Heathkit IO-4550 - Page 17

Page 15
THEORY OF OPERATION
The dual-trace capability of this Oscilloscope allows two
different signals to be displayed on a conventional CRT
(cathode ray tube) that has only one set of vertical
deflection plates. Two identical vertical preamplifier circuits,
a switching circuit, and a vertical deflection amplifier make
this possible. Each vertical preamplifier circuit attenuates its
input signal by a known factor, amplifies it to a usable level,
and provides the necessary positioning bias. The switching
circuit (a diode-type switch), which is automatically
controlled by the display control circuit, alternately allows
the output signals from the two preamplifier circuits to pass
to the vertical deflection amplifier. The vertical signal, a
composite of both input signals, is amplified further by the
vertical deflection amplifier before it is applied to the
vertical deflection plates of the CRT. The signal at the
vertical deflection plates, which produces the display on the
CRT screen, thus represents both input signals as one
“time-shared”’ signal.
The horizontal portion of the trace displayed on the CRT
screen is produced by the sweep and trigger circuits in
conjunction with the horizontal deflection amplifier. The
sweep circuit produces the linear signal (ramp) used to
sweep the electron beam across the CRT screen from left to
right at a constant rate. This circuit is switch controlled (by
the TIME/CM switch) to provide nineteen accurate sweep
rates needed to view and measure almost all input signals.
This circuit must be triggered either by a portion of one of
the vertical input signals, by an external signal, or by a
Portion of the line frequency signal.
In the absence of a trigger signal, an automatic baseline
circuit causes the sweep circuits to operate while in the
automatic mode. This ensures that, even though no signal is
applied, a reference baseline (trace) will appear on the CRT
screen. The sweep signal is coupled to the horizontal
deflection amplifier where it is amplified before being
applied to the horizontal deflection plates of the CRT.
Other circuits within the horizontal amplifier also provide
the necessary positioning bias.
At the end of each horizontal sweep, the blanking circuits
(which are triggered by the sweep circuits) turn the trace off
(blank it). This prevents a line (retrace) from being displayed
as the electron beam returns to the left side of the CRT
screen to start a new trace.
Regulated power supply circuits ensure overall accuracy as
well as excellent control of the electron beam size and
intensity.