Advertisement
Section IV
Paragraphs 4-5 to 4-21
4-5.
RANGE ATTENUATOR.
4-6.
The signal from A1S1 is fed to the first section
of the RANGE- DB switch, A2S1, and then to the input
amplifier.
The second section of A2S1 is located be¬
tween the input amplifier and the second amplifier.
The 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, A2S1, 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
Rl, the coarse control, is a250Kohm 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, A2S2, to the third amplifier
A3Q8 and A3Q9.
4-11.
EXPAND CIRCUIT.
4-12.
The function of the EXPAND switch A2S2, 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 of DC-offset current from
A3Q17 to the meter and simultaneously increasing the
signal to 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,
A3Q8 and A3Q9, is shaped by negative feedback. The
feedback path includes a Wien-bridge and amplifier
A2Q7.
At the null frequency of the Wien-bridge, the
negative feedback path is open and the gain of the am¬
plifier is maximum. Off center frequency the negative
feedback through the 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
A3R29. 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,
Model 415E
and gain is independent of its setting. Center frequency
is set by varying resistors R4 and R5 (these resistors
are ganged and comprise the front panel FREQ control).
4-16. FINAL AMPLIFIES.
4-17.
The output amplifier consists of four transis¬
tors.
The two output transistors, A3Q12 and A3Q13,
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 across resistor A3R51: The cur¬
rent through A3R51 is supplied by A3Q12 and A3Q13
conducting one at a time on alternate half cycles (Class
B operation ) and the output signal sine wave is
a 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 and a 1000 ohm resistor, R6,
to develop a DC voltage for the recorder output.
4-18. GROUND LOOPS.
4-19.
The grounding technique used in the 415E con¬
sists of 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,
R7, and a 0.05 uf capacitor, Cl, 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 can 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 normally used with it.
This results
in lower noise figure and the highest possible input
signal to the 415E. For example with the 415E INPUT
switched to LOW, the input impedance is approximately
2000 ohms while the output or source impedance of a
bolometer is approximately 200 ohms.
4-2
02152-3
Advertisement
