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Old 18th March 2006, 05:58 PM   #1
percy is offline percy  United States
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Default Capacitor (filter) phase question (pic attached)

Here is a picture of a 2200f 50V capacitor's reactance and phase response and I have a question about the phase response (yellow line). I expected the phase to remain flat at 90 across the entire frequency range but it starts changing at about 100hz. Is this normal ? I measured a 10f polypropylene cap and the phase was indeed flat at 90 throughout. Can someone please explain what's going on ? Thanks.


P.S:- Is the peak around 2hz the self-resonance of the capacitor ?
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Old 18th March 2006, 06:14 PM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
I think the hi imp peak is a measurement artifact.
The low frequency impedance should keep rising until leakage becomes the limiting effective impedance (10s of kohms).

I am slightly surprised that you have not detected the rise in impedance by the time you got up to 20kHz. That would be more typical of electrolytics.

I think the self resonace of larger electrolytics is in the region of 10kHz to 20kHz. It is at or above this that you should see that rise in Z that I mentioned earlier.
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Old 18th March 2006, 07:44 PM   #3
Eva is offline Eva  Spain
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The phase changes from 90 towards 0 as the frequency is increased because the capacitive impedance becomes so ridiculously small at high frequencies that the series resistance of the capacitor and the test fixture dominate instead.

Then, the phase continues rising towards 90 because the inductive impedance of the capacitor and the test setup is no longer negligible in comparison with the corresponding resistive impedances, and it ends up dominating at higher frequencies.

Try to reduce the resistance and inductance added by your test setup and you will get more realistic results.

Concerning the 10uF film capacitor, as its capacitive impedance is 220 times higher than in the 2200uF capacitor, the effects of parasitistic series resistance and series inductance become 220 times less important in the 1Hz to 20Khz measurement . However, you will see similar results (phase sweeping from -90 to 90) if you measure the film capacitor between a few Khz and a few Mhz with the same setup.
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Old 19th March 2006, 01:03 AM   #4
lndm is offline lndm  Australia
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This is the reason that small capacitors sound better than large ones (oops! isn't that supposed to be the other way around?).

Large values can be used if care is taken in design but going overboard is IMO discouraged. Bypassing with small value caps may create more complex resonances and can sound worse (sometimes much worse).
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Old 19th March 2006, 03:44 AM   #5
percy is offline percy  United States
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Eva, I affirm your thought that it has to do with additional resistance/inductance in the test setup. I totally removed the two leads that I connect my DUT with which are about 12-14" long each and connected the dut (cap) directly to the measuring device with a very short cable(less than 6") and the phase response certainly changed. The reactance is also more accurate now. Goes to show how longer than neccessary connections in this area affect behaviour in this area.
I am anxious to see what would be the result if this cap was bypassed by a small value (100uf ?) cap.

Notice the -180 to +180 phase shift at 11.5Khz.
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Old 19th March 2006, 06:54 AM   #6
Eva is offline Eva  Spain
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Indm:

Please, keep your imagination under control. I only said that lower value capacitors are much easier to measure because the resistance and inductance added by the test setup is much less relevant. The impedance of a 10uF cpacitor is 220 times higher than the impedance of a 2200uF one.


Percy:

Could you explain which equipment are you employing for that measurement? Now it looks as if the impedance of the capacitor became much smaller than the output impedance of the own measurement system, thus fooling also the measurement. Is the measurement system trying to put a constant AC voltage across the capacitor and measuring the current, or is it puting a constant current and measuring the voltage?
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Old 20th March 2006, 12:42 AM   #7
percy is offline percy  United States
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Default Speaker Workshop!

Speaker Workshop, with a Turtle Beach Santa Cruz soundcard.
It has the ability to measure passive components. Its range is limited but as you can see it can measure something like a 2200f cap with reasonable accuracy.

I believe it measures the voltage across the DUT. Look at the jig schematic that I use. That should give you an idea.

I tired a couple of different low value caps in parallel with the big electro, and lower the value of the parallel cap, better was the phase response. With the last 8.2f polyprop cap that I tried, the phase response was almost flat to 10Khz then rose to +90 by 20Khz.

I cannot say whether lndm is right or not but it sure does raise a question in my mind - what effect would this type of phase behaviour have on the sound ? Because, agreed that these anamolies are being caused by parasitic inductance/resitance, but then again isn't this actually close to the "real world" scenario where such parasitics are bound to exist ?
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Old 20th March 2006, 12:44 AM   #8
percy is offline percy  United States
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forgot to attach the jig....
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Old 20th March 2006, 02:08 AM   #9
Eva is offline Eva  Spain
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Well, the problem here is that the output amplifier of the soundcard is trying to apply an AC voltage to a 2200uF capacitor at high frequencies, and obviously it can't do that directly. Also, another problem is that due to the extremely low impedance of the 2200uF capacitor, the feedback signal turns ridiculously small at high frequencies and ends up buried in the noise floor.

Try placing a 10 to 100 ohm resistor in series with the output of the soundcard before the test fixture. Try also using a 1 ohm reference resistor instead of 10 ohms. Increasing the recording level in the recording mixer control may also help.
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Old 20th March 2006, 07:15 AM   #10
AndrewT is offline AndrewT  Scotland
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Hi,
that connection schematic is identical to the Steber paper posted recently. It was published in 2005. Someone was quick to incorporate it.

Just shows what the computer designers are capable of.
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