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Low-distortion Audio-range Oscillator
Low-distortion Audio-range Oscillator
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Old 16th February 2019, 04:05 PM   #7361
RNMarsh is offline RNMarsh  United States
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Quote:
Originally Posted by Monte McGuire View Post

With non-polar caps, especially miniature multi-layer C0G capacitors, adding extra DC bias can add even order nonlinearities, by making the peak field strength across the dielectric higher for one polarity of signal peak vs. the other polarity. This is only an issue with multilayer ceramics, where the dielectric is already exposed to extremely high field strengths, since the dielectric layers are so thin. The theory is that the dielectric starts to saturate at extremely high field strengths, and biasing the cap away from a net zero field strength makes the peak levels worse and also asymmetric relative to the AC signal across the cap.

A nice side effect of this behavior is that one can use this to determine the linearity of an MLCC dielectric in a distortion test by changing only the field strength across the dielectric, without changing the generator level and analyzer levels (which would alter the analyzer fundamental). If you see a change in 2nd harmonic while changing only the DC bias, you have found a nonlinearity caused by dielectric saturation. Presumably, if the same amount of 'squish' happens at a higher DC bias level for one cap compared to another, that's a cleaner cap.




-RNM

Last edited by RNMarsh; 16th February 2019 at 04:09 PM.
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Old 21st March 2019, 05:55 PM   #7362
Oneminde is offline Oneminde
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Originally Posted by vicnic View Post
I found my old tests for 10nF capacitors at 1kHz 2,7V RMS.
Typical figures:
1. COG 1206 200V, 2. COG 1206 630V, 3. Wima FKP2 100V
Thank you for this data, I really appreciate it. I was looking at FKP as signal capacitors and might be better than C0G.

Do you know / remember the TCR (ppm(C) value of does C0G you tested ???
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Old 22nd March 2019, 03:54 PM   #7363
vicnic is offline vicnic  Latvia
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Originally Posted by Oneminde View Post
Do you know / remember the TCR (ppm(C) value of does C0G you tested ???
The data sheets of the measured COG capacitors:
https://www.farnell.com/datasheets/1...816.1512842515
http://www.wlxmall.com/images/item_p...1J080AA-cn.pdf
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Old 22nd March 2019, 04:14 PM   #7364
1audio is offline 1audio  United States
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If you look at the datasheet for the Kemet caps its adamant that you keep the voltage constant during testing. Is that because some parameter changes on caps that are supposed to be stable with voltage? I guess I'll need to set up an SMT test fixture soon.
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Old 17th April 2019, 08:15 AM   #7365
triode_al is offline triode_al  Netherlands
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Quote:
Originally Posted by plk View Post
For your inspiration
Click the image to open in full size.
This circuit looks great. I have a Radiometer distortion analyser.
There the notch filter is variable (there is a balance knob to tune the notch) and this needs to be aligned a few times before it stabilises.
Question: can your filter be 'tuned' a bit to be able to match small drifts in frequency? Which resistors should be changed or is that not do-able, 1600/1600/800 cannot be changed in high enough matched quality)?
I am looking to buy this generator:
Sine wave signal generator 1KHz ultra low distortion | eBay
+ and need a corresponding high quality (and high Q) notch filter with low noise.

albert
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Old 17th April 2019, 05:42 PM   #7366
HpW is offline HpW
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Originally Posted by plk View Post
For your inspiration
Do you have also plotted, on real or simulated circuit, the idle noise floor?

While on ordinary active T-Win Notch, the noise floor rises as a bell around the notch frequency...

Hp
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