Posted by
Don Wilhelm-4 on
Jan 14, 2008; 5:33pm
URL: http://elecraft.85.s1.nabble.com/K2-100-Power-investigation-tp458672p458681.html
The 10X probes I have are rated at either 300 volts or 600 volts, so no
problem.
Vic does bring up a very good point that you should know the ratings for
anything that you use in electrical measurements - not only to keep from
destroying it, but also to know whether the device will behave as you
expect it to. Sometimes what you see on the 'scope face is not really
what you are measuring. I know my 100 MHz rated probes create some
loading at 30 MHz, so I don't use those for measurements at that frequency.
I should have stated that 10X probes were definitely to be used, 1X
probes load too much.
At lower power levels, an RF Probe will provide very good results. The
DL1 is a good dummy load to use and it has a peak detecting RF Probe
attached across half of it.
I don't understand why some folks seem to continuously convert from peak
to RMS and back again - when using a 'scope, the display will show the
peak voltage - I use the derived formula for power as Vp-p^2/8R when
working with a 'scope - that is square the peak to peak voltage and
divide by 400 if the load is 50 ohms - I find it easier than doing all
the conversion formulas.
73,
Don W3FPR
Vic K2VCO wrote:
> Vic K2VCO wrote:
>>
>> Be careful if you use this method! I learned the hard way (destroyed
>> THREE probes) that the voltage rating for scope probes goes down as
>> frequency goes up. There is a little piece of paper that comes with
>> the probes; look at it, compute the voltage across your dummy load at
>> the power levels that you will be testing (100 watts is about 70.7
>> volts), and make sure that the probe can handle it at the frequency
>> of interest.
>
> I should have said 70.7 v RMS; the peak voltage will be 1.414 x 70.71
> = 100v, which means that the peak-to-peak voltage will be 200v.
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