58452 VXIbus Universal Power Meter
3-6
Publication 21555, Rev. E, September 2002
3.4.3
Oven
The measuring thermistor is maintained at a constant 60 °C by being mounted on the Q1 heater
transistor, which is driven from the sensing thermistor RT2 by way of the Q7 current amplifier. RT2 is
mounted very close to RT1 so that both are maintained at the same temperature. When RT2 reaches
60 °C, the voltage across it is just enough to maintain drive to the heater. This condition will be
maintained regardless of the ambient temperature.
3.4.4
Thermistor Bridge
RT1 is connected in a self-balancing bridge circuit which delivers just enough power to the thermistor
to keep it at 500
Ω
. Thus, if part of the power delivered to it is from the RF generated by the oscillator
and the rest is from the DC current of the bridge, then by reducing the amount of DC power, the circuit
will increase the drive to the oscillator as needed to keep the total power in RT1 constant. It is necessary
to measure only the amount of DC power reduction to know the amount of RF power present. In this
way, a precisely known RF output level can be established.
3.4.5
Track & Hold and DAC
In order to know how much power is being added by the oscillator, it is necessary to measure the power
delivered to the thermistor with no RF present. This is done by turning off the oscillator power (closing
switch U2C), and then measuring the voltage out of the control bridge. This is known as the ambient
bridge voltage. To make this measurement, the following conditions are established: U8D and U2B are
switched open, and U8A & C are switch closed. By using the U13 DAC, a successive approximation
measurement of the voltage is made. The output of the DAC is connected to one input of U4 and the
bridge is connected to the other. Thus, it becomes a comparator that makes it possible for the computer
to tell when the output voltage of the DAC is greater than the bridge voltage, and so complete the
successive approximation. Once this is done, the DAC is set for 0 V output, U8A is opened, U8B, U8D
and U2B are closed, and the track and hold capacitor, C39 will charge up to the voltage which
represents the zero RF power condition of the bridge. When the oscillator is turned on by U9C, then the
sampling switch, U2B, will open and allow C39 to supply this “RF Off” condition to the measuring
circuit. Any voltage from the DAC will now reduce the amount of DC power being delivered to the
thermistor bridge, and the control circuit will add just enough current to the oscillator to cause its
output to add back that much RF power into the bridge.
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