Tektronix DC 502, Instruction Manual

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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