HP 8414A Operating And Service Manual page 74

Section VIII
Service
DETAILED BLOCK DIAGRAM DESCRIPTION
HORIZONTAL AND VERTICAL AMPLIFIERS
Test Channel Amplitude Input
The test channel signal is applied to the Polar Dis¬
play Unit from either the Network Analyzer main¬
frame for the standard 8414A or from a rear panel
BNC connector for the H26-8414A. The amplitude
of this signal and phase relationship with respect to
the phase reference signal contains the information
that is displayed on the CRT.
Switch, Beam Center
The test channel input signal is grounded when the
Beam Center pushbutton is pressed or for the
H26-8414A when the Auto Beam Centering circuit
is activated. With the input signal grounded there is
no vertical or horizontal deflection voltage to the
CRT, the CRT display is a dot and the front-panel
centering controls may be used to locate the dot in
the center of the CRT.
Preamplifier
The preamplifier is a feedback-pair amplifier with a
voltage gain of about 25. At the output of the
preamplifier the test channel signal path divides
into two branches. The signal in one branch is fed
through a -90 degree phase shifter and driver to the
sine detector. The signal in the other branch is fed
through a driver to the cosine detector.
Phase Shifter
The phase shifter retards the phase of one output
of the preamplifier 90 degrees. The test channel
input signal is then two signals separated in phase
by 90 degrees or sine and cosine signals.
Drivers
The sine detector driver and cosine detector driver
isolate the sine and cosine detectors preventing
interaction with the -90 degree phase shifter and
preamplifier. The gain through each driver is about
unity.
Phase Splitter-Limiter
The phase splitter-limiter amplifies the phase ref¬
erence signal from the mainframe. Its two output
signals are limited to about 5V p-p and fed to the
sin 0 and cos 0 detectors. These signals turn the
detectors on one at a time.
Sin 0 and Cos 0 Detectors
The sine 0 and cosine 0 detectors are balanced
modulator phase detectors. The signals from the
phase splitter-limiter turn the detectors on one at a
time during alternate half cycles of the turn-on
signal. When the turn-on signal from the phase
splitter limiter causes the voltage at A2TP6 to be
positive and A2TP7 to be negative the cosine <p
detector is turned on. During the next half cycle of
the turn-on signal the sine 0 detector is turned on.
Any signal appearing at the input, A2TP3 or TP15,
when a detector is turned on will be passed
through the detector and the detector's output
capacitor will charge to the average of the signal
passed through the detector. When the input signal
(A2TP3 or TP15) is in-phase with the turn-on sig¬
nal the detector's output is maximum positive.
When the input signal is 180 degrees out of phase
with the turn-on signal the detector's output is
maximum negative. When the input signal is
exactly ±90 degrees with respect to the turn-on
signal the average of the detector's output signal is
zero. Therefore, a detector's output can vary from
a maximum positive (0 degrees phase difference) to
a maximum negative (180 degrees phase differ¬
ence) and back to a maximum positive (360
degrees phase difference). The magnitude of the
maximum positive and negative detector output
voltages is directly proportional to the amplitude
of the input signal. Because the signal to the sine
0
detector is shifted -90 degrees, the output of the
sine 0 detector is equal to the amplitude of the
signal at A2TP14 times the sine of the angle be¬
tween the reference and test channel input signals
(A sine
0),
and the output of the cosine
0
detector
is equal to the amplitude of the signal at A2TP14
times the cosine of the angle between the reference
and test channel input signals (A cos
0
).
Buffer Amplifiers
The Buffer Amplifiers are differential amplifiers
with a voltage gain of about ten. Phase shift
through these amplifiers is negligible.
Low Pass Filter
The low pass filters filter out frequencies above 10
kHz with a capacitor to ground and a feedback
loop which couples high frequency signals from the
output back to the input 180 degrees out of phase.
The low frequency voltage gain through these
filters is about two.
8-10