On 1/28/2014 2:25 PM, Wimpie wrote:
El 28-01-14 20:03, amdx escribió:
I have beads* on a coax and want to know the R and the L.
I have measured the R at 3.85MHz, It is 3,350 ohms.
I have also measured the phase shift, voltage leading
by 17ns. The period of 3.85Mhz is 260ns.

I want to calculate the impedance of the reactance.

Can anyone solve this for me?
I would like to see the math, because I want to measure again
at 7.5MHz.

My first step was to find the phase angle, 23.5*.
Do we agree there?

Thanks, Mikek


* it is actually a bit more than beads. Years ago, we were sent a box
of ferrite potcores, the cores arrived broken. I slide 42 broke halves
onto a piece of RG59, and now I'm measuring it.

Did you actually measured R (say Re(Z) ) , or |Z|?

I measured the voltage, the current and the phase relationship.


Can you provide us some info on your setup?


I can, it might turn into a word war, but have used it very
successfully 100s of times at 600kHz, I'm not sure of the accuracy
at 10 MHz. But with time I will refine it as needed.

Here's a diagram of the setup.
http://s395.photobucket.com/user/Qma...awing.jpg.html
Ignore the green lines and print for now.

Here's the board with scope probes attached.
http://s395.photobucket.com/user/Qma...notes.jpg.html
(Note the curly Qs holding the probes, very useful for measurements were
the leads cause ringing.)

You connect a frequency generator and "Device to be Measured", adjust
the frequency, and set the output level. I like to set the voltage at
1vpp, but it doesn't matter.
On the scope you will see the voltage and the current as measured
across the sense resistor.
Say for your current you have 5 units pp on .02v scale, across a
100 ohm sense resistor. 5 x .02 / 100 = 0.001 amps pp. Then 1vpp /
0.001amps pp = 1000 ohms. Your 100 sense resistor is in series, so must
be subtracted out, 1000 - 100 = 900 ohms. You can use the scope to see
the phase.
You can drop the pp, the numbers all come out the same.

Now the fun part.

I original used a device like this when I was measuring the R and C
of bonded piezos in water. That brings me to the green lines in the
first drawing, where it shows the capacitor, I had a variable inductor.
I would adjust the inductor to tune out the capacitance of the piezo,
(set the inductor so the scope shows zero phase difference.) I would
then use the scope readings to calculate the R of the piezo. Then I
would short the piezo connection and use the scope readings to calculate
the impedance of the inductor, which is the same as the capacitance of
the piezo with reverse sign.
Then the higher functioning brains would calculate transformer and
inductors for the amplifier.
Any questions?
Learned this from Henry.
Mikek