Back to back aluminum electrolytics

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

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.

Could you try it at 1V and 0.1V and tell us what results you get?

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

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

ohms.

Yes, one use.

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

instruments?

caps

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

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

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

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.

--
John Popelish

full

with

like

ohms.

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?"

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

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

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

--
dd

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