[AMPS] Re:

[Rich Measures]

Ian 

>Rich Measures wrote:
>>>When the FCC measures power, they use a regular peak reading 
>>>meter. .....
>>
>>€  wanna guess how such meters are ultimately calibrated?.  
>>
>Not against a 'scope, but ultimately against a terminating thermal
>wattmeter which is the only system that is absolute and verifiably
>broadband.
>
€  How does one use a bomb-calorimeter to measure PEP during voice 
modulation.  .  

>Scopes and all kinds of "through-line" wattmeters don't actually measure
>power - they measure voltages and/or currents, maybe only proportional
>to the voltages and currents in the line (and the ratios may vary with
>frequency). Then either you or the makers of the instrument have to
>calculate the power level - which requires certain assumptions. 

€  Yes, such as voltage squared divided by resistance equals watts.  //   
  I think it's important to realize that so-called Wattmeters are 
actually either voltmeters or ampere meters.  
>
>For example, if you're using a 'scope, it requires the assumption that
>the Y-amp and probe calibrations are both correct at that frequency, and
>that you've read the peak voltage off the screen correctly (+/- the fuzz
>on the trace). This isn't especially a criticism of the 'scope method;
>all other methods have their own weaknesses and sources of error.
>
€   Indeed, Ian.  This is why I check the compensation of my x100 
multiplier probe, and calibrate the vertical deflection against my 
voltage standard before I make a measurement.  

>All this is for constant (CW) power.  The peak-envelope-power function
>is not an RF-related problem at all - it's a problem of accurately
>finding and holding the peak of the modulation envelope, which in our
>case is down at audio frequencies. 

€  An oscilloscope is used to measure the RF sinewave per se.  .  
>
>The definition of PEP does refers to "one cycle" but that means "one or
>more identical RF cycles" as distinct from "the peak inside a single RF
>cycle". In practice the peak of the modulation envelope contains many RF
>cycles at that are substantially identical, because the modulating
>frequency is thousands to millions of times lower than the radio
>frequency.
>
€  agreed

>With sine-wave modulation, you can go as much as +/- 5.7 degrees away
>from the peak of the modulating envelope (along the X/time axis) before
>the voltage drops enough to cause a 1% error in PEP. For 1kHz modulation
>at 1.8MHz, this allows about 60 RF cycles that are all close enough to
>the peak. A peak-reading instrument has this much time to grab its
>rectified voltage sample, and if it is averaged over the whole period
>the error in PEP is <1%.
>
>I think that in practice the 'scope is no better at identifying the
>modulation peak, because the user does the same averaging process by
>eye.

€  The scope may be set to trigger at any voltage.  
>
>The real problem is that there are so many meters out there with poor
>peak-reading circuits. Some of them don't accurately grab the peak of
>the modulation, and then some don't hold it for long enough for the
>meter needle to respond accurately - or if they do, the total discharge
>time is too long. A really good peak-reading converter would hold for
>about one second after the peak, to give you time to read the meter, and
>then discharge quite rapidly to allow the meter to respond again.
>
€  which is why a calibrated oscilloscope is used to calibrate peak 
reading RF wattmeters.
  
-  cheers, Ian


Rich...

R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures  


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