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Old 13th February 2013, 07:31 PM   #101
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Just curious,
Has anyone tried teflon capacitors?
How do they compare to polyester?
Any lower in distortion?
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Old 13th February 2013, 08:37 PM   #102
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Much better performance than PE... extreamly stable, too.

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Old 14th February 2013, 05:06 PM   #103
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If that is the case, wouldn't it be worth while to replace the caps in the twin-T with teflon caps?
I know they are considered better in most regards. I just don't know about THD.

Oops, above I meant extended foil polystyrene, not polyester.
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Old 14th February 2013, 06:13 PM   #104
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Quote:
Originally Posted by myhrrhleine View Post
If that is the case, wouldn't it be worth while to replace the caps in the twin-T with teflon caps?
I know they are considered better in most regards. I just don't know about THD.

Oops, above I meant extended foil polystyrene, not polyester.
Polystyrene is also excellent and for this app, just as good as Teflon. For deep nulls and less drift - either is better. -Thx-RNMarsh
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Old 14th February 2013, 07:42 PM   #105
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I'd be careful with attributing fixed distortion rankings to generic capacitor types, particularly polymer film types. Distortion is not necessarily a very consistent parameter, i.e. we must expect considerable spread from specimen to specimen. I have seen Polystyrene parts with higher distortion than good Polypropylene types, and the very few Teflon capacitors I looked at weren't immaculate either.

Ceramic C0G seems to be much more consistent than polymer film types (and is also more stable, comes with lower tempco and is available with 1% precision at reasonable cost). I've observed very little spread amongst different specimen, and even between various types/brands (of course with the same voltage rating/size). I suspect this is because forming a ceramic capacitor involves much less ill-defined mechanical processing steps than winding a polymer film part. As far as I can measure this at the moment, C0G rated at 100 V or higher has consistently less than -140 dB harmonic distortion with +20 dBu across the capacitor. At lower AC levels, this quickly drops to vanishingly low levels.

The most effective and at the same time cheapest solution to improve a standard passive notch filter is to add a carefully designed input attenuator. This is to make sure the filter operates at modest voltage only (say 1-2 Vrms maximum) to keep distortion in the passives low. The input attenuator is probably best constructed from multiple series resistors (again to keep their voltage drop and thus distortion contribution low). Take e.g. ten 1k resistors in series--the last tap forms the output of a 10x divider.

Samuel
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Old 15th February 2013, 01:29 AM   #106
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All very good advise on the particulars. Thx-RNMarsh
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Old 15th February 2013, 02:11 AM   #107
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Stable TC of caps are important consideration for high Q, deep notch/null circuits. Especially for non-auto null notched circuits. Polystyrene is excellent in this regard. However, for best performance for such a circuit, I think the Polyphenylene Sulphide (PPS) would be excellent. Drift away from a deep, sharp. High-Q notch will be minimised.

Thx-RNMarsh

Last edited by RNMarsh; 15th February 2013 at 02:13 AM. Reason: See Kemet
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Old 16th February 2013, 06:01 PM   #108
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At lower AC levels, this quickly drops to vanishingly low levels.


So then, would a 100v device have lower distortion than a 50v device at the same signal level?
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Old 16th February 2013, 06:43 PM   #109
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Quote:
> At lower AC levels, this quickly drops to vanishingly low levels.

So then, would a 100 V device have lower distortion than a 50 V device at the same signal level?
That's not what I'm saying here. I'm saying that any given capacitor has lower distortion at 0 dBu than it has at +20 dBu. Relative to the fundamental, you can expect the 2nd to be 20 dB lower, and the 3rd 40 dB (4th 60 dB etc.). 2nd falls proportional with level, and 3rd with the square. That's why it can be so extremely effective to add an input attenuator to a passive notch filter.

However, it is also my experience (which is backed up by Bruce Hofer's findings, see his "Building Analog in the 2010s" slides) that C0Gs rated for higher voltages tend to have lower distortion for a given AC level. This is perhaps a result of the higher thickness of the plates, and may not hold for other capacitor technologies.

BTW, I think I've remembered the distortion figures for C0Gs I quoted in post #105 wrong. Will need to check my measurements again, suspect it's a couple of dBs higher than -140 dB at +20 dBu. But surely mighty low at 0 dBu!

Samuel

Last edited by Samuel Groner; 16th February 2013 at 06:48 PM.
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Old 21st February 2013, 11:22 PM   #110
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Hmmm. Just as I got the QA400 running and getting some data -- I am about 5dB different in one vs another test for harmonics... Then the QA400 FFt went nuts on me. stopped working and only works sporatically now. Waiting to see what can be done about it -- fix/replace or what?

But before it bit the dust, it appeared that the ShibaSoku 725D analyser had a fundemental notched null that was at -150dB (!!).

Thx-RNMarsh

Last edited by RNMarsh; 21st February 2013 at 11:29 PM.
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