Thanks for you detailed explanation.
Yes, actually three resonant frequencies are observed, either in the original publication setup or the modified one. Two positive plus one negative. But for my purpose, the negative one does not magnify voltage and thus is useless. Varying C3 from 20pF to 30pF or even 80pF does not change the overall characteristics of those frequencies, but the only difference is the ratio of two positive frequencies shift slightly, which is a useful parameter for my purpose.
The purpose of linear amp is to amplify a RF signal from a special waveform generator. I am not sure whether a direct connection between that waveform generator with the circuit will work properly. This is a problem I need to take some thought.
I think that RG8 cable is not used for the connection between linear amp with the circuit, instead it was used in between the circuit and the downstream output, for signal transmission. But I do not understand your comments on impedance of RG8 coax. I have never consider that. Thanks for your pointing out that.
I am not sure whether my explanations to your comments are right?
1. An unloaded output with RG8 cable is actually loading combinations of capacitors and inductors of that cable. The total impendance of RG8 cable is around 50 Ohm. Since the impendance is so small, there will be large reflection effect from the RG8, which affects the circuit.
2. A loaded output with RG8 cable will be better, only if the loading impedance is far more than 50 Ohm. That larger impedance will produce negligible reflection effect. Therefore, the characteristic of the original LC circuit will be unchanged.
By the way, would you like to provide me more hints on stress analysis? I don't know anything of that.
Thanks for your patient for reading my long posting.
Your circuit does not reflect the values used by the author. All comments and component names below refer to the authors schematic.
I converted all series and paralleled components to their equivalent (effective) values. I varied C3 from 20pF.- 30pf. This produced only relatively small changes in my plots.
When two resonant circuits are coupled in this manner it forms a much more complex circuit, where each resonant block is highly dependent on the other.
My AC analysis results actually indicate 3 distinct points of resonance, (one negative going and two positive going) which doesn't surprise me at all. Change your sweep settings to logarithmic from 10Hz to 10.0MHz You will then see all three very sharp points of resonance.
I'd like to also mention that I don't understand the authors use of an RF Linear Amp as referenced in his design. This circuit works.. IE, generates very high output voltage because it's virtually unloaded on the output. The author is connecting the linear to the input of the circuit with a length of RG8 coax that's not terminated into 50Ohms on the input of the circuit. The output Z of the circuit is far to high to reflect any load back to the input that would look anything like 50Ohms.
This creates two extremely undesirable issues. The first being an unloaded linear is an unhappy linear and a good idea only if your doing stress analysis!
The second is an un-terminated transmission line is no longer a transmission line. It's a length of cable that looks like lumped capacitance, inductance and resistance. Without a 50Ohm termination on the input a 3 ft length of RG8 will produce very different results than a 6 ft. length. In other words we usually don't want our transmission line to become part of any circuit for any purpose other than being an RF pipe!
Chris