MM0474
the power from the two measurements. Figure 21 shows the instrument setup for
A digital voltmeter with at least a 0.05% DC
DC substitution measurement.
accuracy.
(2) Measurement Procedure.
(a) Connect the DVM to the 432A rear panel VCOMP and VRF outputs.
Be
sure that the digital voltmeter input is isolated from chassis ground.
(b) Turn off, or disconnect the RF power from the thermistor mount.
(c) Zero the 432A with the COARSE ZERO controls.
(d) Depress the FINE ZERO
toggle,
and
measure
the
differential
voltage (V0) between VCOMP and VRF
(e) Release the FINE ZERO toggle, and turn on, or reconnect the RF
power to the thermistor mount.
(f) Measure
again
the
differential
voltage
(V1)
between
VRF
and
VCOMP.
V1 = VCOMP - VRF
(g) Measure VCOMP to ground.
h. Simplified Description.
(1) The 432A Power Meter consists of two major sections: the bridge and
meter logic assemblies.
The instrument also contains an auto zero circuit
which provides for automatic zeroing on any range. A simplified block diagram
of the 432A is shown in Figure 22.
(2) The bridge section contains circuits which form two self-balancing
bridge circuits when a suitable thermistor mount is connected to the 432A.
Each bridge is automatically brought to balance by the action of a high gain DC
the RF bridge, VRF, is responsive to both input RF power and ambient
temperature changes. The voltage at the top of the compensation bridge, VCOMP
is responsive only to ambient temperature changes. Knowing VRF and VCOMP, the
RF power can be calculated.
(3) The meter logic section processes VRF and VCOMP to produce a meter
current proportional to RF power. The sum (VRF + VCOMP) controls the width of
5 kHz pulses. The difference (VCOMP - VRF) is chopped,
42
Previous Page
