[Elecraft] Elecraft RF Probe

[Geoffrey Mackenzie-Kennedy]

Ron, AC7AC, wrote:


As you make the RF probe better able to handle high RF voltages you will
also cause it to be less accurate at lower RF voltages.

The common RF probe diodes have a turn-on voltage of about 0.2 volts. Below
that, they are almost open circuits and have no output. Higher PRV diodes
have turn-on voltages of closer  to 0.6 volts - three times that of the more
sensitive diodes. So it takes three times the RF voltage to even begin to
show a DC output and you'll need to be well above the turn on voltage to
show an output that accurately indicates the RF applied. Put two or three
diodes in series and you'll need 1.2 to nearly 2 volts  to just start
showing some indication of an output.

Bottom line, don't try to use such a probe for low RF levels.

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Quite right, but there is a way to overcome this problem which is to apply 
an offset DC bias to the diode(s) which cancels the diode's natural 'bias'. 
The diode(s) will then conduct with low applied 'signal' voltage. Requires a 
stable and 'quiet'  DC source e.g. a battery, and stable resistors in the DC 
'level adjust' circuit, plus care when setting up. Perhaps it is not worth 
doing this unless there is a REAL need for a probe that can measure low RF 
levels.

73,
Geoff
GM4ESD