3. When the mode selector switch is set to AUTO, the AC indicator illuminates and
the AC signal under measurement is coupled to the thermoelement in the same manner
as for manual balance operation.
The DC output of the thermoelement is chopped,
amplified, and applied to a memory circuit which automatically develops a DC
voltage which balances the thermoelement output.
At this point, the voltage
amplifier output is 0 volts and the meter indicates a null. After a short delay
interval, the transfer circuitry is automatically triggered and the output of the
voltage amplifier is coupled to the attenuator and the applied AC signal is
disconnected. The resulting action is identical to the manual transfer mode. An
automatic periodic recycle mode from DC to AC is switch selectable. This recycle
mode acts as an auto zero circuit to minimize the effects of thermal drift.
4. The manual transfer mode is virtually identical to the automatic transfer mode
except that the transfer is accomplished by depression of the TRANSFER-RESET
pushbutton switch instead of being automatic.
Learning Event 4:
DESCRIBE THE NBS THERMAL VOLTAGE CONVERTER SET
1. The NBS Thermal Voltage Converter (TVC) set, shown in Figure 3-8, consists of
three instruments which are used as a standard source of known RF voltages in the
frequency range of 100 MHz to 1 GHz.
The TVCs are rated at 0.25 -1 volt, 1-2.4
volts, and 2.4-7 volts.
They can be used to accurately calibrate any type of
voltmeter that can be attached to their output.
A device that can detect the
output of a thermocouple is needed to operate the instruments.
2. Each instrument consists of a frequency-compensated thermocouple with a "T"
connector built in. The basic diagram is shown in Figure 3-9. The RF voltage is
measured at the reference plane indicated. This place coincides with the end of a
type "N" female connector screwed into the standard output.
This RF voltage is
detected by the TVC which is made up of a thermoelement (TE), a dropping resistor
(R), and a compensation network (Rc and Cc). The voltage at the voltage reference
plane is dropped across R and the heater of the thermoelement.
The thermocouple
output of the TE is a function of the voltage drop across its heater and the
reference voltage.
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