Re: Optimize pow() function

From	pozz <pozzugno@gmail.com>
Newsgroups	comp.arch.embedded
Subject	Re: Optimize pow() function
Date	2016-09-26 23:30 +0200
Organization	A noiseless patient Spider
Message-ID	<nsc40b$2sf$1@dont-email.me> (permalink)
References	<ns0mo1$7j2$1@dont-email.me> <DZNFz.52445$ba.36815@fx25.iad> <ns8jte$kn4$1@dont-email.me>

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Il 25/09/2016 15:36, Tauno Voipio ha scritto:
 > On 25.9.16 13:50, Richard Damon wrote:
 >> On 9/22/16 9:36 AM, pozz wrote:
 >>> I have to convert voltage measure (Volt) into power (Watt).  As you
 >>> know, the physics says I need to square the voltage measure.
 >>>
 >> Whoever told you that didn't give you the full message, as it applies in
 >> only very limited cases.
 >>
 >> Electrical Power is Voltage * Current, not Voltage squared.
 >>
 >> IF you have a resistive load, then we have that
 >>
 >> Voltage = Current * Resistance.
 >>
 >> From this we can get to Power = Voltage * Voltage / Resistance.
 >>
 >> This means that for a system with constant resistance we can say that
 >> Power goes as Voltage squared, with a scale factor of 1/Resistance.
 >>
 >> If the resistance of the system isn't constant, this relationship
 >> absolutely doesn't hold. I have built systems where (within an operating
 >> band) the Power consumed goes up with dropping voltage.
 >>
 >> If all you can measure in the field is the Voltage, and the system if
 >> fairly stable, then what you can do is in the lab go to a NUMBER of
 >> points and measure voltage and power to determine how your system
 >> responds and come up with some sort of curve to fit the results.
 >>
 >> You need MORE points than unknowns to be able to sanity check your fit.
 >> You also should perform it under varying conditions (like ambient
 >> temperature) to make sure the results are stable.
 >
 >
 > The situation of the OP is more complex: He's attempting to measure
 > the output power of a RF amplifier by measuring the DC feed voltage
 > and relating the output power to it.

I think many misunderstandings derive from this assumption.  I never 
wrote (or I think I never wrote) that I need to calculate the output 
power of an amplifier knowing only the DC feed voltage.

I'm trying to calibrate a measurement device that, in my case, is a 
directional coupler.  For me (the firmware engineer), it is a voltage 
signal that is related to the output power by a square mathematical law 
(at least, this said the RF engineer).

By comparing our results with a professional instrument, we noticed that 
the square law (P=kV^2) is somewhat good for our necessity. 
Unfortunately, if it is calibrated at the nominal power (k), at low 
power levels the results deviate a little from the theorical square law.

This is why I'm trying to introduce a second parameter in a mathematical 
law P=f_k,h(W) that is *the* square law when the second calibration 
parameter (h) isn't touched.
That formula is:
    P = k * (V / Vn) ^ (2 + h)
When h=0 (second calibration parameter not touched) the law *is the* 
theoric square law.

If the operator notices a big error at, i.e., Pn/2, he tries to change 
h. In this way, he is sure the measure at Pn/Vn will not change.

 >
 > There are simply too many moving parts in the equation, it will not
 > work with some mathematical trickery.
 >

Thread

Optimize pow() function pozz <pozzugno@gmail.com> - 2016-09-22 15:36 +0200
  Re: Optimize pow() function Don Y <blockedofcourse@foo.invalid> - 2016-09-22 07:17 -0700
    Re: Optimize pow() function pozz <pozzugno@gmail.com> - 2016-09-22 23:10 +0200
      Re: Optimize pow() function rickman <gnuarm@gmail.com> - 2016-09-22 17:31 -0400
        Re: Optimize pow() function lasselangwadtchristensen@gmail.com - 2016-09-22 14:44 -0700
        Re: Optimize pow() function pozz <pozzugno@gmail.com> - 2016-09-23 09:03 +0200
          Re: Optimize pow() function Paul Rubin <no.email@nospam.invalid> - 2016-09-23 00:17 -0700
          Re: Optimize pow() function Tauno Voipio <tauno.voipio@notused.fi.invalid> - 2016-09-23 22:45 +0300
      Re: Optimize pow() function rickman <gnuarm@gmail.com> - 2016-09-22 17:44 -0400
        Re: Optimize pow() function pozz <pozzugno@gmail.com> - 2016-09-23 09:08 +0200
          Re: Optimize pow() function rickman <gnuarm@gmail.com> - 2016-09-24 01:54 -0400
      Re: Optimize pow() function Don Y <blockedofcourse@foo.invalid> - 2016-09-22 14:50 -0700
        Re: Optimize pow() function pozz <pozzugno@gmail.com> - 2016-09-23 09:23 +0200
          Re: Optimize pow() function Don Y <blockedofcourse@foo.invalid> - 2016-09-23 00:32 -0700
          Re: Optimize pow() function Evgeny Filatov <e.v.filatov@ieee.org> - 2016-09-23 12:56 +0300
            Re: Optimize pow() function Don Y <blockedofcourse@foo.invalid> - 2016-09-23 10:41 -0700
          Re: Optimize pow() function Hans-Bernhard Bröker <HBBroeker@t-online.de> - 2016-09-23 21:07 +0200
          Re: Optimize pow() function Don Y <blockedofcourse@foo.invalid> - 2016-09-23 12:12 -0700
    Re: Optimize pow() function pozz <pozzugno@gmail.com> - 2016-09-22 23:12 +0200
      Re: Optimize pow() function Don Y <blockedofcourse@foo.invalid> - 2016-09-22 14:58 -0700
  Re: Optimize pow() function Tim Wescott <tim@seemywebsite.com> - 2016-09-22 10:01 -0500
    Re: Optimize pow() function Jack <jack4747@gmail.com> - 2016-09-22 08:08 -0700
      Re: Optimize pow() function Tim Wescott <seemywebsite@myfooter.really> - 2016-09-22 11:53 -0500
        Re: Optimize pow() function pippo2@disney.com (Jack) - 2016-09-22 21:12 +0200
          Re: Optimize pow() function lasselangwadtchristensen@gmail.com - 2016-09-22 13:12 -0700
          Re: Optimize pow() function Tim Wescott <seemywebsite@myfooter.really> - 2016-09-22 15:51 -0500
    Re: Optimize pow() function pozz <pozzugno@gmail.com> - 2016-09-22 23:15 +0200
      Re: Optimize pow() function Tim Wescott <seemywebsite@myfooter.really> - 2016-09-22 17:52 -0500
      Re: Optimize pow() function David Brown <david.brown@hesbynett.no> - 2016-09-23 09:32 +0200
    Re: Optimize pow() function David Brown <david.brown@hesbynett.no> - 2016-09-23 09:28 +0200
      Re: Optimize pow() function Paul Rubin <no.email@nospam.invalid> - 2016-09-23 00:47 -0700
        Re: Optimize pow() function David Brown <david.brown@hesbynett.no> - 2016-09-23 11:05 +0200
        Re: Optimize pow() function Tim Wescott <tim@seemywebsite.com> - 2016-09-23 11:44 -0500
  Re: Optimize pow() function Jack <jack4747@gmail.com> - 2016-09-22 23:15 -0700
    Re: Optimize pow() function Hans-Bernhard Bröker <HBBroeker@t-online.de> - 2016-09-23 20:49 +0200
  Re: Optimize pow() function Richard Damon <Richard@Damon-Family.org> - 2016-09-25 06:50 -0400
    Re: Optimize pow() function Tauno Voipio <tauno.voipio@notused.fi.invalid> - 2016-09-25 16:36 +0300
      Re: Optimize pow() function Chris <xxx.syseng.yyy@gfsys.co.uk> - 2016-09-26 17:30 +0000
        Re: Optimize pow() function Tauno Voipio <tauno.voipio@notused.fi.invalid> - 2016-09-26 21:49 +0300
      Re: Optimize pow() function pozz <pozzugno@gmail.com> - 2016-09-26 23:30 +0200
        Re: Optimize pow() function Tauno Voipio <tauno.voipio@notused.fi.invalid> - 2016-09-27 18:58 +0300
  Re: Optimize pow() function Robert Wessel <robertwessel2@yahoo.com> - 2016-09-26 22:44 -0500

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