MM0474
or
(7) Thus an RF power measurement reduces to setting VCOMP = VRFO (with
zero RF power) initially, measuring VCOMP and VRF, and computing with the above
formula. The 432A carries out the computation by forming the indicated sum and
difference, performing the multiplication and displaying the result on a meter.
(8) The meter logic circuits change the two DC voltages to two pulse
signals which contain all the RF power information. One of the signals will be
a square wave whose amplitude is proportional to VCOMP - VRF. The other signal
will have a pulse width proportional to VCOMP + VRF.
(9) The VCOMP - VRF signal is obtained by taking the DC voltage outputs
from the A1 assembly and applying them to a chopper circuit.
This chopper
circuit is driven by a 5 KHz multivibrator. The output of the chopper is a
square wave signal whose amplitude is proportional to VCOMP - VRF. The output
of the chopper is coupled to the range amplifier and then to the calibration
factor amplifier.
The amplification that the signal receives in these two
amplifiers depends upon the setting of the RANGE switch and the CALIBRATION
FACTOR switch.
The output of the calibration factor amplifier is V.
This
current is fed to the electronic switch. A square wave current with amplitude
proportional to (VCOMP - VRF).
(10) The VCOMP + VRF signal is obtained by taking the two DC voltages
from A1 assembly through a summing circuit and feeding this voltage to a
voltage-to-time converter. The voltage-to-time converter is driven by a 5 kHz
multivibrator. The output of the voltage-to-time converter is a signal whose
pulse width is proportional to he sum of VCOMP + VRF. The signal controls the
electronic switch. From the VCOMP - VRF and VCOMP + VRF inputs, the electronic
switch provides a 5 kHz pulse train whose amplitude is proportional to VCOMP -
VRF and whose pulse width is proportional to VCOMP + VRF. The pulse width is
always 90 msec or less.
(11) The bias circuit switch and filter provide a zero current reference
for the meter circuits. This is accomplished by controlling the DC bias to the
first stage of the calibration factor amplifier.
This circuit, in effect,
restores the DC component to the square wave which has been amplified by AC
coupled amplifiers.
47
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