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Author Topic: Accurate Measurements on pulsed system's harder than you think.  (Read 52769 times)

Offline EMJunkie

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Re: Accurate Measurements on pulsed system's harder than you think.
« Reply #180 on: December 14, 2015, 05:45:26 AM »
Now you are oversimplifying.  Sometimes the use of RMS values for measurements will be more appropriate, depending on what you are doing.

But as I've said, and as Verpies and Poynt99 have also confirmed, the proper way to do _power_ calculations, if you have the data as you do from a DSO, is to do the instantaneous multiplication of the actual per-sample values of voltage and current, which generates an instantaneous power waveform, then take the _average_  (not RMS) value of that IP waveform. This procedure works for all waveforms, whether complex or not, all load types, and all power factors. All the phase shift and other complications are taken care of by the sample-by-sample calculations performed in the scope, typically at hundreds of thousands of samples per second or more, so the errors caused by this "numerical methods integration" are very small.


Yes, I realise. Possibly a good idea to check with both equations anyway. Its not hard. Just a case of gathering the data which one will be doing anyway.

As one equation partially verifies the other.

   Chris Sykes
       hyiq.org

Free Energy | searching for free energy and discussing free energy


Offline EMJunkie

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Re: Accurate Measurements on pulsed system's harder than you think.
« Reply #181 on: December 14, 2015, 05:48:02 AM »
So turn up your power supply until the scope/DMM reading is 2.5 V and then re-do your power calculations using the scope's new RMS value and let's see how it comes out.

It looks like your Duty Cycle is accurate across the scope screen. I have found that horizontal measurements are usually more accurate than the vertical ones, since there is less noise and you aren't limited to the 8-bit ADC precision levels.


Yes, this what I did in the first place, some drift slipped in there.

I should have rechecked it again before posting. Was only five minutes between experiment and taking Pics. Still shows others something else to watch for also.

   Chris Sykes
       hyiq.org

Offline EMJunkie

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Re: Accurate Measurements on pulsed system's harder than you think.
« Reply #182 on: December 14, 2015, 06:25:26 AM »
For Giggles:

The 1KHz Signal is from a High Side Mosfet Driver. The 1K Resistor is measured to be 1K.

Mean Calculation: Red Channel

V / R = I = 5 / 1000 = 0.005 Amperes

I2 * R = P = (0.0052 * 1000) * 0.5 (Duty Cycle) = (0.000025 * 1000) * 0.5 (Duty Cycle) = 0.0125 Watts

RMS Calculation: Yellow Channel

V2 / R = P = 3.4582 / 1000 = 11.957764 / 1000 = 0.011957764 Watts

Now I must have something wrong here? We see: 0.000542236 Watts difference!

   Chris Sykes
       hyiq.org


I am still seeing a possible problem if the resistance changes in time, or am I seeing something that is not there?

In all of the Equations we use, Ohm's Law, if R changes during the operation of the circuit, then the Power through the Circuit Element in question will change.

Eliminating the problem, like TK has pointed out, can only be done with a Current Sensing Resistor with absolute minimal Inductance and using this Voltage Drop, Across the Non Inductive Current Sensing Resistor, CSR.

   Chris Sykes
       hyiq.org

Free Energy | searching for free energy and discussing free energy

Re: Accurate Measurements on pulsed system's harder than you think.
« Reply #182 on: December 14, 2015, 06:25:26 AM »
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Offline TinselKoala

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Re: Accurate Measurements on pulsed system's harder than you think.
« Reply #183 on: December 14, 2015, 06:55:31 AM »
Actually the Ohm's Law relationships are definitions of each term in terms of the others. So, since the current depends on the resistance (if voltage is constant) then even though the resistance isn't explicitly included in the particular formula you may be using, it is still there hidden in the other variable values. So P=VxI does have the resistance in there, because I = V/R.

Here's where it becomes important to understand how the scope calculates "Average" or mean values. It is adding up values from very short timeslices and then dividing by the number of slices. So even if the resistance (or voltage, or current) may vary during the "on" time of a pulse, the sampling system will catch it, as long as it isn't varying too fast. But at 1 gigasamples per second..... well, you can see that even very small, very fast changes will be caught by the system and will give the true average. This is actually a problem sometimes, since the scope may be including noise or random glitches in the average. Hence, the scopes will have "Bandwidth limiting" that can be selected which will cut down on the presence of this kind of noise in the input samples to the averaging function.

On my Rigol, the 20 MHz bandwidth limiting is indicated by a "B" symbol in the Channel V/Div display at the bottom of the screen.

Offline EMJunkie

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Re: Accurate Measurements on pulsed system's harder than you think.
« Reply #184 on: December 14, 2015, 08:00:38 AM »
Actually the Ohm's Law relationships are definitions of each term in terms of the others. So, since the current depends on the resistance (if voltage is constant) then even though the resistance isn't explicitly included in the particular formula you may be using, it is still there hidden in the other variable values. So P=VxI does have the resistance in there, because I = V/R.

Here's where it becomes important to understand how the scope calculates "Average" or mean values. It is adding up values from very short timeslices and then dividing by the number of slices. So even if the resistance (or voltage, or current) may vary during the "on" time of a pulse, the sampling system will catch it, as long as it isn't varying too fast. But at 1 gigasamples per second..... well, you can see that even very small, very fast changes will be caught by the system and will give the true average. This is actually a problem sometimes, since the scope may be including noise or random glitches in the average. Hence, the scopes will have "Bandwidth limiting" that can be selected which will cut down on the presence of this kind of noise in the input samples to the averaging function.

On my Rigol, the 20 MHz bandwidth limiting is indicated by a "B" symbol in the Channel V/Div display at the bottom of the screen.


Yes, this is the way I see it also, one can be interchanged to work out the other. Each are complimentary.

Ok I see what you mean, the over all Mean Sampling is fast and accurate enough to see any circuit changes most of the time. I must admit, I was not think clearly when posting the last post and really did not need to post it, so I am sorry for the Interlude there.

I think still, the oscilloscope is the most awesome machine we Humans have ever built. We would be lost without them!

   Chris Sykes
       hyiq.org

Free Energy | searching for free energy and discussing free energy

Re: Accurate Measurements on pulsed system's harder than you think.
« Reply #184 on: December 14, 2015, 08:00:38 AM »
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Offline TinselKoala

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Re: Accurate Measurements on pulsed system's harder than you think.
« Reply #185 on: December 14, 2015, 09:05:19 AM »
Yes, the scope is awesome. I've often said that the Lathe is the King of Tools, and the Oscilloscope is the King of Test Equipment.

However.... the low-end scopes are not especially good voltmeters. Most of them have 8-bit ADC front-ends, which means you only have 256 voltage levels across the full height of the screen in the best of circumstances and usually even fewer than that. So the precision is not that good. And as far as accuracy is concerned, they are subject to drifts due to ambient temperature, component ageing, and who knows what else.

I've just spent some time checking my scope, comparing manual calculations from the screen data with what the scope comes up with. The three screenshots below show what I've been doing. I used one data capture from the "Woopy" circuit, and used cursors to read off the heights of the CH1, CH2 and Math traces to get values for the manual calculations. And I told the scope to provide Measurements of the Duty Cycle, the CH1 and CH2 RMS, the Math and CH2 Averages.

Here are the results.

Offline TinselKoala

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Re: Accurate Measurements on pulsed system's harder than you think.
« Reply #186 on: December 14, 2015, 09:21:23 AM »
So as you can see the cursors in the CH1 test are showing 12.30 V for the peak values. Multiplying the Peak by the square root of the 19.44 percent Duty Cycle gives
12.30 x sqrt(.1944) = 5.42 Vrms, and the scope is reporting 5.40 Vrms. Not too bad, an error of less than 1 percent.

In the second test with the CH2 readings... the cursors are giving me a value of 128.0 mA for the peak values. Multiplying this by the square root of the Duty Cycle gives 0.1280 x sqrt(.1944) = 0.0564 or 56.4 mA rms, and the scope is reporting 63.1 mA rms. WTF? This is a large error. And for the Mean value, the 0.1280 x .1944 = 24.9 mA, whereas the scope says 30.8 mA. Again, a large error. WTF? Is this due to an inaccurate channel, or bad calibration, bad cursor positioning, or what? Perhaps the channel is just not accurate at very sensitive settings?

In the third test with the Math readings, the cursors give me 1.540 W for the peak value, and factoring in the duty cycle gives 1.540 x 0.1944 = 299.4 mW as the average. And the scope is reporting 305 mW as the average. Again not too bad, about 2 percent error.

These three shots are using the same sample set, I have stopped the scope and am just moving cursors around between the screensaves.

The poor result on CH2 has made me start the Self-Calibrate routine; maybe it will get better, or maybe not.

Free Energy | searching for free energy and discussing free energy

Re: Accurate Measurements on pulsed system's harder than you think.
« Reply #186 on: December 14, 2015, 09:21:23 AM »
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Offline TinselKoala

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Re: Accurate Measurements on pulsed system's harder than you think.
« Reply #187 on: December 14, 2015, 10:05:19 AM »
Well, I ran the self-cal routine and the CH2 discrepancy did go down, from about 11 percent to around 8 percent. The discrepancies on the other two readings went up slightly to about 2 1/2 percent difference each. I've noticed that the cursor position values seem a little off from what I think they should be, so maybe the error is in the reported cursor positions rather than in the scope's computations themselves.

Yes, I just checked the cursor positions carefully. There is definitely an error in the reported cursor positions.

In the screenshot below, the CH1 marker is exactly on the screen center horizontal marker (offset 0.000). The Cursor B is positioned exactly on this line and reads -100.0 mV. The Cursor A is exactly on the second horizontal line below center and should read -10.00 V but actually reads -10.20. There is a similar error on other channels.

Grrr. I'm reporting this as a bug to Rigol.






Offline EMJunkie

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Re: Accurate Measurements on pulsed system's harder than you think.
« Reply #188 on: December 14, 2015, 09:03:46 PM »


TK - A nice example of how we can see some inaccurate measurements on the Oscilloscope. Thanks for sharing this.

What's the Channel like at higher voltage levels? Does this error margin come down some?

   Chris Sykes
       hyiq.org

Free Energy | searching for free energy and discussing free energy

Re: Accurate Measurements on pulsed system's harder than you think.
« Reply #188 on: December 14, 2015, 09:03:46 PM »
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Offline TinselKoala

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Re: Accurate Measurements on pulsed system's harder than you think.
« Reply #189 on: December 15, 2015, 12:13:02 PM »

TK - A nice example of how we can see some inaccurate measurements on the Oscilloscope. Thanks for sharing this.

What's the Channel like at higher voltage levels? Does this error margin come down some?

   Chris Sykes
       hyiq.org

The error in the cursor position reporting seems to be 1 or 2 ADC counts or maybe vertical pixels at most, regardless of the v/div settings, but depending on where the cursor is positioned vertically on the screen.  It's annoying but like many errors in measuring instruments (they all have them) it can be worked around once it is known that it exists and how it behaves.

As far as I can tell the Measurements from the measure menus aren't subject to this kind of error (which is why my computations using the Cursor data didn't agree with the Measurements), but of course they too are limited by the resolution of the 8-bit ADC front end of this and all other low-end DSOs. You could think of this ADC resolution limitation as "rounding error". 

 

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