Theory of Operation
Coaxial RF Power Standards Instruction and Service Manual
4-2
Temperature Variable Resistance
Each RF Standard contains a pair of thermistor beads whereby the resistance changes as a
function of temperature. Thermistor bead temperature is a function of the combined DC and RF
power applied to the beads and the ambient temperature surrounding the beads. The level of
power applied to the beads is controlled externally. A heating element, controlled by an external
controller, provides ambient temperature stability.
Figure 4.1 depicts the thermistor assembly electrical configuration. DC biasing of the dual
thermistor beads to 100 ohms each provides a nominal 50 ohm parallel RF resistance and a 200
ohm series DC resistance. A DC blocking capacitor and bypass capacitors isolate DC from RF
signals. Filtering capacitors provide low VSWR in the lower end of the frequency range.
Application of approximately 30 mW of power to the thermistor beads produces a 200 ohm DC
resistance. As the power applied to the thermistor beads increases, their effective resistance
values decrease (refer to Figure 4.1). This is due to the negative temperature coefficient of the
beads. Initially, the beads are DC biased to 200 ohms. Application of RF power increases the
power level present at the beads and causes the effective resistance value of the beads to drop.
The Type IV bridge circuit reduces DC power until the initial effective resistance value is restored
(200 ohms). The total amount of DC power removed is proportional to the amount of RF power
that was introduced. Quantitatively, the total power applied to the thermistor beads equals the
sum of the two types of power.
Power Splitter
The Models F1125, F1130, F1135 contain a two-element resistive power splitter. The T-shaped
divider contains a series 50 ohm resistor in each of the two legs (refer to Figure 4.1). The test
port is a Type N precision female. Use of the splitter in a closed loop configuration that applies
constant power causes the common point (divider) to become a constant voltage point. This
means the source impedance at both splitter output ports is determined by the 50 ohm resistor
and the output connector. This provides very good source match. In addition, the power is split
equally between the two ports as shown in Figure 4.1.
Bypass
Capacitor
T
Bypass
Capacitor
- DC Bias
+ DC Bias
T
Filtering
Capacitor
Power Splitter Only On
Models F1125, F1130, F1135
DC Block
Filtering
Capacitor
Figure 4.1 Simplified Schematic of RF Power Standards
