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In Reply to: RE: Choke VS Transformer posted by Triode_Kingdom on January 31, 2016 at 09:49:33
Because of the coupling, any current flowing through one half of the winding creates an equal but opposite current in the other half.
Correct. It behaves as a 1:1 inverting transformer and since it needs to be inverting, bifilar is not an option. There was a large discussion here a number of years back about this and it is still my belief that using the terms "Autoformer" or "CT choke" stem from how the circuit is often drawn and not how it actually operates. My description of the circuit has the primary of a 1:1 inverting transformer acting as a grid choke for the in phase signal to one of the output tubes and the inverted secondary providing signal for the other output.
dave
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Your drawings are correct and the "in phase" bifilar will have very wide bandwidth. A bifilar works because even though there is a large capacitance between windings the voltage gradient is such that there is very little AC potential between adjacent wires. When you invert on of the windings you have have that capacitance to ground which makes a nice path for your high frequency information.
Here is the simplified circuit in spice.
the 5n of capacitance is arbitrary and going to a slightly more elaborate model shows the same behavior.
Now to be fair I removed the obvious offending 1N cap to ground and the behavior is still there.
The moral of the story is do not invert a bifilar unless you want a nice lowpass filter. In practice the inverted bifilar typically used as an IT kills everything above 10Khz but the simple nature of this filter can actually be used to your advantage at the output of a Dac where a 40khz brickwall filter spells sonic death.
dave
Well, I guess the impedance in this application is a deal breaker for out-of-phase bifilar. I've designed and measured such transformers out to frequencies approaching 1 GHz, but that was always much lower impedance. I do see your point about about strays and opposing voltages.
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Buy Chinese. Bury freedom.
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