Measuring coil inductance and parasitic capacitance.

[Teleno]

When optimizing a PI it's necessary to know the coil's inductance and parasitic capacitance.

This can be done with an extremely simple circuit and an oscilloscope (alternatively a Freq. meter).

Full description: http://www.robkalmeijer.nl/techniek/...e48/index.html




PCB







Nomograph (for 150 pF)




Partlist

B1, 2	9V
C1	150 pF 5%
C2	33 pF
C3	47 nF
Q1, 2	2N5245
R1	10 kΩ
R2	470 Ω
R3	1 kΩ

[WM6]

Nice approach, I am using similar solution, applying some of applicable wide-band oscillator (in general wide-band X-tal testing oscillator can be simply adaptable for this purpose, by replacing X-tal with coil), but without additional fixed capacitor in tank circuit and without need for coded formula.

So for measurement coil resonant frequency, we take real coil parasitic C and coil inductance.
Those with FFT digital scope, can easily establish coil resonant frequency in Spectrum analyzer mode.
Most PI coil will fall in 100kHz to 1Mhz range. No having scope we can build old known and simple multipurpose measurement means called Grid-Dip Meter (or nowadays Gate-Dip Metar).

Then from coil inductance and measured coil own resonance, we can get coil parasitic capacitance using this nomograph (A, & B line here as example how to use nomograph):



On paper printed Nomograph is not power depended and can be used everywhere, but in same way we can use some web coil resonance calculators as well.

[rickb]

WM6
What book was that nomograph copied from?

[WM6]

Not sure about its primary source, rickb.

I take it from old (former) YU magazine "Radioamater" and a little adapted to here more useful range.

My present solution is with wide-band (easy start) oscillator integrated in out-of- module-made frequency meter (photo):
After measured coil inductance (with L meter) and coil resonance with F-meter, we can get parasitic capacitance value in above Nomograph.

[barwildi]

A calculator for the coil tester...

When optimizing a PI it's necessary to know the coil's inductance and parasitic capacitance.

This can be done with an extremely simple circuit and an oscilloscope (alternatively a Freq. meter).

Full description: http://www.robkalmeijer.nl/techniek/...e48/index.html




...

I just noticed this thread from January of last year. I actually made and use that little circuit for coils too. Very handy and useful!

Here's a little spreadsheet calculator I recently made in Excel for ease of use. Just plug-in your numbers in the "GREY" areas and voila! Makes the task quick for beginners or experts. The estimator at the bottom is pretty useless, but I included it just to mock-up the program that the author provided.

coil_tester.zip

[homefire]

Why not just sweep the coil with a function generator monitoring with a scope to see when it spikes ?

[barwildi]

Why not just sweep the coil with a function generator monitoring with a scope to see when it spikes ?

Hello homefire,

That certainly works also - for everyone that has a scope and a signal generator. As you probably know, many members here unfortunately do not have access to that equipment. The beauty of the circuit Teleno posted is that it requires neither a scope, nor signal generator. Only a cheap frequency counter as bare minimum ($10 or less from Ali? ...and the circuit, of course) is needed. A scope is optional. Simply connect your coil to the circuit and it oscillates. Then, after 2 frequency measurements (cap in-circuit / cap out-of-circuit), plug the 2 frequencies into the equations provided or use the spreadsheet I posted. Inductance, and distributed capacitance are calculated. The SRF is read directly from the f-counter (cap out-of-circuit).

That's all. It's just a handy little 3-in-1 tool to provide some very useful info about our coils and no expensive equipment is required.

b

[Polymer]

@barwildi

Thank you for the excel file, this makes doing this much easier as my basic skills are very rusty.

@Teleno

Thanks for putting this nice simple method here! This is really handy. Can finally measure SRF and parasitic capacitance now.

[Polymer]

Thought I'd reply to say I have built this and am really happy with the lovely little circuit.

Hope this may animate other rookies like myself who have not been able to measure SRF yet,
as they may not have a signal generator either.

In case you can't get the 2N5245 like me I have used BF245A as a replacement after doing a little datasheet reading.

Here a PDF of the original info for easy printing:

SRF_Measurement_N4TMI.pdf

And a picture of my build, so you know I really did it

Simple_SRF_Oscillator.tif

[dbanner]

Thought I'd reply to say I have built this and am really happy with the lovely little circuit.

Hope this may animate other rookies like myself who have not been able to measure SRF yet,
as they may not have a signal generator either.

In case you can't get the 2N5245 like me I have used BF245A as a replacement after doing a little datasheet reading.

Here a PDF of the original info for easy printing:

[ATTACH]44652[/ATTACH]

And a picture of my build, so you know I really did it

[ATTACH]44653[/ATTACH]

What do you use to measure the frequency? I don't have anything to measure higher frequencies. Is there any way around this?

[green]

What do you use to measure the frequency? I don't have anything to measure higher frequencies. Is there any way around this?

Do you have an oscilloscope?

[Polymer]

@ green & dbanner

Yes, I have a simple oscilloscope.

You might be able get away with measuring frequency with a multimeter like the Fluke 87 V which can measure up to 200kHz at spec
but up to 1.2 MHz when amplitude is high enough. Found this info about it on the eev blog:

https://www.eevblog.com/forum/review...-limit-1-2mhz/

I was looking for a way to measure the oscillations simply, without having to switch on my slow netbook, login etc etc, for the usb oscilloscope. Sometimes I'm impatient.

I do have the Fluke and want to make an amplifier for the output of the oscillator, so I can make a quick measurement of frequency if I want to.
Don't have any clue as to how I will be doing that yet.

Just looked on the net, there are many cheap chinese DYI frequency counters available.
There is a good quality one from a small outfit in Germany and they are reasonably priced as DYI kits.

Dunno if I can put a link here, just search for "ascel electronic". Hope this is OK, George. Otherwise delete it.

[green]

I was wondering if dbanner has a scope and what high frequency he was wanting to measure.

I know Polymer has a good scope, http://www.geotech1.com/forums/attac...3&d=1512769848

[Monolith]

I was wondering if dbanner has a scope and what high frequency he was wanting to measure.

I know Polymer has a good scope, http://www.geotech1.com/forums/attac...3&d=1512769848

Looking at picture [email protected] in the sea and picture test@1meter, my guess is that the coil was closer to the ground under water for the 1 meter test.
Could you confirm that?

[Polymer]

Looking at picture [email protected] in the sea and picture test@1meter, my guess is that the coil was closer to the ground under water for the 1 meter test.
Could you confirm that?

That's a good perceptive question Monolith!
The water there is much deeper than it would appear. It's about 3 meters.

I might just get a very long bamboo pole like 6m and attach only the coil at the end with a long cable going to
the surf pcb on land for measuring. Surely the delay time will suffer, but it's the only simple way to do this that I can come up with for now.