Help deciding on PicoScope for FRA for Switch-Mode PSUs

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TimNJ
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Help deciding on PicoScope for FRA for Switch-Mode PSUs

Post by TimNJ »

Hi all,

Company I worked for bought a 2204A a while ago. Nice scope for the price. Recently, I attempted to do a frequency response analysis (gain/phase) of an offline AC-DC switch-mode control loop with 2204A and got non-sense results. I'm using a roughly Lm = 100mH (nom.) injection transformer. The transformer has almost perfectly flat response between 10Hz and 1MHz, when driven from 600-ohm source impedance and into a 22-ohm injection resistor.

After hours of fiddling with the FRA software, I still could not get meaningful results. So, I gave up.

Now, I am wondering if perhaps 2204A is simply not a powerful enough scope to extra the signal from the noise. Does anyone have an experience in this application? I am not sure what specifications to look for.

Perhaps a 2207B or 3203D would be a better choice? If so, please explain.

Thank you very much,
Tim

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Re: Help deciding on PicoScope for FRA for Switch-Mode PSUs

Post by TimNJ »

I found some good advice in this thread, searching the keyword "stability": search.php?st=0&sk=t&sd=d&sr=posts&keyw ... 1&start=15

Based on this feedback, it seems that the main priorities are resolution (ENOB) and memory depth. I don't quite understand the memory depth requirement with respect to PC scopes. I read through the Picoscope article, but still not sure how it affects this application.

So, I am now looking at 4224A as the main candidate. Any feedback?

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Re: Help deciding on PicoScope for FRA for Switch-Mode PSUs

Post by TimNJ »

I hope to make a purchase this week, but am hoping someone can chime in with some feedback first.

Thanks.

bennog
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Re: Help deciding on PicoScope for FRA for Switch-Mode PSUs

Post by bennog »

the memory depth is depending what the highest frequency is you want to measure.
And the duration you want to see the data of.

The sample rate of the scope needs to be 5 - 10 times the frequency you want to see.

The lower end 2000 models can max sustain 1 M/samples per second. So you can use the pc memory up to 100 - 200 khz signals.
If you want to see higher frequencies over longer times, the you need to sample this on the internal memory first, This is the part where internal memory comes to play.

The more expensive usb-3 scopes 3000 and up can use PC-memory up to 17 M/samples per second (in picoscope software mode). With the SDK you can go higher but you need to write the code yourselves.

Benno

Gerry
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Re: Help deciding on PicoScope for FRA for Switch-Mode PSUs

Post by Gerry »

Hi Tim,

The problem will be that the Buffer size of the 2204A is too small. I have explained this in detail below, but if you just want the summary, that is towards the bottom of this reply.

A Bode phase and frequency plot essentially plots swept Spectrums of values. In order to plot a Spectrum you can specify the number of 'Spectrum Bins' used to perform the plot. The Spectrum Bins correspond approximately to half of the number of samples used to collect the data. The number of samples used to collect the data is limited to a maximum defined by the size of the memory buffer in the Hardware PicoScope. For the PicoScope 2204A this buffer size is only 8k samples. This means that the maximum number of Spectrum Bins that you can use is 4096 Bins.

In Spectrum Mode, the conversion to frequency is done in a way that is optimised to show signal frequencies accurately. This optimisation has a side effect where the noise floor can be reduced by using more Bins to do the plot. (This optimisation is known as 'Coherent Gain' or 'Process Gain' - for more detail you can go here: viewtopic.php?p=86081#p86081).
Spectrum bins represent the average amplitude of all of the frequency components within the Bin width of frequencies. So, if you have captured signal harmonics they will be averaged, and if there is also any noise, you will have a resulting value which is not just the signal harmonics but also the included noise. Every time you double the number of Bins for the same Bandwidth, you halve the Bin width (because the complete Bandwidth of the spectrum plot is split up into the Number of bins). So, every time you double the number of Bins you are removing additional signal harmonics and removing the amount of noise that gets averaged along with any signal harmonics. So the more bins you have, the more accurate (more representative of one single harmonic) will be each bin value for the whole Spectrum plot.
Noise is a random process that generates constantly changing values, so as you remove these from the Spectrum Bins, your plotted values have less variance due to noise, and therefore become more stable. Spectrum Bins can be selected starting from 128 Bins to 1048576 Bins, doubling at every successive selected value. So, the limit of 4096 Bins for the PicoScope 2204A means that you can't even get to halfway down the
list of selectable 'number of Bins', so the plotted Bins for this Scope will have a lot of noise, and therefore a lot of instability.

SUMMARY
In a Spectrum Plot, using more Bins means that you can get more accurate signal levels, stabilize the signal levels and reduce the noise floor, as you found in your research, but the PicoScope 2204A and 2205A buffers limit the Number of Bins too severely to benefit in this way. These are the only PicoScopes that limit you in this way (as they are cost-effective, entry-level PicoScopes) so, to be able get the plots that you want, you just need to select any other PicoScope, however.......

....you should still consider what your measurement goals are, so that you can select a PicoScope that will satisfy your needs, and there is some information here that can help you with that: https://www.picotech.com/library/applic ... e-tutorial, as well as our Help Desk (at support@picotech,com) and this forum (using advanced search here: search.php). Then once you have narrowed down your requirements, you can make a faster selection by eliminating scopes that don't meet your requirements using our selection tool here: https://www.picotech.com/products/oscil ... ifications.

Regarding Effective Number of Bits (ENOB, which is the Resolution minus the Noise and Distortion), this is relevant, but only if noise is going to be a problem in your measurement. As already explained, for Spectrum Plots, the noise floor can be easily lowered, and this can be to well below the level of any Distortion of the PicoScope. If getting good spectrum plots is all that would concern you for your measurement then you would be better off considering just the Resolution of the Picoscope minus its Distortion of the signal. If you will also be performing Time Domain plots, or you will be performing Power Spectrum plots in another tool (e.g. after exporting the data) then ENOB will be more important.

I hope this clarifies thngs for you,

Regards,

Gerry
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TimNJ
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Re: Help deciding on PicoScope for FRA for Switch-Mode PSUs

Post by TimNJ »

Here was my original intended reply to bennog, which I was preparing while Gerry responded:

Thank you. Understood about the memory depth. But, I guess I don't understand why the other posters recommend higher memory depth when most power supply control loops have cutoff frequencies usually not above 50KHz. It seems Pico 2000A should be able to handle this.

There's a solid chance my original setup was no good. I checked the AWG output (Ch. A) and the response (Ch. B) with a bench oscilloscope during this test. The injected waveform amplitude is quite low, as is the measured response, so the SNR is not great.

It's been a while since I studied measurement theory, so I forget a lot about the relationships between oversampling, quantization noise, ADC resolution, etc.

Would a Pico 4000A series scope have a better shot at handling low amplitude signals and especially those which are drowned in a lot of power supply switching noise?

Thanks a lot for your response.

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Re: Help deciding on PicoScope for FRA for Switch-Mode PSUs

Post by TimNJ »

Thank you Gerry. I think that probably sums up the issue I was having. The company purchased the 2204A on a whim, mostly on my recommendation to use it as a low frequency component/network analyzer. (At $130, they couldn't say no!) To be honest, I didn't even realize that its sample depth was only 8K. I must have thought it was 8M, given that 2206B has 32M. For noise-free analysis, it works great, of course.

Thank you for the links to the comparison table and buying guide.

Regarding noise, the output voltage of a typical AC-DC power supply usually has 50-500mV AC component (ripple, switching noise). The injected signal (from the AWG, via injection transformer) has an amplitude on roughly the same order. Whether moving to a true 12-bit scope would really be beneficial/worth it, I'm not so sure. Maybe, as you seem to suggest, the most important idea is to maximize the spectrum bins.

Thank you.

hexamer
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Re: Help deciding on PicoScope for FRA for Switch-Mode PSUs

Post by hexamer »

Gerry is right ... For FRA, deep buffers, allowing high DFT sample count is about increasing processing gain (narrow bandwidth for signal selection and good noise rejection).

For power supply stability analysis, there are cases where you need the stimulus signal to be small to keep the control loop in a linear region. With high switching noise, the SNR is low, and so processing gain helps. But the stimulus signal may need to be so small compared to the range to cover the ripple that it could be less than a few ADC counts. This is where extra resolution can help too.

In theory if there is some Gaussian noise present, even if the signal is less than a single ADC count, enough processing gain would be able to recover the signal, but I'm sure there are practical limits.

That said, I've never really tried to characterize the trade-offs between buffer depth and resolution. I've always wondered whether an 8 bit scope with huge buffer depth would be just as good as a 12 or 15 bit scope with modest buffer. It probably depends on the nature of the SMPS and how large of a stimulus you can get away with. I can tell you that a PS5000D scope is a good choice for small DC SMPS stability analysis. But a lower resolution scope with very large buffer might work too.

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