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[AMPS] Re:

To: <amps@contesting.com>
Subject: [AMPS] Re:
From: G3SEK@ifwtech.demon.co.uk (Ian White, G3SEK)
Date: Tue, 27 Jul 1999 20:12:34 +0100
Rich Measures wrote:
>
>
>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.  .
>
One doesn't. As I said, one calibrates the power measuring device on CW,
and then one tackles the *separate* question of grabbing the true peak
of the modulation. 

It isn't so much a question of which method you use, as where you get
your calibrations from.

>>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.  

That's true - but now talk to me about the error budget for your voltage
and resistance measurements. How accurately do you know those values at
the frequency you're measuring... and how do you know that?

>//  
>  I think it's important to realize that so-called Wattmeters are
>actually either voltmeters or ampere meters.

All wattmeters except the thermal kind... and that includes the 'scope
method.

>>
>>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.
>
You have an *RF* voltage standard? Sorry, just being picky, to make the
point that all methods of RF power measurement are on pretty shaky
ground unless they can provide a calibration that is traceable to a
thermal standard.

>>
>>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
>
Peter disagreed, and he's right in principle. I should have said "but
*for our purposes* it *can* mean one or more identical RF cycles."

>>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.
>
A 'scope is an excellent way to check the peak-reading function. It's a
poor way to calibrate the actual RF power measurement.

73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                          'In Practice' columnist for RadCom (RSGB)
                           http://www.ifwtech.demon.co.uk/g3sek

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