Help with 4262 and Pico6 ...

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aethertech
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Help with 4262 and Pico6 ...

Post by aethertech » Fri Apr 12, 2019 6:00 pm

I am new to the forum and I am not sure where to post this but here goes.

I use a 4262 with Picoscope 6.12 exclusively in spectrum view to measure analog amplifier performance. My desired measurement bandwidth is up to 200kHz (sometimes a bit higher). I typically setup Pico6 for a 2Mhz bandwidth and use the zoom function to display only the desired 200kHz measurement bandwidth. Doing this allows me to use a gentle low pass anti-alias filter just above the 200kHz measurement BW. Using 8192 bins and a Blackman window with averaging, I achieve quickly updating and very clean spectrum views.

With the setup as above, let's say I am adjusting amplifier bias for lowest distortion. To do this requires me to trim bias with one hand while constantly stopping and restarting the sampling to update the continuously "averaging" display. This is somewhat of a hassle requiring the Pico platform and DUT to be within a two arm spread of each other. It also does not allow full attention to be given to the bias trimming tool and components in close proximity screaming "please don't slip and touch me".

So, as a first question, is there any way to set up Pico6 so that when in average spectrum view, the displayed average is a constantly updating "running average" of a selected number of previous samples? This would allow averaging to be used to smooth the spectrum display while updating the average sufficiently fast to make the effects of adjustments appear more "real time". What would be most handy is to be able to select the number of waveforms being averaged for either a more "real time" feel or cleaner looking waveforms as desired.

(I have considered using a keyboard macro that effectively stabs the space bar at 1 or 2 second intervals to affect some semblance of the above, but that seems like a bit of a kludge at best.)

As a second question, has there been any updates with regard to allowing the use of average views of spectrums as reference waveforms?

I know this has been previously discussed, but it would be very desirable to be able to use averaged spectrum views as reference waveforms. For example, when set up as above, I could measure an amplifier for best THD, noise performance or frequency response, obtain a long clean average of that spectrum and then use that saved measurement as a reference waveform to compare the performance of that amplifier with additional modifications or adjustments. Unfortunately, it seems that reference waveforms cannot be "average" views. Without averaging, the reference waveform for spectrum views is pretty worthless. Looking at THD spectrums (spurs) of less than .01% THD without averaging is difficult at best.

Any help regarding either of these two questions would be greatly appreciated.

Thank-you...

Edit: replaced erroneous "shift key" with "space bar"
Last edited by aethertech on Tue Apr 16, 2019 3:40 pm, edited 1 time in total.

Gerry
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Re: Help with 4262 and Pico6 ...

Post by Gerry » Mon Apr 15, 2019 3:17 pm

Hi aethertech,

To answer your first question, what you can do is set a fixed number of buffers to use for capturing data in the Preferences, then set an alarm to restart the capture after filling the buffers, then use a repeat trigger to perform enough captures for your averaging. This will then average your data and reset the capture at regular intervals. I created an example (see data file and animated Gif below) which uses 50 captures for a sample rate of 2MHz (zoomed in to a Bandwidth of 200kHz) and 8192 bins to give a period between captures of a bout 2 seconds. So you can tune the operation to be (a) more real time by reducing the number of captures, and or the number of bins, or (b) more accurate by increasing the number of captures, and or the number of bins.

Adjustable fixed length averaging.psdata
(678.29 KiB) Downloaded 7 times

Unfortunately, I'm not aware of any update to enabling the reference waveform creation out of an averaged value of a channel in Spectrum Mode, but I know that there is some work being done to make changes to selected areas.

Regards,

Gerry

P.S. Be careful using the animated Gif on Debian/Ubuntu Linux and derivatives as it consumes a lot of CPU resource.
Attachments
Adjustable fixed length averaging.gif
Gerry
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aethertech
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Re: Help with 4262 and Pico6 ...

Post by aethertech » Tue Apr 16, 2019 2:50 pm

Gerry,

Thank-you for your reply. I was hoping there was a more elegant solution that created a more fluid "running average" such as setting up the buffers to act like a variable length FIFO shift register, with a new average calculated as each new capture is added and the oldest capture dumped. This would allow the display of a constantly updating running average with the length of the "FIFO" determining the amount of smoothing/averaging applied to the displayed spectrum. I would think anyone who uses the hi-res scopes in spectrum mode for signal analysis would consider this a very useful feature.

For the past 15 years, I have used a hi-res Picoscope exclusively in spectrum mode going back to the ADC216. Your solution, and in particular, your animation, produces results that look very similar to what I look at for many hours a day, but with my right hand doing the stop/starts. A recent injury to my left shoulder has finally forced me to find a better solution.

Over the weekend I experimented with the use of a keyboard macro to "stab" the space bar for me to do the stops and starts. Because I also need to routinely save clean/long average spectrums, the keyboard macro may be a bit more handy than your solution. I can press the "Z" key to start the macro and when I want to save a cleaner waveform, I can hit the "Z" key again to stop the macro and allow for continuous sampling/averaging for as long as desired. Varying the time delay between virtual presses of the space bar via the macro allows for variable smoothing and continuous updating.

Although both your solution and the key macro work to a degree, neither is as elegant as I had hoped.

Regarding the "averaged spectrums" as reference waveforms issue, I know this was brought up by at least one user some time ago. Using the average function to reveal low level signal spurs buried in the noise floor is a necessity. Averaging is also very convenient when using a noise source to measure frequency response/bandwidth. Being able to use the very clean averaged spectrum displays the hi-res Picoscope generates as reference waveforms would be extremely useful for design and development. Perhaps the software team could up the priority on this a bit.

Thanks again,
Aethertech

EDIT: As an old guy who apparently is losing his mind (or, as I prefer, has just too much going on up there), I just realized I have repeatedly used "shift key" when I meant to say "space bar". In my initial post I meant to say the "space bar" to stop/start sampling, not the "shift key". I have corrected that in this post.

Gerry
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Re: Help with 4262 and Pico6 ...

Post by Gerry » Wed Apr 17, 2019 2:19 pm

Hi Aethertech,

If you don't force the change by restarting the averaging process in the way I demonstrated, then with a 50 waveform buffer capture, the Spectrum mode Average plot will always be averaging the last 50 waveforms as long as you have the trigger on Repeat, Auto, or None. So it will then change between the waveform harmonics smoothly. The issue with what you describe is that the Averaging process doesn't update the split waveform buffer one buffer at a time (instead it updates all buffers with the last waveform captured), which appears to be a fault that our development team are aware of.

I'll add a note to these issues to raise the awareness.

Regards,

Gerry
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