[Elecraft] Test Equipment Procedures WARNING

[Ed Milcarsky]

Hi Roy,

From experience I know that a scope probe's maximum voltage must be de-rated
with increasing frequency.  I toasted one of my probes doing just what was
suggested to you, i.e., measuring the voltage at a 50 ohm dummy load
directly with the scope probe.  Hopefully your probes came with a voltage
de-rating curve.

My example:

My scope probe is rated at 600V for DC but at 30MHz the maximum voltage is
only around 25 volts.  So if you try to measure 100 watts at the 50 ohm
dummy load on the 10 meter band, like I did, the probe sees about 71 volts
rms.

PoooooF went my probe!  (It failed open circuited by the way)

You can make a voltage divider with say a 100K resistor in series with 1K
and measure the voltage across the 1K.  You put this series combination in
Parallel with your dummy load.  Then you do the math to figure the true rms
voltage over the dummy load.

Ed  KG4ARN