
All,
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...........
Jon
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
email jurgen.r.krannich@tek.com
Phone  5036273713 or 18008359433, ext.7 3713
8/20/97 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) is VALID ONLY WHEN THE VOLTAGE AND CURRENT SIGNALS ARE PURE SINUSOIDS. 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 produced". 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 WattSeconds (i.e. joules), WattHours, or KilowattHours. Since electrically, RMS (i.e. rootmeansquare) 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. pointbypoint) 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 Data". 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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