7-12. Circuit Description
from final stage Q11 is converted to a current by
Q12B. Adjustment of crest factor gain is with R38.
d. Integrator Amplifier. The integrator am-
a. Range Amplifier. The range amplifier scales
plifier consists of U7 and U8. It. is a complex
the applied input voltage to the ac converter to 1
volt rms for a full scale input. Basically, the range
filter. Currents at the summing junction of U7-2
amplifier is an inverting operational amplifier
are integrated by U7 and its three poles of feed-
whose gain changes as the range changes. Voltage
back, C30, R68 and C32, and R69 and C33.
gains of -1, -10, -100, and -1000 correspond
Additional filtering is provided when the FILT
with the 1 through 1,000-volt ranges of the
button on the voltmeter front panel is depressed.
AN/GSM-64B. Active elements of the range
This action initiates a command through P13-10
amplifier are Q1, Q2, Q5, and U3 with protection
and enables Q18 and Q15. With these transistors
diodes CR6, CR7, CR12, CR13, CR18, CR20,
enabled, FET switches Q13, Q14, Q16, and Q17
CR21, and CR22. Transistor Q2 drives the guard
switch additional feedback capacitors across the
at the summing point potential of the operational
existing three poles. Capacitor C29 is switched in
amplifier. FET Q5 switches C5 into the feedback
parallel with C30; C34 with C32; C35 with C33;
path in the 10-, 100-, and 1,000-volt ranges.
Diodes CR6, CR7, CR21, and CR22 provide
amplified is buffered by U8. Output from U8 is
clipping for input overload protection to Q1.
distributed to the square root amplifier. The
Diodes CR18 and CR20 provide overload clipping
output is also directed through K2B and P13-12 to
at the output of the range amplifier for protection
the A-to-D converter.
of the rest of the ac converter circuits.
e. Square Root Amplifier. The square root
b. Balance Amplifier. The balance amplifier is a
amplifier consists of input resistor R57, trimming
rectifying amplifier with a gain of -1. This am-
and selection resistors R61, R62, R63, and R64,
plifier changes all negative-going signals from the
feedback transistor Q8B, protection diode CR19,
and amplifier U6. Diode CR19 acts as a feedback
Balance gain adjustment (R32) controls the input
path to prevent saturation of U6, should input
to the balance amplifier. The control is adjusted so
polarity to the amplifier reverse. The amplifier uses
that positive and negative signals at the output of
logarithmic feedback to produce the square root of
the balance amplifier (junction of R31, R33, and
its input in a way similar to that used in the
U5) will be equal in magnitude. Active components
squaring amplifier. The square root process is
of the balance amplifier are CR15, Q6, Q9, Q10,
accomplished by matched transistors Q8B and
and U4. Balance zero adjustment, R42, is an
Q12B. Resistors R48, R79, and R80 are factory
adjustment for zeroing the input of U4. The
selected for overall ac converter temperature
primary or low frequency signal path is through
coefficient and linearity. Some, or none, of these
U 4 , CR15, and final output stage Q10. At
resistors may be installed.
frequencies above a few hundred kilohertz, the
f. Relay Controls.
signal path is through Q6 and Q10. Transistor Q10
is controlled manually by switch S1. Closing S1
has a current source load, CL1, which maintains a
(dc coupling) activates relay K3, which causes
high amplifier output impedance for driving rec-
capacitor C1 to be bypassed. This action allows
measurements of composite ac/dc signals. Placing
CR14 and CR16 is compensated by Q9. Amplifier
S1 to ac coupling deactivates K3 which places the
feedback is through Q9 and C17.
dc blocking capacitor (C1) in the input circuit.
c. Squaring Amplifier. The squaring amplifier is
Input and output relays K1 and K2 are operated
an inverting operational amplifier with nonlinear
from the vac function control line. The range
feedback through matched transistors Q8A and
relays, K4, K5, and K6, are operated by driver
Q12A. These transistors transform the input
circuitry, consisting of NAND gates U2A, C, D
current into an output voltage that is twice the log
and current source transistors Q3 and Q4. These
of the input current. Since two times the log of a
range control gates accept the buffered outputs of
number is equal to the log of the number squared,
the range counter on the display board (A14).
Integrated circuits U1B and U2B deliver the up
operation. The main signal path is through U5 and
and down range stop commands, for vac function,
Q11. At high frequencies, Q7 parallels U5. Zero
to the range delay board. NAND gates U1A and
offset at the input to U5 is compensated by ac zero
U1C deliver the set and reset commands to the
adjustment, R45. This adjustment has the greatest
range counter, on the display board, for prevention
effect for overall ac converter zero. Output voltage
of a disallowed range in the vac function.
Change 1 7-5