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MODEL
8559A
SERVICE
DIGITAL PANEL METER ASSEMBLY Al, CIRCUIT DESCRIPTION
The Digital Panel Meter (DPM) Assembly AlAl/AlA2 is a dc voltmeter that measures a tuning voltage from
Marker Assembly A8, and converts it to a front
-
panel frequency readout. The DPM electronics are contained
on two assemblies: the DPM Display Assembly
AlAl and the DPM Driver Assembly AIM.
DPM Display Assembly A1A1
The DPM Display Assembly comprises five seven
-
segment displays with Darlington
-
transistor switches, Ql
through Q5. The seven
-
segment displays (DSl through DS5) are the common
-
cathode type. The cathode of a
display is pulled negative (to about
-
10.5V) when the Darlington
-
transistor switch associated with it is turned
on. With the cathode at a negative potential, the output of
AlA2U4 can light the display segments. The
transistor switches are
strobed so the displays light sequentially. The refresh rate is determined by the clock
(block C) and is fast enough (about 300 Hz) that the displays appear to be lit simultaneously.
DPM Driver Assembly A1A2
Contained on the DPM Driver Assembly AlA2 are the analog
-
to
-
digital converter, power supplies, and display
interface circuits. Analog processor IC
(U2) and digital processor IC (U3) are each one
-
half of an analog-to-
digital converter (ADC). Analog comparator circuits in U2 control counter logic in U3. To accomplish the
analog
-
to
-
digital conversion, U2 and U3 interact on three control lines: the M/Z (measure/zero logic) line, the
COMP (comparator) line, and the U/D
(up/down) line. The ADC, U3, produces two outputs. The first com
-
prises five sequential four
-
line BCD outputs that are fed to BCD
-
to
-
seven
-
segment converter U4. The second
consists of five sequential digit strobes that are fed to Darlington
-
transistor switches AlAlQl through AlAIQ5
on the DPM Display Assembly A1 A1
.
The input signal applied across connector pins 51-3 and 51-6 of the DPM Driver Assembly A1A2 is a dc level of
OV to
-4V,
representing an instrument tuning
-
range of 0 to 20 GHz (a 1V change of the input level represents a
tune frequency change of 5
GHz). This OV to
-
4V input signal is divided by precision resistors R33 and R27,
providing a OV to
-
2.0V signal across pins 2 and 15 of the analog processor IC, U2.
Transistors
Q1, 42, and Q9 interface the "sign/or/urV
(sigdover-rangehnder-range)
output of U3 with seg
-
ment
"g"
of numeric display AlAlDS5. Transistor 42 and CR2 provides a
"
wired AND
"
function so that the
minus sign is shown only in the most
-
significant
-
digit position (when both "D5" and "sign/or/ur" are high).
Transistor
Q1 serves to shift the signal level and Q9 supplies drive to the segment when a minus sign is displayed.
Field
-
effect transistor Q8 and its associated circuitry form a Colpitts oscillator that provides a clock of about
225 kHz. Inductor
L1 and the series combination of C1 and C2 determine the nominal clock frequency.
Power Supplies and Reference (G) (A)
The power supply circuitry provides the necessary voltage reduction, protection, and filtering for the dc supply
voltages:
+
12V,
-
12.6V, and
+
5V. The supply voltages are filtered as they enter the board to reduce interfer
-
ence between the DPM and the rest of the instrument. The
+
15V supply is used to derive the
+
12V supply and
the
+
5V supply. Zener diode VRl is used to reduce the
+
15V supply to
+
12V, while regulator U8 reduces the
+
15V supply to
+
5V. The
-
12.6V supply is filtered to offer two supply lines:
-
12.6VF, and
-
12.6VF2.
Operational amplifier
U7
and its associated circuitry provide a constant dc voltage reference of approximately
+
6.2V to the analog IC, U2.