Tektronix DC 502, Инструкция по эксплуатации

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Theory of Operation — DC 502
TIME BASE AND CONTROL CIRCUIT
1 MHz Clock
A precise one megahertz clock provides the reference for
operation of the gate-generating circuits. The output of
crystal oscillator Y200 is adjustable by C201 to exactly one
megahertz. The four parts of U200 form a shaper-buffer
stage to produce square-wave clock pulses and to isolate the
oscillator from the 1-MHz output line.
Optional Clock
An optional 1 MHz clock is available, using a very stable
5 MHz crystal oscillator and a divide-by-five counter. This
combination is shown on the schematic as Y201 and U201.
Decade Divider Units (DDU's)
The DDU's consist of seven cascaded divided-by-ten
counters, U209 through U215. They produce four gate
times, 0.01 sec, 0.1 sec, 1 sec and 10 sec, which are made
available via the MEASUREMENT INTERVAL switch to
the gate generator to establish the precise time interval the
GATE is open. The 1-MHz clock signal is applied to pin 14
of U209, whose output is connected to the input of the
subsequent decade. Each decade is clocked with a negative­
going transition. The DDU's are reset by a CLEAR pulse,
which places a 0 count in U209 and a 9 count in each
subsequent decade.
Gate Generator
The gate generator produces the GATE control signal
and initiates the CLEAR, CLEAR, and LATCH pulses. The
generating portion consists of U220A, U222A, U220B, and
U222B. The display time control portion consists of Q230,
Q238, and Q240. The circuit will be described first in the
normal gate mode (MEASUREMENT INTERVAL switch in
one of the four gate time positions).
Assume that the To conditions are as given in Fig. 2-1.
The Q outputs of U220A, U222A, U220B, and U222B are
all LO. Q230 is off and the emitter of Q238 rises as C235
charges. At Tv Q238 reaches its firing potential and
discharges the capacitor. This results in a short-duration LO
pulse on the direct-set input (pin 2) of U220A, forcing its Q
output HI and its Q output LO. With two HI inputs on
NAND gate U230A, its output goes LO and the output of
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2-2
REV. MAY 1974