The bifilar pancake coil at its resonant frequency

[tinman]

I get 1.02ghz.

Are you sure on that resistance of .1ohm? Just that I have 100ft of 14ga and it is .25ohm.  Just really looks like some stout wire there and fewer turns than 55 or so i have.

Mags

Thats the best i can measure Mags.

If i short my DMM leads together,i get .3 ohms,and across the coil,i get .4ohms.
My DMMs only have a resolution of .1 ohm

So how are we determining the self resonant frequency ?
Keep increasing the frequency until maximum voltage amplitude is reached across the coil?

Bellow is a pic of the two coils together .


Brad

[Magluvin]

I get 1.02ghz.

Are you sure on that resistance of .1ohm? Just that I have 100ft of 14ga and it is .25ohm.  Just really looks like some stout wire there and fewer turns than 55 or so i have.

Mags

When checking very low ohms and I consider 1 ohm pretty low, the connections of the meter leads to the wire really need a strong connection as a lack of solid contact can alter the true value to a higher reading than what it actually is.

Back it the early 90s I bought my first quality multimeter. Wavetek 2030. $329 back then and it seemed to be the only handheld at the time that could go to .01 accuracy. Well in amps and such there are resistors that go to that second decimal in accuracy and the best fluke couldnt read it to that accuracy back then. Still have it. It has a relative buttion that will record and subtract the test lead resistance to zero it out to 0.00 ohms leads shorted.

Other guys in the electronic repair shop didnt believe it would be accurate but IT showed them.

You can find the shorted IC or other component on a circuit board with many ICs and caps on a say 5v line on the board just by using the meter. To be able to read to .01ohm you will be able to track that part down without desoldering everything to isolate that bad part. Not many people know about that.

Mags

[tinman]

yeah you can do a freq sweep to find it. I use circuit sim to do simple tests like these to help find the freq with very good results. Thats how i found the 1.02ghz for u. it should be right on or close enough to find the area, just incase there is a slight discrepancy in a component value reading that could throw of the sim a bit if the values are not dead on. I like that sim. Simple stuff can be checked quick and reliably. The default circuit is an rlc so i just plug the values and apply power then disconnect and it rings and the res freq result is at the bottom right of the screen.

Mags

Ah,see we were typing at the same time.

If it's 1.02GHz,then were screwed,as my SG only go's to 20 meg.

From last nights quick play,maximum voltage amplitude was reached at a far lower point-around 40KHz ,if i remember correctly.

Time for some testing. Back soon.


Brad

[Magluvin]

Thats the best i can measure Mags.

If i short my DMM leads together,i get .3 ohms,and across the coil,i get .4ohms.
My DMMs only have a resolution of .1 ohm

So how are we determining the self resonant frequency ?
Keep increasing the frequency until maximum voltage amplitude is reached across the coil?

Bellow is a pic of the two coils together .


Brad

Yes the voltage on the coil will be highest at resonance.

Mags

[Magluvin]

Ah,see we were typing at the same time.

If it's 1.02GHz,then were screwed,as my SG only go's to 20 meg.

From last nights quick play,maximum voltage amplitude was reached at a far lower point-around 40KHz ,if i remember correctly.

Time for some testing. Back soon.


Brad

Your inductance seems quite small compared to mine and TK, if that value is correct at .06nh where I have7nh and tk got around 1 or 2nh. On sim I may be wrong and it may be inaccurate at those freq or what ever. There are online calculators and you can plug in the values and get an answer. It does seem like a huge jump in freq in comparison so maybe we should try another calc to confirm that no.

Mags