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Section IV
Paragraphs 4-5 to 4-21
4-5.
RANGE ATTENUATOR.
4-6.
Тһе signal from А151 is fed to the first section
of the RANGE-DB switch, А251, and then to the input
amplifier.
The second section of A281 is located be-
tween
the input amplifier
and the second
amplifier.
Тһе
RANGE-DB
Switch positions
are
marked
in 10
db steps.
4-7.
INPUT AMPLIFIER.
4-8.
After passing
through the first section
of the
range
attenuator,
А251,
the signal goes to the input
amplifier
(A3Q1/Q2/Q3/Q4)
which
consists of four
transistors in cascade.
The input signal is applied to
the base of A3Q1 and the final amplifier signal is taken
from the collector of A3Q4.
The GAIN and VERNIER
controls are associated with this amplifier and vary its
gain over a range of more than 10 to 1. GAIN control
Ri, the coarse control, is a 250K ohm variable resistor
which
adjusts
the amount
of negative feedback
from
the collector of A3Q4 to the emitter of A3Q1.
VER-
NIER control,
R2,
is a fine gain control and changes
gain by inserting 0 to 5000 ohms
in series with the
output signal.
4-9,
SECOND AMPLIFIER.
4-10.
Transistors A3Q5 and A3Q6 amplify the signal
from the second section of the range attenuator.
AC
feedback provides gain stability and high input im-
pedance.
The output of the amplifier is applied through
the EXPAND attenuator,
A282, to the third amplifier
А398 and A3Q9.
4-11.
EXPAND CIRCUIT.
4-12.
The function of the EXPAND switch A282, is to
allow any signal level to be measured on an expanded
scale with continuous
coverage while
maintaining the
original reference level.
Expansion is accomplished
by applying a precise amount ої DC -offset current from
A3Q17 to the meter and simultaneously increasing the
signalto the 3rd amplifier.
This increased gain allows
a 2 db change in signal level to deflect the meter across
its full scale. The offset current places the zero signal
indication off scale to the left.
4-13.
FREQUENCY
SELECTIVE
CIRCUITS.
4-14,
The frequency response of the third amplifier,
A8Q8 and A3Q9,
is shaped by negative feedback.
The
feedback path includes a Wien-bridge and amplifier
АЗОТ.
At the null frequency of the Wien-bridge,
the
negative feedback path is open and the gain of the am-
plifier ismaximum.
Off center frequency the negative
feedback
through
Ње Wien-bridge
reduces gain.
The
amount of the
"off resonance" gain reduction depends
on the setting of the BANDWIDTH control, R3.
4-15.
The Wien-bridge is adjusted for a sharp null at
center frequency
with BRIDGE
STABILITY
ADJUST
АЗВ29.
Actually,
this control is set for a very slight
bridge unbalance to produce just enough positive feed-
back
so that signal current
to the base
of A3Q8
is
supplied mainly by A3Q7. Thus, at resonance, negligible
signal current flows through BANDWIDTH control, R3,
4-2
Model 415E
andgainis independent ofits setting. Center frequency
is set by varying resistors R4 and R5 (these resistors
are gangedand comprise the front panel FREQ control).
4.16.
FINAL
AMPLIFIER.
4-17.
The output amplifier consists
of four transis-
tors.
The two output transistors,
А3012 and А3013,
operate
as a push-pull
class
B amplifier
with
both
collectors AC grounded.
The emitters of these tran-
sistors are tied together and the AC amplifier
output
is taken from this point through a coupling capacitor,
A3C28.
Large
negative
feedback
makes
the gain of
the output amplifier very nearly unity.
The AC output
voltage is developed acrossresistor A3R51:
The cur-
rent through
A3R51 is supplied by A3Q12 and
A3Q13
conducting one at atime onalternate half cycles (Class
B operation)
and
the
output
signal
sine
wave
is
а composite of this half-cycle operation.
In addition,
the collector
current of A3Q13
can drive the meter
directly. No rectifier diodes are needed,
This meter
driving current is filtered by capacitor
A3C26
and
passes through the meter anda 1000 ohm resistor, R6,
to develop а DC voltage for the recorder output.
4-18. GROUND
LOOPS.
4-19,
Тһе grounding technique used in the 415E con-
sists
ої an input connector
ground,
a circuit board
ground,
and output connector grounds.
These
are
"floating" grounds that are tied together and isolated
from chassis ground except for a 46.4 ohm resistor,
ЕТ,
anda
0,05 uf capacitor,
СІ, connecting ground
and chassis.
A solid
connection to chassis-or-earth
ground permits
troublesome
ground
loop currents to
flow causing
erroneous
instrument
operation,
For
this reason,
connecting grounded instruments
to the
415E output connectors сап cause erroneous readings.
Most
recorders and
oscilloscopes that might be used
with the 415E outputs have differential inputs available
with neither side grounded (see Paragraph 3-11).
4-20.
INPUT
IMPEDANCE.
4-21.
The Model 415E
is designed to have an input
impedance much higher than that of any crystal detec-
tor or bolometer normaily used with it.
This results
in lower noise figure and the highest possible input
signal tothe 415Е.
For example with the 415Е INPUT
switched to LOW, the input impedance is approximately
2000 ohms
while the output or source impedance of a
bolometer is approximately 200 ohms.
02152-3
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