Product: 492 Ser.1
Date: 2-12-85 Change Ref: M55287
U2070 tracks the output of U1065 when the circuit is in the track
mode and serves as the inverting amplifier in the feedback system
shown in Figure 5-28. Normally the incoming signal is routed
through R2067. To inprove the integrator's slewing rate, CR1067 and
CR1069 conduct and connect R1068 across R2067 when input signals in
excess of IV are present.
Write-Back Circuit
This circuit consists of comparator U1055 and enabling transistor
Q1058. When it is necessary to do a carry between the low and high-
order DAC's, the circuit is put into the hold mode by turning off
Q1065. U2060 is incremented one bit and U2055 is reset to all 0's.
The output of U1065 is now at something other than OV. The purpose of
the following approximation routine is to get U1065's output as
close to OV as possible before switching the circuit back into
track mode by turning on Q1065. Comparator U1055 detexts whether
U1065's output is above or below coarse tune ground. The instrument
microcomputer begins to exercise the low-order DAC bits one at a time
from MSB to LSB. After each bit is turned on, U1055 is enabled by
turning off Q1058. If U1055 detects that U1065's output has crossed
OV, that bit is turned off and the next lower bit is turned on. This
continues through all 12 bits and when completed, should have U1065's
output very close to OV. Q1065 can now be turned back on without
causing excessive signal jump on screen.
-10V Reference Buffer
The circuit uses the voltage reference developed on the 1st LO Driver
board as a reference for the DAC's. Differential amplifier U2045
receives the -10V reference and -10V reference return and removes
any common-mode signals present. Resistor pairs R1048-R1049 and
R1050-R1051 are TC matched to minimize reference voltage drift over
temperature.
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