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Old 7th February 2014, 02:50 PM   #1
Piotr is offline Piotr  Poland
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Default How to measure capacitor impedance graph

Hi guys. Is there an easy method to get capacitor impedance vs frequency graphs? Reliability of measurment and frequency range would be important too.
I am not experienced with any measurement software, neither PC hardware for such measurements.
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Old 8th February 2014, 01:54 AM   #2
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You need a jig to measure the voltage across the capacitor, while driving it with an ac constant current source. A 1k resistor is good enough for this purpose. This will only be practical for large value capacitors as high frequncy parts need rf test equipment
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Old 8th February 2014, 07:21 AM   #3
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Thank you for your interest.
What does RF mean? Radio frequencies?

Lets say 500kHz for analog audio purposes would be ok.
100kHz range would be good too - so to start somewhere.

I'm really determined to do this. I've got hundreds of capacitors and spending hundreds of hours making flat impedance response banks - by ear. This is possible, but I'm going gray. I can built any JIG, this is not a problem.
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Old 12th February 2014, 03:43 AM   #4
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In a earlier post I have a link to a simple Opamp interface that I use to do impedance measurements using my sound card.

In the link there is also some simple software that you use with it.
You can also use this very same interface with REW and Visual Analyzer to do RLC impedance measurements with great accuracy.

I show such examples later in the thread as well.

Here is the post and the link to the interface,

Exploring Visual Analyser (VA)

WB6DHW

I hope this helps you.

jer
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Old 12th February 2014, 06:15 AM   #5
Piotr is offline Piotr  Poland
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Thank you Jer it seems lovely tool I just launched it.

Actually I found impedance easiest thing to measure. Used TrueRTA this week.
It requires only two resistors as voltage divider, then I am calibrating input+output system on these resistors. Then I am swaping the lower output resistor to measured impedance (capacitor in this case) and I am getting lovely graphs. Pink noise is doing well for getting live spectrum results. But TrueRTA is not supporting 192kHz sampling soundard, only 96kHz, which gives 10Hz to 48kHz spectrum range. So it's good to observe 1000uF caps or bigger. To analyze smaller caps I need higher frequencies.

Last edited by Piotr; 12th February 2014 at 06:18 AM.
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Old 12th February 2014, 07:05 PM   #6
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It is better to use the opamp buffers as this isolate's the sound cards input Impedance ( that also changes with frequency) from the component being measured.

I have been able to measure accurately below 100pf down as low as 1pf by changing the reference resistor using VA.
It may be possible to go below 1pf as well if noise doesn't corrupt the signal.

jer

Last edited by geraldfryjr; 12th February 2014 at 07:09 PM.
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Old 12th February 2014, 07:24 PM   #7
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I guess you say about HF filtering by low input impedance and parastic capacitances. I didn't get with frequencies that far. Yes, this can be crucial. AD811 could be a good buffer, this is video buffer. Or maybe CA3130, it has over 1Tohm input impedance and good frequency bandwidth.

Last edited by Piotr; 12th February 2014 at 07:30 PM.
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Old 12th February 2014, 07:37 PM   #8
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Yes, they may be good candidates.

I am in the process of building a more permanent fixture myself.

Use an Opamp that will be unity gain stable as I had a few issues with the TL072's breaking into oscillation's every once in a while.

I have some LT1007's that I think I will use, and, I just got a few LM1875's that I am thinking about using for driving speakers and transformer windings and such that needs a little more current to drive properly.

jer
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Old 13th February 2014, 03:43 PM   #9
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To test at >100 kHz frequencies, the soundcard is useless. You need something like a Pico usb oscilloscope/function generator
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Old 13th February 2014, 04:01 PM   #10
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Quote:
Originally Posted by geraldfryjr View Post
Use an Opamp that will be unity gain stable as I had a few issues with the TL072's breaking into oscillation's every once in a while.
jer
Hi Jer, allow me to recommend the OP07 opamp. Even though it's a Precision Monolithics (now Analog Devices) design, the price is shockingly low: 34 cents in quantity one @ Arrow. Lots of other companies make the OP07 too (just like lots of companies make the Philips/Signetics NE5532 opamp).


What is great about the OP07 is its enormous phase margin: 80 degrees. See attached image. It is one of the most forgiving, unity-gain-stable opamps around. And cheap too! (Link to price at Arrow)
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