Section
2
—
DC
502
SECTION
2
THEORY OF
OPERATION
INPUT
CIRCUITS
Direct
Input
Attenuators.
Signals
to be counted are applied via
front-panel
DIRECT INPUT
connector
J100 to the attenu
ators.
The attenuators
are frequency-compensated voltage
dividers
consisting of resistors R102-R107
and capacitors
C102-C107.
Switches
S100A
and S100B allow front-panel
selection of X1, X5, X10,
or X50 attenuation of the input
signal.
C110
provides AC coupling.
FET
source
follower Q115 and emitter follower
Q122
present
a high impedance
to the input signal. The diodes in
the
base circuit
of
E. F. Q128
form a series-limiter and
clamping
network, which reduces
the input signal to limits
suitable
for
driving the
shaping
circuits. The
clamping
diodes
limit the
voltage at the
emitter of Q128 to a
dynamic range
of
about
1.2 volts.
Signal-Shaping.
U150B, an OR
gate integrated circuit
with push-pull
outputs, is connected as a Schmitt trigger. It
shapes
the
input signal into a
square wave. Its
“hysteresis
window“ is
a width of about 200 mV. The output changes
states
when the signal voltage passes
through the upper
threshold,
then
reverts to its
original state when the signal
voltage
passes through the lower threshold. For this reason,
an
input signal smaller in
amplitude than the width of the
hysteresis window cannot
activate
the counting
circuits.
The
quiescent level
at the input of U150B can
be
adjusted
to overcome some of the triggering difficulties
arising
from
various input-signal shapes and frequencies.
Integrated-circuit
operational amplifier
U135 and its
associated
discrete components are connected as a voltage
follower. TRIGGER
LEVEL potentiometer R135 selects a
voltage between ground and
about —2 volts and
applies it
to
pin 3 of
U135. This level
is then established at pin 2, and
hence,
the input of U150B, through
the action of the
operational amplifier.
The
output of
U150B is applied to U150A,
whose
push-pull
outputs
drive Q160
and Q162, which
are connec
ted
as a differential pair. This circuit
provides
a level shift
to
TTL level, and further shapes the signal
to
be counted. A
waveform with
fast rising
and falling
edges is produced at
the
collector
of Q160.
CR165
limits the amplitude of the
count
signal to 5 volts, clamping the negative-going portion
of
the signal to ground. The signal is then passed through
emitter follower
Q170 to U160B, where it
receives a final
phase inversion
(to correspond
with the input signal)
and
becomes the
decade input.
4-10
Pre-Scale Input Circuit
50
MHz
to 550 MHz
signals applied to J180 are
AC-coupled into
a
50Ω environment. A quiescent
10 mA
current, set
by
R185 and
R187,
keeps CR185, 186, 187
and
188 in the diode
clamping
bridge
turned on until the
input
signal amplitude reaches about ±0.4
V. While the
bridge
is turned
on, the signal
source sees about 20Ω
equivalent resistance
of the bridge
in series with R189,
24Ω, and
the emitter resistance of Q190, about 5Ω. As the
signal amplitude
exceeds ±0.4
V, one pair of diodes
(CR185,
CR188) begins to turn
off during positive-going
excursions,
while
the other
pair (CR186, CR187) turns off
during
negative-going excursions. As
these diode pairs
alternately turn off
with signal amplitude changes, a
matched
pair of diodes (CR181A, CR181B) on
the input
side
of the
coupling
capacitor, C182, alternately
turn
on
to
maintain the
50Ω input impedance. C181 prevents
CR181A
&
CR181B
from being biased on or off by DC
levels
which
may be part of the input signal.
The
clamping action of the bridge diodes limits the
changing
DC level at the bridge output to a maximum 0.8 V
peak-to-peak.
This signal is then coupled by
C188 to Q190
via
the high-frequency peaking network, C189-R189. C192
couples
the amplified signal to pin 10, the high impedance
input of the 4-10 counter, U190. C192
is mounted on the
circuit
board with special lead
dress to aid high frequency
response at
minimum
signal amplitudes. Bead L192 is
mounted
on one lead
of C192 to suppress high frequency
oscillations.
R191 establishes a
bias
at
pin 10 of U190.
C191,
193,
and
194 decouple pins 14, 13, and 12,
respectively, right
at the terminal connections to the circuit
board.
L199 and
C199
suppress power supply noise and
decouple
the Vcc
input, pin 14.
Pin 4 of
U190
produces one output cycle, a level change
of
about 0.5 V, for every ten
input events to pin 10. The
base of Q195
swings approximately
between 3.4 V
and
3.9
V. The emitter-follower action of Q195 provides a
healthy
current
drive
to the
base of Q198, which would
saturate in
no-signal conditions except for
CR196. The
collector
of Q198 can, therefore, respond immediately to
signal changes, producing a TTL-compatible output to drive
pin 9
of U160B in
the gating logic to the Decade Counting
Units.
2-1
Summary of Contents for DC 502
Page 4: ...DC 502...
Page 24: ...39 39 Relocated on back of board SN B070000 REV MAY 1974...
Page 33: ...FIG 1 EXPLODED...