Sujet : [MEG_builders] THS720P measurement error 
Date : 10/05/2002 03:06:44 Paris, Madrid
De : Jon Flickinger
A :
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Below is an interesting response plus an attached document that I received from the technical support folks at Tektronix that came as a final answer to my continued questioning about the Math channel inaccuracies of my TDS3034. Did you ever wonder why you couldn't make any sense of Bearden's and JLN's THS720P scope readings, well...........


Hi Jon,
Even though our scopes allow this it turns out the RMS measurement on a math waveform is bogus and has no value. I have a attached a document discussing this.

Best Regards
Jurgen Krannich
Tektronix Customer and Sales Support Center
Phone - 503-627-3713 or 1-800-835-9433, ext.7 3713


There's a difference between WaveStar's True Power calculation and the way the THS720P's Harmonics FW does it. The difference can lead to
correlation questions.

First however, two important points should be be noted:

1. The classic formula for True Power i.e. (RMS volts X RMS current X Cos Theta)

2. RMS values of power are meaningless - THERE'S NO SUCH THING AS RMS WATTS.

       By definition, power is, "The rate at which work is done".
       Mathematically, power is expressed as, "The derivative of
       work with respect to time" and is commonly measured in
       units such as Watts or Horsepower.

       Practically, the derivative of work may be defined as,
       "The instananeous rate of change of work with respect
       to a variable" (i.e. in electrical applications the
       variable is time).

       Further, power may also be defined as, "The rate per
       unit of time at which electrical energy is consumed or

       The energy supplied by a current to a load (appliance)
       enables the load (appliance) to "work" or to provide other
       forms of energy such as light, heat, etc.

       The amount of electrical energy a load (appliance) uses is
       found by calculating the product of it's power rating
       (Watts) by the operating time.  Units of electrical energy
       are usually expressed as Watt-Seconds (i.e. joules),
       Watt-Hours, or Kilowatt-Hours.

       Since electrically, RMS (i.e. root-mean-square) is defined
       as the, "The value of an AC current or voltage, equivalent
       to it's DC value, there is obviously no such quantity as
       RMS power (i.e. power is power - no equivalent).

WaveStar's "Measurement" function calculates several RMS power values
which, as shown above, are meaningless and should be ignored.
These include:
       RMS Watts
       AC RMS Watts
       Cycle RMS Watts
       Cycle AC RMS Watts

The correct way to calculate True Power is to perform an instantaneous
multiplication (i.e. point-by-point) to calculate the instantaneous
power waveform - then take the mean value of the power waveform.

In WaveStar, this is accomplished by:

       1. Acquire the voltage and current waveforms.

       2. Use the "Harmonics Graph" function to "Calculate Harmonic

       3. Use the "Harmonic Tabular" function and read "Power"
          (above the table).

An alternate WaveStar method is to:

    1. Acquire the voltage and current waveforms

    2. Use the "Harmonics Graph" function to "Calculate Harmonic
       Data" and "Create Power Waveform"

    3. Use the "Measurement" function and read "Mean (CH1*CH2)"

WaveStar performs it's calculations using floating point math and so, is quite accurate.

THS720P's Harmonics FW calculates True Power the same way but doesn't employ floating point math and so, truncates values. This leads to somewhat different answers.

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