Theory of Operation—492/492P Service Vol. 1 (SN BQ30000 & up)
Fig. 5-25. Simplified digital-to-analog converter.
CMOS inputs are each protected by a series input resistor.
Since the sweep signal is applied to the Vre( input, U1042
serves as a digitally controlled attenuator for the sweep
signal.
The attenuated sweep signal from U1042 is applied to
U2042, an operational amplifier. It in turn drives an output
current buffer, consisting of complementary pair
Q2062/Q3056. The pair is biased to produce an output cur
rent of about 10 mA in the absence of an applied signal.
This eliminates crossover distortion of the output signal. Di
odes CR2051, CR2053, CR1051, and CR1049 provide tem
perature stabilization for the bias current in the stage. When
high current is passing through the pair, diodes CR1056 and
CR1061 clamp the voltage across the emitter resistors to
reduce voltage drop.
Feedback for the output stage is provided by R1056,
plus an internal resistor in U1042. The internal feedback re
sistor ensures better temperature tracking. The internal re
sistor provides a gain slightly less than unity; R1056
increases the stage gain and permits gain calibration, as
described below.
One-of-four decoder, U4025, using the data from the Q4
and Q5 lines from U2015, controls three sections of a quad
FET switch, U3025. (RC circuit inputs of each FET control
line filter out noise from the digital circuits.) The code is ex
clusive; i.e., only one FET is switched on at a time. See
Table 5-12 for a listing of the codes. When a FET is
switched on, it connects a calibration adjustment potentiom
eter to the summing node of the operational amplifier. Ad
justment R1065 sets the 1st LO tune coil sweep, R1071
sets the 1st LO FM coil sweep, and R1067 sets the 2nd LO
span.
Table 5-12
CALIBRATION CONTROL SELECTION CODES
U4025
Pin 3 Pin 2
Selected Adjustment
low low
R1065 (main coil)
low high
R1071 (FM coil)
high low
R1067 (2nd LO)
5-60
REV AUG 1981
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