The corresponding high
Q
is available at reset signal multi
plexer U4401A. Upon receiving the clock pulse (U4412A low
Q),
U4412B Q goes high, switching U44018 to pass the reference fre
quency and setting the counter system to count downward. In
the RF mode, after a 15 microsecond delay, the reset circuit
resets U44128 to a low
Q,
high
Q
condition. However, in the
audio mode, reset does not occur for an additional 10 time-base
pulses while U4412A completely recycles and clocks U44128 back
to low
Q,
high
Q.
Therefore, the reset signal to U44128 is
functional only in the RF mode.
In the RF mode, multiplexer U4401A passes every high
Q
signal
from U4412A to the reset one-shot multivibrator U4413A. In the
AUDIO mode, the
Q
from U4412B is selected, which coincides with
alternate high
Q
signals from U4412A. When the reset master
multivibrator U4413A is clocked, the
Q
output produces a 15
microsecond low pulse to trigger the counter system. The
leading edge of the following high clocks the slaved multi
vibrator U4413B. Simultaneous 15 microsecond pulse outputs
(high from
Q,
low from
Q}
reset the counter system and the
slaved timing multivibrator U44128.
D. COUNTER SYSTEM (On Digital Counter PC Board)
A logic network consisting of NOR gates U4414A, U4414B, U4414C
and U4414D, NANO gates U44168, U4416C and U4416D, and inverters
U44178, U4417D, U4417E and U4417F, is controlled by binary codes
from the Frequency Error Range Switch. This network establishes
the starting count for chained counters U4402 through U4406.
Outputs from counters U4402 and U4403 are coupled to latch
U4408. When triggered by reset multivibrator U4413A, latch
U4408 transfers the count to DAC U4409. Outputs from counters
U4404, U4405 and U4406 go to a logic network comprised of NOR
gates U4410, U44158, U4420C and U4420D, and inverter U4421D. If
the overflow count from U4403, plus the starting count, at all
three overflow counters is not zero, the over-range logic feeds
a· corresponding over- or under-range signal to dual flip-flop
U4407A and U44078. When triggered by reset multivibrator
U4413A, latch U4408 transfers the count from U4402 and U4403 to
DAC U4409 and any value from overrange flip-flop U4407A or
U44078 to op amp U4419B.
The DAC supplies two differential signals to op amp U4419A
(which is a current-to-voltage converter), which feed op amp
U4419B. U44198 subsequently furnishes the signal to deflect
the Frequency Error Meter needle. If an overrange count occurs,
the output of either U4407A or U44078 will override the DAC
circuit output and peg the needle either high or low according
to the error direction.
7-50
Содержание FM/AM-500
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