The bifilar pancake coil at its resonant frequency

[Magluvin]

I have a bifi coil made of 26ga.  The dimensions  are 1in w 2 1/8in dia and 5/8 air (some plastic) core. Dont have the turns no.  It is 1.16mh and 16.07nf.  Switching on dc then cutoff the scope says 14.88khz for the wave developed.  Calc says like 35khz  Anyway,  Im wondering if this coil needs to be run in the mhz range like what you guys are finding to get the results you get.  Could it be more the length of the wires that is dictating that upper freq range above the normal resonant freq that needs to be hit?

Mags

[Magluvin]

No....no.....no....

Hmmm.  Well Im trying to figure it out.  Seemed like a relationship of what was said and what is happening was developing. 

But Ill nix it for now...

Back to the drawing board...

Mags

[TinselKoala]

Mags, that Hantek is a nice looking unit... but does it really say "Forse" on the Force Trigger button?  That's even worse than the Rigol's "pluses" for pulses error, because at least you don't have to always see the "pluses" in the Rigol!



A decent stand-alone DDS AWG or FG will produce the standard 15 v p-p into 50 ohms and should be able to provide a Watt or two of power I should think. Some older analog units like my Interstate F43 are considerably more powerful than that... the F43 will do 40 v p-p into 50 ohms. But it is limited to about 4 MHz max frequency and at the top output and top frequency the sine wave starts to distort. That may just be down to old age though... I should probably re-cap that thing at some point.

[TinselKoala]

Hmmm.  Well Im trying to figure it out.  Seemed like a relationship of what was said and what is happening was developing. 

But Ill nix it for now...

Back to the drawing board...

Mags

Actually it depends on the circuit and the exact type of coil. The "ordinary" TBF pancake or solenoid coil is quite stable when voltage is varied when only connected to a minimal circuit, say, to establish resonant frequency by itself. But we have seen more complicated circuits where these coils and other coils do change their characteristics depending on the voltage supplied to the circuit and also to surroundings which can interact through capacitance, and this IS highly voltage dependent. SO... no no no, and also yes yes yes.

[TinselKoala]

I have a bifi coil made of 26ga.  The dimensions  are 1in w 2 1/8in dia and 5/8 air (some plastic) core. Dont have the turns no.  It is 1.16mh and 16.07nf.  Switching on dc then cutoff the scope says 14.88khz for the wave developed.  Calc says like 35khz  Anyway,  Im wondering if this coil needs to be run in the mhz range like what you guys are finding to get the results you get.  Could it be more the length of the wires that is dictating that upper freq range above the normal resonant freq that needs to be hit?

Mags

To get the actual distributed capacitance you have to work backwards from the resonant frequency and measured inductance. Just separating the two half-coils by disconnecting the centertap link and measuring capacitance between the two halfs doesn't give you the correct distributed capacitance value. SO get the ringing frequency from your scopetest, which is 14.88 kHz (nice and low due to your high inductance of 1.16 mH) and go to the Resonant Frequency Calculator here:
http://www.1728.org/resfreq.htm
and enter the freq and inductance and solve for capacitance. This will be your correct interturn distributed capacitance. If you have the inductance and capacitance values correct that is.


Yes, at the high frequencies being used to see the "effect", inductances of probe connections and other wiring become significant.