> 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
--
FAQ on WWW: http://www.contesting.com/FAQ/amps
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
|