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DIY Injection Transformer for Power Supply Control Loop Response Measurements

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rx8pilot:

--- Quote from: rbola35618 on October 13, 2016, 08:04:15 pm ---The only problem that I see with the Keysight is , if I am not mistaken, the resolution of the ADC is only 8 bits. The dynamic range and the ability to get the signal out from the noise is not as good as a regular FRA or the Cleverscope.

My Cleverscope has a 10bit ADC and I can upgrade it to a 14 bit ADC. So theoretically, the Cleverscope should be more sensitive. I have done some test where I can a good loop plot using only 1mV on input stimulus signal. I then tried doing that on out 30K Venable and it was not able to get a good plot. 

I think some of the higher end Keysight may have a higher resolution ADC. I would assume that those with higher ADC would work much better.

RB

--- End quote ---

To be fair, the resolution of the ADC is only a useful spec if the analog circuits in front of it are better. With full manual control on the 8bit Keysight using averaging and filtering I was able to pull out some really small signals.

rbola35618:
Hi rx8pilot,

What would be an interesting test would be to see how low of a stimulus signal will get you a good plot.

If you get a chance and are interested in finding out, do some plots and decrease the signal and see where it starts to plot noise. I would be interested to find out how sensitive it is.

RB

Kleinstein:
The 8 Bit converters in the scopes are really fast. So they can do a large amount of oversampling to reduce noise. An 8 Bit resolution looks low, but at GHz sampling the noise density is not that bad. It is well possible that the noise from the (switching) supply itself is higher than the ADC / Scope noise. Often 1:10 probes already give more noise than the scope itself. There is not much to gain with a low noise scope, if there is more noise from the DUT.

The important part could be how good averaging and harmonic analysis is done and thus what is the effective bandwidth. A slower measurement with longer averaging on each point can reduce the noise - especially if it is slower for the points with poor S/N only. So it could be just a question of how much time is spend for the curve. It's only improving like the square root of the time, but there is still a last resort in using extra time - up to the point of over night.

rx8pilot:

--- Quote from: rbola35618 on October 13, 2016, 08:34:21 pm ---Hi rx8pilot,

What would be an interesting test would be to see how low of a stimulus signal will get you a good plot.

If you get a chance and are interested in finding out, do some plots and decrease the signal and see where it starts to plot noise. I would be interested to find out how sensitive it is.

RB

--- End quote ---

Using the built in application, there is essentially no solution for SMPS. By the time I got a usable SNR, the loop will have long since been clipping. The 6000 scopes do not yet have the new power application that the 3000's and 4000's have. I could try to setup some manual tests to see what happens on one of my boost converters. I will be in the CNC shop for the next few days, so possibly on Sunday.

It would be great to throw the results and method into this discussion. Not a whole lot of conversation to be found on loop response. TALKnerd(1000) = AUDIENCEsmall.

Kleinstein:
I looks like isolated generator is an $400 option. So if you have that type of scope, this is an attractive option.

If you want to build the transformer yourself, I would consider a more limited frequency range and than the option to chose from a few. It should be easier to make two of them for separate frequency ranges than one with very high bandwidth. Depending on the DUT you might have to choose the suitable one.

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