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

Thu Dec 8 19:22:32 EST 2011

Thanks Jack for that additional information and caution.

I believe that is an accuracy limitation for the probe specs, and not a 
probe damage rating.

I routinely leave my 'scope probe connected to the dummy load at the 
workbench and it is subjected routinely to powers up to 180 watts.

I use the RF voltage across the dummy load to accurately assess the 
power output of the KX1, K1 and K2/10 transceivers that I repair.  At 
higher power levels, I use the LP-200 wattmeter which has been 
calibrated at the 20 watt and the 80 watt levels to make quick decisions 
on the power output of a transmitter.  If I want a quick indication, I 
just look at the LP-200.  It is one of the digital wattmeters that is 
good to within better than 10% of the actual reading (it is better than 
5% at the 20 and 80 watt calibration points).  If I need greater 
accuracy or NIST traceability, I invoke the LP-100 which Larry (N8LP) 
has calibrated with his NIST traceable standards.  I use that wattmeter 
as the "master" for calibrating all the others at higher power, but at 
low power levels, I rely on the 'scope peak to peak readings and my 1% 
50 ohm dummy load and punch the buttons on my calculator.  With this 
combination of tools, I can commit to better than 5% accuracy in power 
measurement over the 1 to 100 watt range.

I also have accurate power measurement capability for low levels (-75 
dBm through 100 watts), with the W7ZOI Power meter,  but that is a 
different subject - if anyone is interested, contact me off-list.

73.
Don W3FPR

On 12/8/2011 6:40 PM, Jack Smith wrote:
> 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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