doubt that the best way to counter thermal drift is by having two identical oscillators, one as a reference, and one as a variable, and heterodyning these - So its not just about designing an oscillator and then tacking on ESD protection and heterodyning later if one feels so inclined - Likewise the issue of oscillator sensitivity - the requirement for sensitivity is hugely influenced by the way one derives difference behaviour - One could have less than 1/10th of the oscillator sensitivity if one was using heterodyning, and still end up with greater resolution.
" The coil, and possibly everything else connected to the oscillator, is picking up 60Hz. First order of the day then is to construct a counter-wound coil and give that a go." - Dewster
It may be the coil - but dont be too sure about that!
I found that electrical fields (as opposed to magnetic) from local wiring was the major cause of most problems I had on this matter, and that the "antenna" was the primary source.
An easy way to determine whether its capacitive or inductive coupling is to have an insulated wire connected to your live mains feed, move this wire towards your antenna in a way so that capacitive coupling to the antenna is maximised, but inductive coupling to the coils etc is minimised..
If you find the mains "hum" increasing as you approach the antenna, its a strong indicator that the problem is electrical (capacitive) coupling IMO.
(Just thinking bout it, the other way is to remove the antenna and replace it with an equivalent capacitor - if hum is from the antenna, it should drop hugely.. why didnt I think of that before... ;-)
"I found that electrical fields (as opposed to magnetic) from local wiring was the major cause of most problems I had on this matter, and that the "antenna" was the primary source." - FredM
Yeah, I think you're right. I made a coil consisting of two 0.3mH windings on the same form but wound in different directions (CW & CCW) spaced ~20mm apart so the destructive coupling is only a few percent. With my scope trigger delay at 41.66ms (2.5 60Hz periods) I'm seeing pretty much the same 60Hz wobble. I've got a bit of a rat's nest going, but I'm inclined to think it's coming in the antenna because it is much lower when (as you suggest) I replace the antenna with a capacitor.
"An easy way to determine whether its capacitive or inductive coupling is to have an insulated wire connected to your live mains feed, move this wire towards your antenna in a way so that capacitive coupling to the antenna is maximised, but inductive coupling to the coils etc is minimised.."
Makes sense, I'll give it a shot - thanks Fred!
There's something going on with the RF in our house. In the morning it's pretty quiet, then all hell seems to break loose until the evening. Working on these exquisitely sensitive circuits is kinda weird. I'm pretty sure I can see when my wife is walking around upstairs in the room above my workbench!
Putting a 10pF in series does seem to cut hum, but it also cuts sensitivity. I think the reduction in both is likely the same, and is governed by the ratio of the antenna capacitance to the added series capacitance. So I'm inclined to believe this series C thing isn't all that applicable to this circuit. And I kind of wonder if it is applicable to any Theremin circuit, because the antenna has to be a high impedance for the whole Theremin thing to work, but the only way a small C can form a high pass filter is if it "sees" a fairly low impedance.
Though there may be some value in letting the antenna "float" up to the voltage potential found at that height off the earth.
Another thing I see with the series C is a fairly long settling time before the frequency is really stable. Could be any number of causes but I want to snap the Theremin on and start playing, and not wait around for it to "warm up".
There are several ways to get around the 60Hz field problem:
1. Integrate measurements over an exact period. But this limits response time too much.
2. Add a high Q comb filter like I pointed to above.
3. Make the antenna somehow respond differentially to 60Hz fields, but not to hand capacitance.
People who make EEGs and such incorporate all of these methods to squash 60Hz hum.