l
l
l
l
l
l
l
l
l
l
l
Theory of Operation—492/492P Service Vol. 1 (SN B030000 & up)
When the Q5 line of U5031 goes high, a positive pulse of
about 100 ms in duration, generated through RC network
C3021/R3021, appears at the input of U1011B. The output
of the operational amplifier drops to about -1 2 V, and a
positive pulse is passed through the transistor pair, select
ing the lowpass filter. When the Q
6
line goes low, a negative
pulse of the same duration is passed to U1011B. The ampli
fier output rises to about +12 V, and a negative pulse is
passed through the transistor pair to select the preselector.
When the circuit is quiescent, neither Q3025 nor Q4025
conduct, since the sum of the zener voltages of VR3011 and
VR3012 is greater than the combined supply voltages.
When the output of the operational amplifier comes near
one of the supply voltages, the transistor that is connected
to the other supply becomes saturated, furnishing the nec
essary drive current to the relay coil. CR4012 and CR4013
protect the driver transistors from induced voltage surges;
C3028 and R3028 dampen any oscillation that might occur
in the coil circuit.
SWEEP SHAPER AND BIAS CIRCUITS
Refer to the block diagram adjacent to Diagram 38. The
circuit fundamentally consists of a positive and negative
bias supply and a non-linear amplifier that drives the Cavity
2nd LO. The bias circuits supply the oscillator with stable,
re-regulated voltages to minimize oscillator FM’ing. The
shaper amplifier and diode-resistor arrays combine the FINE
TUNE VOLTS and 2nd LO SWEEP signals to form a non
linear driver signal to control the oscillator frequency. The
Cavity Oscillator generates the 2182 MHz signal that is
used in the 2nd Converter circuits.
Bias Supplies
+
12
Volt Regulator.
U2047 is a refrence zener diode
that furnishes a precise, low-noise temperature stabilized,
reference of +6.95 V to the +12 V, —12 V, and +90 V
supplies. This voltage is applied through a filter network to
the non-inverting input of amplifier U3051, which drives se
ries-pass regulator transistor Q2065. Changes in the output
voltage at the emitter of this stage are coupled back to the
inverting input of U3051, amplified, and used to restore the
output to the original value.
—
12 Volt Regulator.
Except for the components and the
polarity, the plus and minus 12 V supplies operate the
same. Refer to the above description for information regard
ing -1 2 V supply operation.
+7.7 Volt “C” Regulator.
This circuit consists of emitter
follower Q3035 and related circuitry. It furnishes a regulated
+7.7 V to the Cavity Oscillator. This voltage is derived from
the reg12 V. The operating bias for Q3035 is set by
the voltage divider in the base circuit, which fixes the emitter
voltage at +7.7 V.
—8.2 Volt “E” Supply.
This circuit consists of compara
tor U3025, emitter followers Q4026 and Q4024, and sur
rounding circuitry. Part of the function of this circuit is to
ensure that the “C” supply voltage is applied to the Cavity
Oscillator first, before the “E" supply. This makes sure that
the Cavity Oscillator is excited into oscillation at turn-on.
When the analyzer is turned on, the output of U3025 is
clamped at approximately +14 V, because the
“C”
supply
voltage is less than the +6.95 V reference. The base of
Q4024 is at about +0.7 V so the transistor is cut off, and
the oscillator receives no current from Q4024. When the “C”
supply voltage rises past the + 6.95 V applied at pin 3 of
U3025, the comparator switches and its output drops to
approximately — 14 V. This cuts off Q4026, Q4024 is biased
on, and the Cavity Oscillator can begin operation.
+90 Volt Supply.
The principal components of this cir
cuit are amplifier U1087 and transistor Q1076. The circuit
takes the +100 V supply output, filters and re-regulates it,
producing a stable +90 V for the shaper amplifier. The
+6.95 V reference is applied to the non-inverting input of
U1087. This is compared to a sample of the output, which is
derived through divider R2087/R1086. The regulated +90 V
is taken off the emitter of series-pass transistor Q1076.
Shaper Amplifier
This stage consists of amplifier U3089, amplifier U1010,
transistors Q1030 and Q2024, and related circuitry. The
FINE TUNE VOLTS signal and 2nd LO SWEEP signal are
summed at the input of U3089, converting the dual differen
tial inputs to a single-ended voltage for the next stage. The
combined signal is applied to the non-inverting input of
U1010, amplified, then applied to the base of Q1030 for
further amplification. The collector of Q1030 drives the base
of emitter-follower Q2024, which in turn provides the tune
voltage to the cavity oscillator, tuning or sweeping the unit,
as appropriate. Feedback for the shaper amplifier stage is
provided through R2018, R1028, and R1026. The gain of
the stage is varied by drawing current away from the
feedback path into the diode-resistor arrays, as discussed
below.
Diode-Resistor Arrays.
U1037, U2037, and associated
components form a divider array that controls the gain of
the shaper amplifier. The input to the amplifier is a variable
dc tune voltage plus a ramp voltage that varies in amplitude
as a function of the analyzer span setting. Total input swing
is within ± 10 V. As the output voltage amplitude increases,
5-66
REV AUG 1981
