Bird Technologies 6091P - Manuale (Pagina 17)

Chapter 2
Theory Of Operation
Measuring AC power at frequencies above 100 kHz can be difficult. Primary voltage
and current measurements become more difficult near the VHF band frequencies.
One method of accurately measuring broad band power is calorimetry. This section
describes how the Model 6091 and 6091P Calorimeters measure RF power.
Basic Calorimetry
The term calorimetry refers to the measurement of quantities of heat. Heat is energy
that can be transferred by a thermal process. The rate of heat exchange can be ex-
pressed in calories per second (cal/sec); a change in energy per unit time. A calorie is
the quantity of heat needed to raise the temperature of one gram of water one degree
Celsius.
The first law of Thermodynamics states that energy can be neither created nor de-
stroyed only changed from one form to another. An RF watt is electrical energy spent
per unit time. By changing incoming RF power to heat and measuring the resulting
temperature difference at a known flow rate, we can determine an accurate measure
of the RF power being generated from a source in watts.
Calorimeter
Overview
The Calorimeter measures the amount of heat created when RF power is dissipated
into a load resistor. The following paragraphs describe how the 6091/6091P Calorim-
eter performs an RF power measurement. Refer to the block diagram in figure 4 dur-
ing the following discussion.
Calorimeter
Functions
An RF power source is attached to the RF input connector on the front panel of the
Calorimeter. RF power is applied, through the front panel connector, to a precision
internal 50-ohm load resistor. The internal load resistor converts the RF power to
heat and transfers the heat into the surrounding coolant.
The center of the load resistor is hollow to allow liquid to flow through the resistor.
The resistor core is connected in line with a closed loop pumping system. Coolant is
continually pumped from the coolant reservoir, through the load resistor, to the
pump. From the pump, the coolant is forced through an air-cooled heat exchanger
and back into the coolant reservoir.
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H E A T E X C H A N G E R
C O O L A N T
R E S E R V O I R
F L O W
S E N S O R
O U T P U T
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S E N S O R
T E M P E R A T U R E
I N P U T
S E N S O R
L O A D
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R F
I N P U T
A N A L O G
C I R C U I T S
D I S P L A Y
M C U
G P I B
T
2
T
1
Figure 4
System Block
Diagram