Voltage Monitor for an LC circuit

[Paul Colby]

series resonance may work better on the primary...

Yes, agreed. This might put more current into the primary.

I've been thinking that a power mosfet might make a good class C or D drive for the primary. Basically use the hp function generator to switch a current off and on at the desired frequency. If the Q is reasonable the voltage at the crystal will be near sinusoid? Anyway, I'm out of town for a week and this will have to wait till I return. Thanks all for the help.

 

[Baluncore]

Try this tapped coupling. Trigger on Channel 1, adjust frequency for zero phase between scope channels.

 

[sophiecentaur]

Okay, ran a inductance calculator on line and I get 762 uH (very close to what my hand held L meter says) http://www.66pacific.com/calculators/coil-inductance-calculator.aspx

I think you need the resonance to be across the whole coil, tuned with the intended load capacitance across it. The resonance of the bottom section will always be low Q because it has 50Ω to ground (the generator source) or a series R which uses up your available volts. The 50Ω in the bottom end would (I think) be transformed by the turns ratio squared( =5k) for the full length resonance. I would be inclined to look at that with a really high impedance take-off (a high resistor or, my suggested loop probe. A higher turns ratio could be worth trying - easy to implement. The Capacitor 'cell' should be connected by a short rigid 'L' of wire to the top terminal and the bottom end, nice and tidy with short wires to the tap. Personally, I would always use soldered joints and arrange things like the construction in that link I gave you.
Was there a reason for the cell to have such a low capacitance? Strays have a lot less effect if the C could be ten times that value. Some sort of compromise on the added source resistance could still give you high volts if, as I suspect, the main source of damping is the generator - despite the reduced source volts.
It looks as though you are getting places at the moment. That can't be bad.

 

[sophiecentaur]

r, series resonance may work better on the primary.

OH yes - I just found that in your earlier post. The Q could be better in the input resonance.

 

[Tom.G]

The circuit that @Baluncore suggested but using the Quartz crystal in place of the 1nF (and with correction for the inductor value) would be a good next step. Resonance will be around 3MHz, with a Q≅50 when driven with the 50Ω generator. This will give a theoretical voltage step-up of 400. Take that into account before you put a 'scope probe on the secondary... 'scope repairs are Expensive.

Try this tapped coupling. Trigger on Channel 1, adjust frequency for zero phase between scope channels.

 

[sophiecentaur]

I've been thinking that a power mosfet might make a good class C or D drive for the primary.

A different RF Power source could always help; 50Ω would not necessarily be the idea source impedance. But do you want to get involved with RF Power Amp spec and design? You need to find a local radio ham who could get involved and bring all his test gear with him (as well as his knowledge).
Can you just confirm that it's high E field across the quartz crystal that you're after and that other circuit parameters can be chosen to produce it? (The gap is presumably determined by the length of the crystal).
PS I think the actual coil design is probably the most important factor in this and that's a pain. Winding large coils is hard work and a good inductance calculator could save time and trouble. I found this one which is all singing all dancing but a bit overwhelming with the information it produces.
Edit: I added the link URL

 

[Paul Colby]

But do you want to get involved with RF Power Amp spec and design?

Money can solve nearly every problem. The real approach to this is spend the time and write a just the facts paper on the intended experiment with some reasonably complete S/N calculation showing the level of difficulty. I kind of view this activity as learning enough to do this with some confidence. To get a paper to the level needed for publication in a real journal is significant work in itself. Frankly, I would likely meet my goals just by getting a paper on arXiv.org but I don't know if it would even get past the spam filters.

Anyway, it's o-dark-thirty and time to go to the airport (I hate flying). I'll be away from the "lab" for a week or so. Thanks

 

[Paul Colby]

Okay, I keep forgetting that voltage dividers don't have to be resistors. Clearly a series combination of capacitors does what I need

Configured this way, the voltage drop across the 350 pF cap is 1/100 that appearing across the 3.5 pF. Also as an added benefit, the net capacitance of the pair is only slightly more than 3.5 pF.

 

[Baluncore]

The problem there is that the signal generator's 50 ohm impedance is in series with the LC circuit and so will spoil the Q.
You need something like post #37 where the inductor is also a fixed ratio autotransformer and the crystal takes the place of the capacitor.

 

[Paul Colby]

The problem there is that the signal generator's 50 ohm impedance is in series with the LC circuit and so will spoil the Q.
You need something like post #37 where the inductor is also a fixed ratio autotransformer and the crystal takes the place of the capacitor.

Yes, the previous post, #43, just shows my favored solution to the initial problem posed, how to monitor the voltage waveform across the 3.5pF cap without cap loading the circuit or blowing up my scope. I've spent my time away MacSpiceing several alternative drive circuits. Modeling the taped coil was fun and informative. I'll post some sims and data after I recover from traveling tomorrow.

 

[Baluncore]

Remove the .txt extensions to make Ltspice .asc and .plt files.
R1 = 10k drops a voltage in phase with the drive current.
R2 = 10Meg is the oscilloscope Chan-2 input set to x10.
R3 = 50 ohm matches the signal generator and may be the oscilloscope Chan-1 input, 50 ohm if available.
Ch2 will give the peak signal, pp / 11.
Tune for zero phase between Ch1 and Ch2. Notice the deep dip at resonance.

 

[Paul Colby]

R3 = 50 ohm matches the signal generator and may be the oscilloscope Chan-1 input, 50 ohm if available.

Thanks for the sims. I'll translate to MacSpice and give it a go. One question though, shouldn't R3 be in series with V1 to simulate the internal impedance? The way it is draw infinite current will flow if the generator (V1 + R3) is shorted?

 

[Paul Colby]

Okay, yesterday was spent playing with the circuit shown in #43. The capacitive voltage divider worked as advertised which is gratifying. However, as noted by several just the preponderance of stray capacitances will make this circuit problematic. Indeed, waving a hand about changes the resonance (a lot) which still shows up 1MHz low by my estimate. So, random capacitances to ground are a serious issue. Perhaps some careful layout and fabrication could minimize this but I think I've determined (hence the point of this exercise) that this approach has dragons. On the up side, this effort suggests changes in approach that I will explore so more later in another thread with a new set of issues.

 

[Baluncore]

One question though, shouldn't R3 be in series with V1 to simulate the internal impedance? The way it is draw infinite current will flow if the generator (V1 + R3) is shorted?

You are correct, the generator will probably have a 50 ohm output impedance. I should have set it's parameter Rser=50 ohm. R3 is either an external termination or the CH1 input to the scope which provides the generator output with a matched 50 ohm load impedance. I did not model that bit accurately, because I had less than 10 minutes to get it together and was more concerned with getting a realistic estimate of the Q. The modeled mismatch does not change the simulated Q.

How could ( V1 + R3 ) be shorted? The model is numerical, the real generator will have an output current limit.

To reduce the stray capacitance, construction would place the crystal capacitor inside the inductor, with the inductor inside a copper box. I would ground the centre of the coil and feed it one turn from the centre.

 

[Paul Colby]

How could ( V1 + R3 ) be shorted?

What could possibly go wrong ? That's just an internal mental consistency check I run. Making equivalent circuit models is an art unto itself.

To reduce the stray capacitance, construction would place the crystal capacitor inside the inductor, with the inductor inside a copper box. I would ground the centre of the coil and feed it one turn from the centre.

Really good thought, however nothing removes the stray coil to ground. A copper box will solidify the problem (make static) but not remove it.

 

[sophiecentaur]

To simulate a 50 Ohm output resistance with a shunt resistor, can't you just have a current source, in place of an ideal Voltage Source? (It's been a while since I did that stuff.)
Also, I seem to remember that self Capacitance of a coil can be reduced by spacing the turns and having uninsulated wire.
It all depends on the precise requirements but there will be an optimum L value (possibly lower than the existing one). Simulation could save any more winding than necessary.

 

[Paul Colby]

Also, I seem to remember that self Capacitance of a coil can be reduced by spacing the turns and having uninsulated wire.

This was somewhat of a revelation for me. I've always thought the self capacitance of a single layer air wound coil was a function of winding spacing. Apparently this isn't true. I'd like to track down the actual papers since the physics sounds interesting. My estimate of the (my) coil is the self capacitance is ~6.5pF or so.

http://coil32.net/theory/self-capacitance.html

http://www.g3ynh.info/zdocs/magnetics/appendix/self_res/self-res.pdf