In stal la tion. 7, Site and shel ter re quire ments. 7, Un packing and in specting. 7 – Bird Technologies 6091P Manuale d'uso

Pagina 17: Set ting the volt age se lec tor switch. 7, Installation, Site and shelter requirements, Unpacking and inspecting, Setting the voltage selector switch, Theory of operation, Chapter 2

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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.

5

P U M P

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

T E M P E R A T U R E

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

R E S I S T O R

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

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