[Elecraft] New K3 built - can I reset the 5w calibration?

[Jack Smith]

Don:

I would add a small caution.  The maximum safe voltage rating of a 10x 
probe is a function of frequency.

For example, Tektronix's model P6106A, a 250 MHz 10x probe, is rated at 
450V below 1 MHz, but only 60V at 10 MHz and perhaps 50V at 30 MHz (hard 
to read the graph accurately). All are RMS values.

100 watts is 70.7 V  RMS, so the 10 MHz - 30 MHz voltage limits will be 
exceeded at that power level. (100V RMS probe limit looks to be around 
6.5 MHz.)

Of course, I'm sure there's a safety factor in the probe specs, but 
before I measured 100 watts output with a 10X probe I would verify the 
safe voltage rating in the probe manual.

Jack K8ZOA


On 12/8/2011 6:09 PM, Don Wilhelm wrote:
> Tom,
>
> OK, it seems now is a good time for me to insert my periodic "wattmeter
> accuracy" rant!  Sub-titled -- "Technician, know your tools but know
> even better their limitations" OR - "No instrument can be relied on
> until it has proven it is capable of telling the truth under the
> conditions of use".
>
> If you want quite good accuracy at the  5 watt level, I suggest you use
> a 'scope with a 10X probe connected across the dummy load.  The peak to
> peak RF voltage can be easily measured by the vertical deflection of the
> 'scope.  To convert that peak to peak voltage reading to power (when the
> dummy load is 50 ohms), use your calculator - enter the peak to peak
> voltage, square it, and divide by 400.
>
> If you do not have an open dummy load that makes clipping the 'scope
> probe across it easily, use a Tee adapter at the dummy load end so you
> can gain access to the center conductor.
>
> 40 volts peak to peak is 4 watts - you might want to use that instead of
> 5 watts since it is even and easy to see on the 'scope face.  If you
> really need 5 watts, adjust for 44.72 volts peak to peak.
>
> In fact, you can use the 'scope probe across the dummy load up to any
> level permissible by your 'scope and probe.  My probes will handle 100
> volts peak easily, and that is the 100 watt level.  The accuracy is only
> limited by the accuracy of your dummy load (mine is within 1%) and your
> ability to interpolate the voltage deflection on the 'scope.  I might
> suggest that you can come within 5% of the actual power level, which is
> much better than analog power meters whose accuracy is expressed as a
> percentage of full scale.  So a freshly calibrated Bird wattmeter with a
> 100 watt slug is spec'ed for 5% of full scale - OK, that is a 5 watt
> potential error - and that applies anywhere on the scale - so trying to
> read a 5 watt power level with a Bird and a 100 watt slug is an exercise
> in futility - use the 'scope, it is a LOT more accurate.
> OTOH, many recent digital wattmeters will be spec'ed for accuracy
> anywhere within its range rather that as a percentage of full scale used
> with the analog wattmeters.
>
> The (Vp-p)exp2/400 formula (actually [Vp-p]exp2/8R) does not seem to be
> well know.  I leave it "to the student" to derive it from standard
> formulas for power, voltage and resistance.  Hint - use SQRT2 rather
> than the 1.414 or 0.707 approximations when converting peak to peak
> voltage to RMS - it is an exact formula, the squares and square roots
> cancel.
>
> 73,
> Don W3FPR
>
> On 12/8/2011 5:12 PM, TJ Campie wrote:
>> I finished building a K3 for the radio club at work and I thought I'd take
>> the bull by the horns and attempt to improve the factory setting for the 5w
>> calibration on the 100w Bird watt meter I had.  I couldn't get it to go up
>> the the point that looked like 5w on the meter regardless of how much I
>> turned the knob on the K3.  The setting ended up at 10 (the minimum) but I
>> think it must have been more correct from the factory.  I *think* it was up
>> at about 37 but I don't remember - is there a way to reset it to the
>> factory setting?  I don't have access to a 5w accuracy meter.
>>
>> Otherwise, the build went great and was a blast :)
>>
>> TJ W0EA
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