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Back to back aluminum electrolytics

Discussion in 'Electronic Design' started by John Smith, Feb 15, 2005.

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  1. John Smith

    John Smith Guest

    The thread on back to back tantalums aroused my curiosity. I had no
    tantalums, so I used a couple of aluminum electrolytics. They are 22
    microfarads at 16V each, Mouser PN 555-16V22 (Elna RVS16V220M-R).


    o-----------> -C+ -- +C--|
    |
    HP 204D Osc 100 Ohms
    |
    o------------------------|

    Using an HP 331A Distortion Analyzer:

    Set 3V at 20 +- 0.5 Hz on 204D output.
    Read .395 V across the resistor.
    Read .193% distortion out of 204D.
    Read .365% distortion across the resistor.

    So, what can I learn from this? Is there a way to verify that the increase
    in distortion is due to the caps rather than my naivety with these instruments?

    If these measurements are accurate, is the distortion introduced by the caps
    equal to the difference in measurements (.365 - .193)?

    And here's a fly in the ointment-- the oscillator with no load measures
    ..295% distortion.

    I appreciate the education you guys are giving me. Thanks.

    John
     

  2. Unfortunately, combined percent distortions do not add arithmetically. For
    example, the capacitor might have a nonlinearity which is equal and opposite
    to the generator nonlinearity - giving a low distortion reading, and leading
    you to believe that the capacitor has *negative* distortion! Since the
    distortion readings you quote are all in the same ballpark, I would'nt draw
    any conclusions about the capacitor distortion.

    Also, the capacitor is dropping most of the voltage, so it it is phase
    shifting the oscillator distortion (harmonics) and that can change the way
    the oscillator distortion reads on the analyser.

    Also, the capacitor - resistor acts as a high pass filter which accentuates
    the oscillator harmonics and thus increases the distortion appearing across
    the resistor.

    My guess is that your capacitor distortion is small compared to the
    distortion of your test setup.

    I don't know what the lower limit of the 331A analyser is, but it is
    probably 0.1% or better. With a better oscillator, you can get your test
    setup residual down.

    I would pick my resistor to get the same voltage across the resistor as
    across the capacitor - at this point, the capacitor is dropping plenty of
    voltage, but there is still plenty of output across the resistor. This
    seems to be a standard setup used for capacitors - that way you can compare
    results with others.

    Roger
     
  3. John Smith

    John Smith Guest


    Thanks, Roger. That's the kind of information I was looking for. I have an
    HP 651 Test Oscillator which, if I recall correctly, has lower distortion.
    It also has a 600 Ohm output and a 50 Ohm output.

    The 331A's lowest switch setting is .1%, so the meter will read .1% at full
    scale on that range.

    I will repeat the test with the better oscillator and an integrating RC with
    a resistor of about 36 Ohms. At 402 Hz, the capacitor should then look like
    36 Ohms and the combination would then have an impedance magnitude of 50 ohms.

    Does this sound like a better try at it?

    Thanks immensely.

    John
     
  4. Robert Baer

    Robert Baer Guest

    Might be better to measure magnitudes of frequency components
    (fundamental, 2nd, 3rd, etc harmonics) at the output of the oscillator
    and at the resistor.
    If the RC is far away from the fundamental, one could get: a) low
    frequency would give apparent high "distortion" at the resistor due to
    RC rolloff; b) high frequency would give same "distortion" due to low
    drop across the caps.
    Hence the suggestion. One should be able to *calculate* the level of
    each frequency component at the resistor, from the RC value and the
    oscillator level of each frequency component.
     
  5. Guy Macon

    Guy Macon Guest

    Could you try it at 1V and 0.1V and tell us what results you get?
     
  6. John Smith

    John Smith Guest

    I do not have and instrument capable of these measurements.
    The RC will be such that Xc = R = 36 Ohms at the test frequency of 402 Hz.
    Is this what a frequency-selective voltmeter is for (or, better, a spectrum
    analyzer)?

    Thanks, Robert.

    John
     
  7. John Smith

    John Smith Guest


    With this setup, the 331A does not have enough sensitivity at these lower
    voltages to calibrate the reference level for a reading. That's the main
    reason I used 3V. However, I also tried to use enough voltage to exceed a
    possible 1V threshold reverse voltage for the caps (if that's important).

    But, yes, I can repeat the measurements using some suggestions from Roger
    and Robert and try to design the setup to for all the requirements as I now
    see them. If I can't get to it tomorrow, it will have to wait until I get
    back into town (the 28th).

    Thanks for asking.

    John
     
  8. Robert Baer

    Robert Baer Guest

    Yes, one use.
     

  9. I get the 36R and 402Hz numbers too, so looks good.

    Maxim have an interesting app note :

    "Do Passive Components Degrade Audio Quality in Your Portable Device?"

    http://www.maxim-ic.com/appnotes.cfm/appnote_number/3171

    In this app note tantalums, aluminium electros and polyester caps are
    measured. In the app note, the graphs show distortion rising as you go
    lower in frequ, which increases the signal voltage across the cap. There is
    plenty of distortion at the frequency where the R and C have equal voltages
    (99.5 Hz on figure 8), so this should be a good measurement point.

    According to fig 8, below the -3db frequency, the distortion rises, but the
    output signal decreases, so the audible effect won't be worse at lower
    frequency.

    Roger
     
  10. John Smith

    John Smith Guest


    Okay, here's what I did...

    o-------C+--+C----|
    |
    402 Hz 50 Ohms
    |
    o-----------------|

    At 3V and at 1V input, .32% distortion at input and .38% across the resistor.

    Swapped the C and R positions (made it an integrator), changed the R to
    33.33 Ohms, and changed the frequency to 440 Hz:

    o-------33.33 Ohms----|
    |
    C
    440 Hz +
    +
    C
    |
    o---------------------|


    3V applied, .31% input distortion, 2.01 V @ .182% across C
    1V applied, .31% input distortion, .67 V @ .176% across C
    ..422 V applied, .32% input distortion, .3V @ .185% across C

    The last measurement was the lowest input the 331A could handle and I'm
    thinking there may be more error in it than in the first two.

    I don't know where I got the idea that the 651A had less distortion than the
    204D. A bit disappointing.

    I'll go read that Maxim page to which you referred and try to learn
    something. Thanks.

    John
     
  11. Phil Hobbs

    Phil Hobbs Guest

    One interesting thing to try would be to apply a variable dc bias to the
    point between the two capacitors, varying it from a moderate positive
    voltage to a very small negative one, and see what that does to your
    distortion measurements. A second interesting thing would be to make
    one capacitor twice as large in value as the other (or use a different
    technology, e.g. ceramic or tantalum). Your circuit will tend to make
    odd-order harmonics cancel, while even-order ones will add. Making the
    capacitors asymmetrical intentionally will eliminate this cancellation.

    Cheers,

    Phil
     
  12. Guy Macon

    Guy Macon Guest

    Excellent idea! If the distortion changes, it pretty much has to
    be distortion introduced by the capacitors.
     
  13. John Smith

    John Smith Guest

    Yes, I agree. I just haven't had time. I've got to go pack now. I'll try to
    get back on this when I get back in town about March 1.

    Thanks for all your help.

    John
     
  14. I think another interesting experiment would be to make a bridge with
    a resistor and back to back electrolytics in one side, and a (possibly
    higher value) resistor and film capacitor in the other side, and
    amplify the difference voltage between the two sides, after adjusting
    the resistors for best phase balance. This should emphasize the
    distortion difference between the two kinds of caps and low pass
    filter and cancel most of the distortion from the source. You would
    need an instrumentation amplifier.
     
  15. dd

    dd Guest

    There is a series of articles in UK Electronics World written by a
    capacitor manufacturing specialist on distortion due to capacitors and a
    dedicated measurement system / circuit (pcb layout included) articles
    published 1/2 years ago most easily available by buying CDs of the last
    few years of Eletronics World
     
  16. Robert Baer

    Robert Baer Guest

    "Distortion" is *not* the term to use!
    "Filtering" is the accurate and correct term.
     
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