its upper limit, level detector U3306A output goes high to peg
the meter and prevent erroneous meter readings resulting from
insufficient signal level.
G. POWER MONITOR (FM/AM-500A only)
Resistor R3345 provides a negative voltage to bias the power
detector in the Output Amplifier, which enables the power
detector to provide feedback to the power monitor. The power
monitor circuit consists of resistors R3346 and R3347, which
form a voltage divider, and op amp U3312B. Potentiometer R3354
provides DC offset for the output of U3312B. One branch of the
output from U3312B goes to the Generator Audio to drive the
receive/generate logic. On the FM/AM-500A only, two more
branches drive the Power Meter function. The first of these
goes through a voltage divider consisting of trimpot R3362 and
resistors R3360 and R3361 for 150 W calibration, then to the
function select multiplexer U3313. The other branch is ampli
fied by op amp U3312A, whose output is calibrated by trimpot
R3365 and resistors R3366 and R3367 for 15 W operation, then
goes to U3313.
H. MODULATION SCALE CIRCUIT
The audio signal selected by multiplexer U3322C is buffered by
op amp U3318A. Its output goes to a voltage divider consisting
of resistors R3398, R3399, R3400, R3401, R3404 and on the FM/AM-
500A only, R343�. One tap goes to the DEMOD jack, the other
t�ps go to multiplexe� U3316. (On the FM/AM-500A only, two taps
first go through multiplexer U3324C to provide the "X60" range
on the Modulation Meter.) The output of U3316 is buffered by
op amp U3314A, then rectified by a precision full wave rectifier
consisting of op amps U3315A and R3315B, diodes CR3314 and
CR33�5, and resistors R3407 through R3411. The rectified sig
nal is then sent to op amp U3323B. A DC offset, which is cali
brated through FM and AM zero trimpots (R3368, R3369, R3370 or
R3371) as selected by multiplexers U3310A, U3310B and U3310C,
is applied to a voltage divider consisting of resistors R3421,
through R3424. Multiplexer U3317 selects the appropriate tap
from the voltage divider and applies it to U3323B. U3323B then
sums the two inputs to provide the FM deviation or AM percent
modulation signal to the Modulation Meter driver circuit.
I. METER DRIVER CIRCUIT
The meter driver circuit consists of multiplexer U3313, a peak
detector comprised of op amp U3311 and associated components, a
buffer using op amp U3314B and related resistors and, on the
FM/AM-500A only, multiplexer U3324B.
On the FM/AM-500, the output of U3323B goes to the peak detec
tor, then to multiplexer U3313. The input signal to U3313,
which is selected by the Modulation Meter Range Switch, is
coupled to the amplifier, whose output drives the meter.
7-61
Содержание FM/AM-500
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