Theory of Operation—492/492P Service Vol. 1 (SN B030000 & up)
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Fig. 5-24. S w eep “interrupt” circuits.
crease or decrease to increase the signal frequency. Thus,
this circuit is a unity gain amplifier that can be changed from
inverting to non-inverting under bus control. When line Q
8
of
latch U2015 is low, Q2023 conducts more and its collector
moves positive to about + 5 V. This in turn causes both
Q2025 and Q2028 to conduct. Pin 3 of U3032 is effectively
grounded, the sweep signal is applied through R3022 to the
summing node of the amplifier, and the gain of the stage is
— 1. If line Q
8
is high, Q2023 does not conduct and the
voltage at its collector falls to nearly — 15 V. Neither Q2025
nor Q2028 are now in conduction, so the sweep signal is
applied to pin 3 of U3032, and pin 2 is disconnected. Now,
the gain of the stage is +
1
.
Digital-To-Analog Converter
The magnitude of the sweep signal is determined by the
desired frequency span, band, and option installed in the
instrument. The microcomputer calculates the proper mag
nitude for each combination, and sends the appropriate
codes to the data latches, which in turn control the attenu
ation factor of the digital-to-analog converter. This stage
consists of converter U1042, amplifier U2042, and a com
plementary pair, Q2062 and Q3056, that form the output
current buffer.
Figure 5-25 illustrates a simplified two-bit digital-to-ana-
log converter. The circuit works by current division. Since
the summing node of the amplifier is at ground potential, the
magnitude of the current through a resistor is not affected
by the position of the switch that selects that resistor. For
example, when switch S1 is at position B, the current is
shunted to ground. When S1 is at position A, the current
through R1 becomes part of the total output current. Thus,
the output current can be 0, 1/4, 1/2, or 3/4 of the total
current available. Because of the resistance ratios, the ratio
of the output voltage to the input voltage equals the ratio of
the output to the total current (V /V. =1 ,/l ,). In this 2-
r
«
' out
in
out total
’
bit converter, there are 2 or 4 output values possible. In the
actual 10-bit converter, there are 2
10
or 1024 output values
ranging from 0 to 1023/1024 of the input.
In converter U1042, each internal resistance is switched
in or out by a CMOS FET (internal to the device). The
REV FEB 1983
5-59
