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Registered Member #540
Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969
I don't know if this has already been said (I don't understand all that is going on...) but I think I have an idea for how pendulums can be 180 degrees out of phase. I would think that they would have to be coupled by something other than a magnet. So say you have two pendulums, equal in length, coupled by a rubber band or something else elastic. You start the pendulums 180 degrees out of phase and the momentum maintained by the pendulum that is coupled to the other causing the rubber band to stretch. The rubber band will slow down the coupled pendulum and then cause it to swing back. They pass over each other at "0" and continue moving until the rubber band is tight again. And it continues. I don't know if I explained that clearly... I also don't know what would cause it to get out of phase in the first place.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hello to all,
Need to make a big post now... I was tired last days and this takes me a lot of time!
coronafix wrote ... Not that i know anything about this, but is a standing wave being created and reflection from the receiver creating the 180 shift? It would seem that it would need reflection to create the 180 shift, and therefore it's not a case of 'memory', but of mirroring and getting locked onto the 'image' of itself in the receiver. Just my thoughts on it, could be far off.
Well, the transmitter can be considered as producing a standing electromagnetic wave. But, with vacuum wavelength at my frequency is so large versus dimensions of the device that the whole notion is very vague I think. Basically the standing wave has a half wavelength equal to length of my wire loop - simply because the loop is of such length, and the capacitor performs electrical lengthening.
I could brag as well that electromagnetic radiation passes from transmitter to receiver via quantum tunneling, but why bother when we can all understand it as a weakly coupled transformer.
Steve Conner wrote ... As for what memorizes the 1 bit of information, consider this: A two transistor flip-flop (aka "bistable multivibrator") can remember 1 bit of information. Yet none of the components it's made from is capable of memorizing anything by itself.
(Of course for my argument I have to ignore the kind of crude write-only memory, all too familiar to 4hv members, where the component is functioning for "1" and blown to pieces for "0".)
So where is the memory of a flip-flop, if it's not in any of the components? If you can answer this, you can answer your own question, since your circuit is just another kind of bistable.
I won't answer my question yet: I'll leave it open here for discussion. Hint: non-linearity.
Conner... Well, flip flop memorizes it's state using positive DC feedback which I find harder to evisage between two AC coupled resonators.
All I could understand from you is that since mosfets are used very nonlinearly - being either off or in saturation - somehow allows the circuit to ''get stuck'' in one mode with more gain. Not sure why though, nor why is gain of the in-phase mode at first high but drops off a lot after coupling is reduced and can't be brought back.
I need a lot more learning to get through this :(
coronafix wrote ... Steve, anymore information on this? Marko, what are you using to drive the transmitter? Can you supply some info on your setup? I have been thinking about this for a week now and no closer to understanding it, I would like to reproduce your experiment. Are you able to supply a schematic? Thanks.
The electronic circuit is a Royer oscillator built around IRFZ44 mosfets, driving a parallel LC tank consisting of a copper pipe loop and 40nF of capacitors. It is fine tuned by inserting a piece of ferrite into the loop. Receiver is an equivalent tank connected to a 24V bulb over a small matching inductor.
Here is the schematic:
For pendulum oscillator, a nearly identical royer circuit is used, except, the parallel LC tank is replaced by a pendulum hung on it's axle, and inductors (which would be impractically large for frequency of only few Hz) are replaced by linear constant current sources.
Rocketry76 wrote ... also, can you check with a spectrum analyzer if there is more distortion in the waves when you are operating 180 deg out of phase.
I thought that some spectrum analysis would be interesting too, but unfortunately I don't own a spectrum analyzer. The scope shots were more than enough for what I needed in school.
If I come to bringing the thing to school again I might try.
Steve Ward wrote ... My guess is that the system moved into a more stable state, kind of like a ball at the top of a hill, but the top of the hill has a little hole that the ball sits in. You just knocked the ball off the hill and cant get it back up there. Now as to why the ball starts there to begin with???
I haven't thought about coupled systems in awhile, seems like a good brain teaser for when things are slow at work.
For anyone who knows controls (or math): can these systems be solved for with a state space model or something like that? Or would your model have to throw away the behavior Marko mentioned before (the non-linearity). Im only familiar with linearizing your model to get rid of these phenomena, but id like to understand it better!
Neato indeed.
I did a quick test just now on a smaller DRSSTC measuring both primary and secondary (Base) currents. With the primary tuned really low, and no real visible beating in the current envelope, the phase stays locked (at 0* i believe, not 180*). Tuning the primary a bit higher so beating is apparent shows a slight phase drift on the secondary from being locked with the primary, its clear to see the energy transfer as the secondary current peaks when the primary current drops off. Tuning the primary even higher leads to a 180* phase shift, almost no sparks, and very high primary currents... seems the system really doesnt like that at all. What im curious about now is, how would the phasing look if i took feedback from the secondary side? I find it funny that i dont really know the answer to this after years of playing with these things. My mediocre controls knowledge fails me... well because this is not a typical control problem.
Steve, it is interesting that what you are mentioning is somewhat different of what Steve Conner told me regarding his coil...
He claimed his coil worked in both modes, but in the in-phase mode it flashed over massively... while 180' one gave good output without flashovers.
Still what you say looks about right -tuning lower with primary feedback excites the lower pole, forcing the in phase mode, and tuning higher excites higher pole and 180' degree shift. Apparently your direct feedback system is simply always picking the in phase mode when tuned for biggest sparks... I don't know why doesn't the higher pole produce sparks though, like Steve Conner's circuit does.
Regarding secondary feedback - after some chatting with Conner, I came to bizarre realization - it apparently just doesn't matter much, regarding the energy transfer, which feedback phasing we take - but rather, we just pick which mode will system operate by changing the feedback phasing!
What I wanted to ask you, Steve, did you actually observe anything like that?
I'm asking this because you're one of few people who actually experimented with secondary feedback before everyone else copied your drsstc1 design which became standard.
Myke wrote ... I don't know if this has already been said (I don't understand all that is going on...) but I think I have an idea for how pendulums can be 180 degrees out of phase. I would think that they would have to be coupled by something other than a magnet. So say you have two pendulums, equal in length, coupled by a rubber band or something else elastic. You start the pendulums 180 degrees out of phase and the momentum maintained by the pendulum that is coupled to the other causing the rubber band to stretch. The rubber band will slow down the coupled pendulum and then cause it to swing back. They pass over each other at "0" and continue moving until the rubber band is tight again. And it continues. I don't know if I explained that clearly... I also don't know what would cause it to get out of phase in the first place.
Well, Myke, you need to realize that the string needs to be stretched all the time -otherwise my coupling wouldn't be constant. To reduce coupling I had to tie it very close to pendulum axles. (I used the coupling band in line with oscillation firstly).
To force 180 degree phase shift with pendulums, I do exactly the same thing as with DRSSTC - tune the excitator pendulum higher
I actually had big trouble attaining the 180' shift mode for longer time, I'd either lose oscillation or revert back to in phase mode (despite lowered coupling). I seem to get a period of heterodyne after which oscillation is knocked out. I have been able to observe this mode for some time though.
Thanks for your replies guys, hope to see a lot more in this thread,
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
Well, flip flop memorizes it's state using positive DC feedback which I find harder to evisage between two AC coupled resonators.
Have you considered the feedback between your coupled systems? I dont think Conner was trying to point out anything specifically about the electronics (like mosfet characteristics), but rather the basic physics.
Regarding secondary feedback - after some chatting with Conner, I came to bizarre realization - it apparently just doesn't matter much, regarding the energy transfer, which feedback phasing we take - but rather, we just pick which mode will system operate by changing the feedback phasing!
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Well, I think it does have something to do with the MOSFET characteristics, though I must confess I don't really understand it myself, and I was hoping someone like Waverider would step in :P Anyway, the best explanation I can think of is as follows:
An oscillator can be thought of as a tank circuit, connected to an amplifier that provides positive feedback. In this instance, the tank circuit is the system of two coupled LCs, and the amplifier is the cross-connected MOSFETs we all know and love.
When power is applied, the oscillator starts because of thermal noise, which gets amplified by the positive feedback. The oscillation will start at whichever frequency has the most positive feedback: which is to say the greatest "loop gain". If no frequency has a loop gain of 1 or more, the oscillator won't start. (Loop gain < 1 is just a mathematician's way of saying that the amplifier doesn't have enough gain to overcome the losses in the tank circuit.)
The double-tuned tank circuit offers a choice of two possible frequencies, and fiddling with the tank circuits changes the relative loop gains of them.
Now, in order for the circuit to hop from one frequency to the other, it must undergo the startup process I described above, where thermal noise grows into oscillations at the new frequency.
My proposed explanation is that the MOSFETs can't "hear" prospective tiny oscillations at the new frequency when they are being "deafened" (driven hard into Class-C) by existing oscillations at the old frequency. They spend most of their time either off, or saturated, and hardly any time in the linear region, where they would need to be to amplify tiny signals.
Or, in more formal terms, the presence of a large signal at one frequency decreases the small-signal loop gains for both frequencies, by driving the amplifier into compression. In fact by definition, the amplifier in such a circuit always gets pushed into compression until the loop gain for the mode it's oscillating in is equal to 1. This is just a formal way of saying that the oscillations don't grow to infinity because the amplifier saturates.
Now, to complete my argument, I just need to propose that this depletion of gain by an existing signal is what punches out the "hole in the top of the hill" for Steve Ward's hypothetical ball to rest in.
If this explanation were true, then there would be no hysteresis if the saturated switches were replaced by a linear amplifier. The circuit would smoothly move between modes, and could possibly even be balanced in the middle where it would oscillate at both frequencies at once.
This could easily be tested in simulation, but in real life it would need some kind of AGC on the amplifier, like the light bulb arrangement in the old HP 200 Wien bridge oscillator. Otherwise small errors in gain would make the oscillations die out, or grow until the amplifier saturated. The AGC circuit, being a source of non-linearity (compression) might then give us hysteresis all over again.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hm.. what can I add...
Conner has put up a very interesting hypothesis... I didn't think that even his understanding isn't complete, but rather just my lack of understanding of his explanation... :P
I don't know how would I test this... I would basically need to build some sort of oscillator like wien bridge oscillator, but with a parallel LC tank in place of wien bridge... now how feasible is that?
Oscillator would need to be high power (at least maybe 5 watts?) which complicates the design due to relatively high dissipation in active elements.
Steve Ward wrote ...
Regarding secondary feedback - after some chatting with Conner, I came to bizarre realization - it apparently just doesn't matter much, regarding the energy transfer, which feedback phasing we take - but rather, we just pick which mode will system operate by changing the feedback phasing!
Yep, that sounds right.
Really - what did you notice, what's the effect on output, and did you do any scoping? I only ever ran DRSSTC with secondary feedback at low power, and there seemed to be a phasing where it worked better than other (which erroneously made me think that the other must simply be some parasitic oscillation).
Thanks you guys a lot for now, but I guess mystery remains..
Registered Member #160
Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938
What if you were to use tubes instead of MOSFETs? Would this help? I still don't see the mode-locking connection though, Waverider. As you say Marko, the frequency you are using is way longer than the distance between tanks so no standing wave can be setup, this would quickly change anyway as the coupling is changed, but yours locks.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Coronafix wrote ...
What if you were to use tubes instead of MOSFETs? Would this help? I still don't see the mode-locking connection though, Waverider. As you say Marko, the frequency you are using is way longer than the distance between tanks so no standing wave can be setup, this would quickly change anyway as the coupling is changed, but yours locks.
Hm. I hope not to get too far off topic, but I believe something needs to be said.
There is really nothing special about electromagnetic waves, and antennas, in overall. Electromagnetic field is just another way of interpreting the behavior of electric and magnetic fields. Electric and magnetic fields, are, in the end, nothing but a way we interpret how charged particles affect each other.
It is pretty much the same to call my oscillating LC tank circuit a resonant cavity, or a small section of waveguide containing a standing wave. Again as I said, there's nothing special about that.
A standing wave in microwave oven or waveguide is really just another way of saying that there's oscillation of energy between some capacitance and inductance.
In other words - When RF engineers say that there is a standing wave in a transmission line, like coax or waveguide, it's really just another way of saying that they have some circulating reactive energy somewhere.
Reflection is simply the result of lack of damping - failure to bring the Q of the cavity or transmission line to 1. The act of doing so is otherwise known as impedance matching.
To get back to the topic, apart from looking on it as a transformer, I also always liked to consider my setup as a waveguide - because it's what it really does; it guides an electromagnetic wave to the load, confining it in with it's low characteristic impedance. I think can even view this as if the waveguide created 'another kind of vacuum', with characteristic impedance of just 2 rather than 377ohms, through which the wave can freely propagate.
It can be envisaged that both LC segments in my system are producing a standing wave of their own - and interference between those two waves if what this thread is about!
With some more mindplay, I can come to understanding of ''real'' waveguide...
I can create a simple approximation of waveguide by spacing my transmitter and receiver further apart and inserting identical LC tanks between them - I will notice that I can even make slight turns and various shapes while not disturbing the power transfer.
Then, I will realize that instead of each LC loop, I can insert two equally spaced ones with 1/2 capacitance for same effect... actually I can divide each section by 2, 4, ... generally until I can't physically fit more capacitors in there; and each division will make the waveguide model more realistic. But, now - if I increase the frequency to some point - I'll be able to get rid of the capacitors altogether! They will be supplemented by distributed capacitance of the copper loops themselves, and my waveguide could consist of bent copper wires hanging in air.
At double frequency from this - which I think, is, the cutoff frequency? - I could even join the wire ends, because it now acts like two parallel LC's which are ''insulated'' from each other by their high output impedances! And for the end, I could realize that, at frequencies over cutoff frequency, such ''insulating parallel LC's'' could be formed even lengthwise, allowing me to get rid of everything and putting a solid copper tube of same diameter in place!
I can't say my understanding of this is closely full - phenomena like refraction, diffraction, and reflection from dielectrics are still heavy unknowns to me (yeah, I'd really love to know how to model a metamaterial from those LC sections!)
But the point I wanted to make with all this, is, that there's really nothing special about all this. I know because I was troubled by this for rather long time.
Once you accept that, everything will start fitting together rapidly!
There's not anything special about tubes either, they aren't really any less or more capable than mosfets for use in linear amplifier, and not any less hard to keep in linear operation like with wien bridge oscillator.
I think I needed to add something like this to my first post in order to make it more understandable to everyone, and for me to recheck my sanity in same time.
I'd also greatly appreciate any honest error reports for this post. Going to do some studying now,
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Hey Marko,
I don't really have anything new to add, but I thought you might like to know that the explanation you give for waveguides is pretty much the same as the explanation for cavity resonators in a couple of old radio books from the 1950s I have in my collection.
One book even has pictures of LC circuits, and then two coils joined in parallel around a single capacitor, and then that capacitor replaced by stray capacitance, and then more and more "coils" (now 1/4 wave lines) joined together until it turns into a solid cavity.
The other one explains a waveguide as a two-wire transmission line suspended on a bunch of 1/4 wave "metal insulators". (It really does call them that.) The cutoff wavelength according to this argument is equal to the guide aperture, since that leaves four 1/4 wave "insulators" with no room for any transmission line "conductors". For frequencies above this, the RF voltage appears in patches on the "ceiling" and "floor" of the guide, with opposite polarity, while a line halfway up each "wall" remains at ground, since it constitutes the "base" of the "insulators".
Waverider may shudder at this crude model, but it helped me a lot to understand waveguides.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
So... anyone with some ideas, about how to construct an LC oscillator with amplifier elements never being in saturation?
I'm having trouble to visualize - if I connect output of a voltage amplifier, like just a voltage follower, to a parallel LC tank, and take negative feedback (over a transformer?) into the inverting input, what would happen? I don't feel such a circuit would work.
And if I have limited supply voltage, and the tank is fed from a voltage source (but only at it's center frequency) why would the amplifier saturate if it's voltage gain is 1?
Registered Member #1143
Joined: Sun Nov 25 2007, 04:55PM
Location: Vilnius, Lithuania
Posts: 721
here 10 minutes job. Simple take SSTC driver, make LC circuit, and set resonance :) wima 2,2uF in shirt time goes very very hot, and L goes hot too (means lot of current from inductor )
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Pendulum experiments and resonant modes, and more
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