HP 8414A Operating And Service Manual page 85

Section VIII
Service
SCHEMATIC 1. CIRCUIT DESCRIPTION
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.
Switch, Beam Center
The test channel input is fed to switch A2CR1,
CR2, which grounds the input when the beam
center pushbutton is pressed, or for the
H26-8414A, when the Auto Beam Centering cir¬
cuit is activated. During non-beam center operation
a positive voltage is applied to the junction of CR1
and CR2, CR1 is biased on, CR2 is biased off and
the input signal is applied to the preamplifier at
A2Q19. When the beam center pushbutton is
pressed or, for the H26-8414A, when the Auto
Beam Centering circuit is activated the voltage at
the junction of CR1 and CR2 is negative, CR2 is
biased on, and the input signal is grounded through
CR2.
Preamplifier
The preamplifier, A2Q19-Q20, is a feedback-pair
amplifier with a voltage gain of about 25. The out¬
put of the preamplifier divides into two signal
paths.
-90 Degree Phase Shifter
The -90 Degree Phase Shifter A2Q1 shifts the
phase of one output of the preamplifier -90 de¬
grees. The test channel signal is then two signals
separated in phase by 90 degrees or sine and cosine
signals.
Drivers
The sine detector's driver A2Q2, Q3 and the cosine
detector's driver A2Q21 prevent the detectors
from loading the phase shifter and preamplifier
output circuits. The gain through each driver is
about unity.
Phase Splitter-Limiter
The phase splitter-limiter, A2Q4, Q6, amplifies the
phase reference signal from the mainframe. Its two
output signals are 180 degrees apart and limited to
about 5V p-p. These two signals are fed to the Sin
0
and Cos
0
Detectors and turn the detectors on
one at a time.
Sin 0 and Cos 0 Detectors
The Sin
0
and Cos
0
Detectors, A2A1 and A2A2,
are balanced modulator phase detectors. The sig¬
nals from the phase splitter-limiter turn the detec¬
tors 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 Cos 0 detector is turned on. During the next
half cycle of the turn-on signal the Sin
0
detector is
turned on. Any signal appearing at the input,
A1TP3 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 signal the detector's output is maximum posi¬
tive. 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 (zero degree phase difference)
to a maximum negative (180 degrees phase differ¬
ence) and back to a maximum positive (360 de¬
grees 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 Sin 0
detector is shifted -90 degrees, the output of the
Sin 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 sin 0), and the output of the Cos 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
A2Q7 through Q10 and A2Q22 through Q25 are
differential amplifiers with voltage gains of about
ten. Phase shift through these amplifiers is negli¬
gible. An adjustable bias voltage is provided
(A2R32, R33) to set the rear-panel horizontal and
vertical output voltages to zero with no test chan¬
nel input signal (Beam Center pressed).
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