[Amps] Power meter calibration

[Dennis12Amplify at aol.com]

In a message dated 10/6/03 7:34:53 AM Central Daylight Time, 
msole at 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.