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Total power in spectrum mode

Forum for discussing PicoScope version 6 (non-automotive version)

Total power in spectrum mode

Postby Tech Hiker » Fri Apr 08, 2016 1:10 am

One of the measurements that can be made with PicoScope 6 in Spectrum Mode is Total Power and I'm trying to understand what that is.

Here is my setup, and spectrum mode settings.
PiccoScope 2205A.
Input signal is 1khz, 7 volts RMS pure sine from a signal generator.
Input range 20V, DC coupling
Spectrum range 24.41khz.
Spectrum bins 16384
Window function Blackman
Display mode Magnitude
Scale Log
Log unit dBu
x scale log10
Num decades 3

When I add a measurement for Total Power(Whole trace) it gives me 6.5 watts. Where did this 6.5 number come from? That number is way too big for a 600 ohm reference load.

Tom
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Re: Total power in spectrum mode

Postby TomVeik » Thu Apr 14, 2016 3:46 pm

Does anyone know the answer to this question?

Tom
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Re: Total power in spectrum mode

Postby Gerry » Fri Apr 15, 2016 8:28 am

Hi Tom,

Unfortunately this is an error in our software. The calculation is done correctly in PicoScope 6.12 and later versions of software, and calculated as:

power = (sum of (V^2)) / impedance

You can download our latest Beta (version 6.12.1 from here: https://www.picotech.com/downloads/_lig ... Scope6Beta) to perform the calculation correctly.

Regards,

Gerry
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Re: Total power in spectrum mode

Postby TomVeik » Fri Apr 15, 2016 12:37 pm

Okay, thanks.

Tom
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Re: Total power in spectrum mode

Postby TomVeik » Thu May 12, 2016 1:37 pm

I installed the beta today and I'm getting the error
No suitable device found.

How do I fix this?

Tom
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Re: Total power in spectrum mode

Postby Gerry » Fri May 13, 2016 3:43 pm

Hi Tom,

First of all, our help desk is probably a better medium for this kind of problem, so it may be better to send an email in to us at support@picotech.com marked for my attention.

How did you install the Beta version? Could you walk us through the steps that you took (including what you did with the non-beta version of PicoScope 6, and at what point you connected the Hardware PicoScope).

There are a number of potential causes for the communication problem, so we need to eliminate some of the more common issues. There is a self-help guide for connection problems like this which you can find here: https://www.picotech.com/download/manua ... uide-1.pdf.
This will take you through a series of checks to perform (in particular establishing if a valid PicoScope device driver exists for your model of PicoScope, and sending in a 'trace.xml' file to us if none of the checks or suggestions help).

Regards,

Gerry
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Re: Total power in spectrum mode

Postby Safar » Fri Sep 30, 2016 4:17 pm

Hi,

I have same question about Total Power calculation.

I feed 10Vpp Sin signal @ 50 kHz to 5444B scope. And I use 6.12 beta.

Measurements in scope window: PP = 10,2V and RMS = 3,573V

OK, if we calc power in 600 Ohm load it should be 21,28mW

In Spectrum window (FFT 131072 bins):

1) in dBm (@600 Ohm) Total Power is 36,74 mW in Blackman window. For Blackman-Harris is 42,65 mW, and most close is Rectangular = 21,28 mW

2) in dBV TP = 22,05 μW in Blackman window. Amplitude in Peak = 10,71 dBV is almost correct (3,42V for 1V base). Don't understand what impedance used in Power calc? 1M input scope? It close for Rectangular = 12,77 mW

3) In dBu is same result with dBV. If I understand dBu is level for 0.775V on 600 Ohm load that corresponding 1mW. Why again 1MOhm? Amplitude in Peak = 12,91 dBu is almost correct (3,42V for 0,775V base).

Could somebody explain me why power so depend of window and why dBu calc not on 600Ohm base?

Thanks,
Alexander
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Re: Total power in spectrum mode

Postby Gerry » Tue Oct 04, 2016 11:44 am

Hi Safar,

To answer your questions:

1) A spectrum analyzer, sweeps a narrow band filter over the frequency range of a signal of interest to look at the magnitude of the frequencies. You are effectively performing the digital equivalent when you perform a Fast Fourier Transform, i.e. applying an FFT calculation to the discrete sample values of the signal enclosed by a windowing function (the digital equivalent of the filter) replicated up and down the spectrum. The windowing function is a complex valued function (or time-variant function) that will change the relative magnitudes and phase of the non-zero values of the signal, creating components not present in the original signal (known as spectral leakage), and leading to components that are reduced/distorted due to signal and window offsets (scalloping loss). These errors interfere with the analysis, for instance, the spectral leakage can make it hard to distinguish between similar amplitude spectral components. So, to increase the visibility of certain spectral components relative to others, different windowing functions can be applied when calculating the FFT. These differences also provide other benefits (this is explained in more detail on the Wikipedia page here: https://en.wikipedia.org/wiki/Window_function).

Most of the windowing functions have errors components associated with what happens at the endpoints (including overlapping) and the flatness of the window. The more leakage, and error components a windowing function has the greater will be the contribution to the measurement. A rectangular windowing function has perfect reproduction of a single frequency sinewave, but also has scalloping loss, and minimal dynamic range (which doesn’t affect a Total power calculation). Because the power is computed as the square of the spectral components, the RMS value of a power spectrum based upon a rectangular window function gives an accurate value. Other windowing functions will give greater values of RMS due to the error components created in addition to the sinewave.

dBm is an expression of a power ratio. It is the power measured relative to 1mW dissipated into a 600Ω load. As you are using the 6.12 Beta version of PicoScope 6, you will notice that when you select the dBm scale the impedance value of 600Ω is also selected, indicating that all references are now to a 600Ω load. So, the actual load that the voltage is driving is not used in the calculation and you should perform the power calculation with this in mind, as follows:
Power = 0.707^2 / 600 = 8.33 * 10^-4 = 833uW
As you can see from the screenshot of the Spectrum plot, and the psdata file when the Logarithmic unit chosen for the scale is dBm the measured ‘Total Power’ is 836uW, which confirms the calculation.
2V p2p sinewave power measurements.psdata
(2.92 MiB) Downloaded 7 times

dBm scale spectrum plot.png


2) dBV is an expression of a voltage ratio. It is the voltage measured relative to 1 volt, regardless of impedance. You will notice that when you select the dBV scale the impedance value is not displayed as there is no fixed impedance value that is relevant to the calculation. So you now consider the dB ratio being referenced to 1V and driving a load that will have an effect on the power calculation, as follows:
Power = 0.707^2 / 1000,000 = 0.5nW
From the screenshot of the Spectrum plot, when the Logarithmic unit chosen for the scale is dBV the measured Total Power is 502nW, which confirms the calculation.
dBV scale spectrum plot.png


3) Although originally derived using a 600Ω load, dBu is an expression of a voltage ratio. It is the voltage measured relative to 0.775V, regardless of impedance. So, once again the Impedance is not shown when selecting it for the scale in the spectrum plot, and using impedance for power calculations relative to a 600Ω load is not applicable. You therefore, once again, need to consider the input impedance of the PicoScope in the power calculation (which will be the same as before), and the measurement of ‘Total Power’, once again, confirms this (as shown in the screenshot for dBu).
dBu scale spectrum plot.png


Regards,

Gerry
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Re: Total power in spectrum mode

Postby TomVeik » Fri Oct 07, 2016 9:12 pm

Speaking of that Beta version, I still can't get it working. Always get the error "No suitable device found." I'd like to try it but it won't work for me. I started an email string with support but that seems to have gotten dropped.

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Re: Total power in spectrum mode

Postby Martyn » Sat Oct 08, 2016 10:47 am

A reply was sent on the email, I have resent the text. There was an exe attached so it may have been blocked. If you you reply to the email asking for a download link I will send this on Monday.
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