[AMPS] Directional coupler design

[Tom Rauch]

> Hi Tom,
>   The point I was trying to clarify in the "note" is that the signal from
> the coupler represents the line voltage (even though it is the sum of the
> sample of the line voltage and a voltage derived from the line current --
> assuming a non-reactive load for the moment) and so must be squared &
> divided by the line impedance to obtain a power measurement.  Though at
> the low levels at which these couplers are typically operated, diode
> non-linearities mess with the math too, making it best to calibrate the
> meter empirically.

I see what you are saying, I guess I assume we all know almost 
every instrument we use does not directly read the thing we are 
measuring.

For example, the current meters in our amplifiers read voltage 
across a shunt. The volt meters read current through a multiplier 
resistor.

When manufactured, meters used to read current with external 
shunts are calibrated for voltage with no attention paid to current 
required. Meters used to indicate voltage with external multipliers 
are calibrated for current through the meter, with little or no 
attention paid to voltage.   
 
>   I didn't want anyone to misconstrue that it perhaps might be the
> product of the voltage and current, thus directly representing power.

I see now what you were saying. I just take that as a given, but 
thinking about it I suppose some people may think the meter reads 
the wattage directly. That is ONLY the case if the meter has two 
coils, one for the field and one for the motor. Then the meter can be 
made to read power directly, with one coil connected across and 
the other through. Unfortunately that works best at low frequencies.


 



73, Tom W8JI
w8ji@contesting.com

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