MM0474
amplified and fed to an electronic switch actuated by the controlled width
pulses. Therefore, the meter current is pulses of variable height and width
with the meter indicating the average current. (This process produces a meter
current proportional to (VRF + VCOMP) (VRF - VCOMP). Paragraph 2i(6) explains
why this is necessary.
(1) A functional block diagram of the 432A power meter is shown in
Figure 23. The instrument comprises two major assemblies: bridge assembly A1
and meter logic assembly A2.
Auto zero circuit A1A1, which provides for
automatic zeroing of the instrument, is included as part of logic assembly A1.
(2) The thermistor bridges are biased with direct current from the
bridge amplifiers.
bring the thermistor resistance to 100 or 200 ohms, depending upon the setting
of the MOUNT RESISTANCE switch on the 432A. If one of the thermistor bridges
is unbalanced due to incorrect thermistor resistance, an error voltage occurs
and is amplified by the bridge amplifier. The error voltage is applied to the
top of the bridge and changes the power dissipation of the negative temperature
coefficient thermistor. The change of power dissipation causes the resistance
to the thermistor to change in the direction required to balance the bridge.
Application of RF power to the RF bridge heats the thermistor and lowers its
resistance. The bridge circuit responds by reducing the DC voltage applied to
the top of the bridge thus maintaining bridge balance.
(3) If ambient temperature causes changes in the thermistor resistance,
the bridge circuits respond by applying an error voltage to the bridges to
maintain bridge balance. The voltage at the top of the RF bridge is dependent
upon both ambient temperature and the RF input. The voltage at the top of the
compensation bridge is dependent upon the ambient temperature only. The power
meter reading is brought to zero with no applied RF power by making VCOMP equal
to VRF so (VCOMP - VRF) equal zero.
Since ambient temperature causes both
thermistors to respond similarly, there will be no net difference between the
amplifier output voltages. Therefore, any difference in output voltages from
the bridges is now due to RF power absorbed by the thermistor mount.
(4) The RF bridge voltage, VRF, and the compensation bridge voltage,
VCOMP, contain the "RF power" information.
To provide a meter reading
proportional to RF power, the DC voltages (VRF, VCOMP) must be further
processed by the meter logic circuits.
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