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Using Measurements on Low Pass filter waveform

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Using Measurements on Low Pass filter waveform

Postby PicoPilot » Tue Apr 19, 2016 6:17 am

I'm using a Picoscope 2205 with software version 6.11.12.1692.

I'm displaying DC voltage ripple and noise on Channel A using AC coupling. I also added some Measurements. Everything looks as expected.

Then I created a Math Channel with a Low Pass filter to remove high frequency spikes from the Channel A waveform. The resulting filtered waveform looks as expected and I have it displayed in a separate scope view in the lower pane.

The problem: When I add Measurements to the Math Channel (which it allows me to do) there are no values displayed for some of the Measurements (e.g., Peak-to-Peak, Frequency). However, just as a test, I noticed it will display values for DC Average on the Math Channel.

Is there some limitation or workaround to getting the Measurements to work on a Math Channel containing a Low Pass filter?

Thanks in advance!
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Re: Using Measurements on Low Pass filter waveform

Postby PicoPilot » Tue Apr 19, 2016 7:26 pm

After more experimenting, I have some partial answers to my own question.

With regard to the Frequency Measurement on a Math Channel Low Pass filter, it simply does not work. This may be understandable as frequency is an attribute of the primary channel signal. The Math Channel series could be a calculated (synthetic) data series that doesn't carry along the frequency attribute. At least, that's how I might rationalize why it doesn't work.

With regard to the Peak to Peak Measurement on the Math Channel Low Pass filter, it does not work at lower time resolutions, e.g., 20 us/div in my scenario. However, when I increase the time resolution to 100 us/div, the Peak to Peak Measurement does work! I can understand why there may be some minimum number of observations to perform a Measurement calculation, but the current limit is not reasonable. I have more than a sufficient number of observations to calculate Peak to Peak at the lower time resolution, and it calculates and displays just fine at this resolution on the primary Channel. So I'm not sure I really understand what's going on here and if this is a bug or poor design choice. The problem with increasing the time resolution to a higher value is that you lose detail in the visual display and the individual waveform cycles cannot be seen as they get crunched together.
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Re: Using Measurements on Low Pass filter waveform

Postby PicoPilot » Wed Apr 20, 2016 11:08 pm

PicoPilot wrote:... The problem with increasing the time resolution to a higher value is that you lose detail in the visual display and the individual waveform cycles cannot be seen as they get crunched together.
I just discovered that by simply zooming in (using the Horizontal Zoom on the second menu bar) I can display a detailed waveform while preserving the Measurement values of Peak to Peak at the higher timebase. I am very pleased this resolves my issue with the Peak to Peak Measurement on the Math Channel Low Pass filter. :D


PicoPilot wrote:With regard to the Frequency Measurement on a Math Channel Low Pass filter, it simply does not work. This may be understandable as frequency is an attribute of the primary channel signal. The Math Channel series could be a calculated (synthetic) data series that doesn't carry along the frequency attribute. At least, that's how I might rationalize why it doesn't work.
I am still uncertain if the Frequency Measurement is supported for a Math Channel Low Pass filter. Is there a workaround? I would greatly appreciate a definitive statement from a Pico staff member. :?:
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Re: Using Measurements on Low Pass filter waveform

Postby Gerry » Fri Apr 29, 2016 8:50 am

Hi PicoPilot,

Regarding Measurements, PicoScope 6 doesn't discriminate between a Signal Channel and a Maths Channel, so it will perform calculations based upon the sample values or generated values that it finds in a buffer. For peak-to-peak measurements the calculation is done for minima and maxima between points. For frequency measurements, it's done according to zero crossings between points.

The key to getting valid measurement data is that there has to be enough sample values between the points for these calculations to be possible (remember that filtering the data is analogous to throwing away a lot of sample values). It is possible to get frequency measurements on a Maths Channel filtered waveform, but you are going to have to slow the time base down significantly to see if you can get enough sample points for the calculation. As you have a PicoScope 2205 that has a limited buffer size, this will limit the combination of timebase, signal and filtering that will give you frequency measurements.

For frequency measurements there also has to be more than one complete cycle between the span being measured for the calculation to be possible. Calculations for frequency can also be prevented where the initial levels used for calculation are significantly larger than the remaining values that need to be used, preventing them from being recognized as values to be used. So, for instance a varying DC level on a higher frequency waveform could cause a problem.

Regards,

Gerry
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Re: Using Measurements on Low Pass filter waveform

Postby PicoPilot » Sun May 01, 2016 5:35 pm

Gerry, thanks for your explanation. I neglected to include a screenshot in my original question. So I'm attaching a screenshot here along with an example that anyone can duplicate using demo mode.

Step-by-step:

- I disconnected my Picoscope from my PC so there are no Picoscope model-specific limitations and the software runs in demo mode.

- I am displaying the default waveform that appears in demo mode on Channel A (a 1KHz sine wave). I added a Freq measurement that indicates the freq values, as shown in the upper pane of the attached screenshot.

- I created a LowPass filter with a threshold of 1MHz which is displayed in a new scope view in the lower pane. The formula is LowPass(A,1000000). This frequency setting is much higher than the 1KHz waveform so the LowPass filter will essentially pass the entire signal through, but it's defined as a math channel filter rather than a primary channel.

- I added a Freq measurement to the LowPass filter. As you see in the attached screenshot, the Freq measurement does not display any values for this LowPass filter. I also performed this test on the Beta version of Picoscope software version 6.12.1.1691 and I got the same result.

Is this a bug? Please advise.
Attachments
Picoscope ScreenShot 250KB.jpg
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Re: Using Measurements on Low Pass filter waveform

Postby Gerry » Wed May 04, 2016 9:15 am

Hi PicoPilot,

The purpose of Demo Mode is to provide basic functionality of the software so that users can experience the PicoScope 6 working environment when performing common, typical functions of a USB/computer based oscilloscope without having the hardware. So, less common complex functions, or combinations of many functions are not necessarily implemented. To perform functions such as measurements of maths channels you really need to be using PicoScope 6 with a hardware PicoScope connected.

The process of performing calculations and then displaying the waveform in real time is quite processor intensive, so to actually get meaningful data you need to give the processor a chance to be able to perform all the calculations. For your specific example the frequency measurements will work, for instance in a PicoScope 3404D, when you slow the timebase down to 500uS/div. For more complex filters such as a bandpass you may have to slow the timebase down even further. Because, the process of filtering involves removing data, there are some instances where the values at the beginning and end of a trace don't produce any filtered data, so using rulers and making measurements between them, to remove the beginning and end of a trace is a way of avoiding corrupted measurements when this happens.

It would be useful to know what it is, specifically, that you want to achieve here?

If you don't have a practical application goal but you want to continue with this topic, we can provide you with better support on our help desk. To do this, just send an email to support@picotech.com and mark it for my attention.

Regards,

Gerry
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