In a message dated 10/6/03 7:34:53 AM Central Daylight Time,
msole@loxinfo.co.th writes:
> I have the WB6BLD meter scale drawing program for which
> the included 200 watt scale uses a linear exponent of 0.55. Having tried
> this it just doesn't seem right. What's the formula?
>
> 73
> Martin, HS0ZED
>
Martin,
I believe the .55 is to compensate for the forward bias voltage threshold of
the diode used in the rectifier circuit in the meter, but number that varies
from diode to diode, and often the capacitance of the diode is overlooked in
the capacitance divider created by the circuit and the board layout.
Most meter circuits are simply rf voltage rectifier circuits where you have
to subtract the diode threshold from the applied peak voltage to get the
voltage across the filter cap feeding the meter movement. The average voltage
maintained across that cap is what the meter movement responds to, but that
voltage does not linearly represent the actual applied RF because some
percentage of
the zero-crossing of the RF wave is 'clipped' by the diode's conduction
threshold, (which is probably where the .55 offset comes from, with the range
usually being .22 for Germanium diodes such as the 1N277, and going as high as
.75
volts for a 1N914 or 1N4148 driving a 10 to 100 milliampere peak load for a
'insensitive' meter movement), and the voltage across the diode is also
affected
by the capacitance of the diode itself and the series and shunt capacitance
of the feeding circuitry.
Not seeing the actual meter circuit diagram I can''t help you much more than
that.
Regards,
Dennis O.
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