Front Unit And Microprocessor - Fluke PM3082 Service Manual

DESCRIPTIONS
Intensity and focusing.
The FINAL Z INPUT STAGE receives input signal for trace intensity (ZS) from the Z CONTROL block
in the time base section. A second input signal (ZT) determines the intensity (Z) component of the
text information. An input signal SW that comes from the microprocessor allows switching between
text and trace intensity. The FINAL Z AMPLIFIER drives the intensity control electrode G1 of the CRT.
This electrode is at a very negative -2.2kV voltage level. It is for that reason that a high-voltage
blocking capacitor is necessary between G1 and the output of the FINAL Z AMPLIFIER. However
only the ac component in the signal can pass through the capacitor. The dc and If components are
applied to the MODULATOR and modulated on a high-frequency carrier. Now they can pass through
a capacitor, are demodulated in the DEMODULATOR and then the dc, If and hf components are
recombined.
The FINAL Z INPUT STAGE drives, in parallel with the FINAL Z AMPLIFIER, the FINAL FOCUS
AMPLIFIER. This has the result that a well-focused spot over a large intensity range is obtained. The
FINAL FOCUS AMPLIFIER drives the intensity control electrode G3 of the CRT. This electrode is at
a very negative -2.2kV voltage level. It is for that reason that a high-voltage blocking capacitor is
necessary between G3 and the output of the FINAL FOCUS AMPLIFIER. The ac component in the
focusing signal passes through a high-voltage capacitor. The If and dc components are derived from
the DEMODULATOR that is also used in the intensity part. Focus control is possible via the FOCUS
LEVEL SHIFTER.
3.2.5

Front unit and microprocessor

The front unit and microprocessor are the sections where all oscilloscope functions are controlled.
Also the generation of text and the automatic calibration is controlled by the microprocessor. The
operations performed by the microprocessor are determined by the ROTARY MATRIX and KEY
MATRIX. Also commands from an external computer connected to the RS232 connector have the
same result.
The ROTARY MATRIX and keys in the KEY MATRIX present at front unit A4 are read by the FRONT
PROCESSOR. Also the ROTARY MATRIX that is present on the CRT controls unit A5 is read by the
FRONT PROCESSOR. The CENTRAL PROCESSOR on the MICROPROCESSOR UNIT A6 is
informed by the FRONT PROCESSOR of the settings selected with the front panel controls.
Incorporated in the CENTRAL PROCESSOR is a complete RS232 interface. Serial communication
is possible via the RS232 BUFFER.
The CENTRAL PROCESSOR has many inputs and outputs and forms the heart of the oscilloscope's
control section. First of all there are a READ ONLY MEMORY (CPROM) and a RANDOM ACCESS
MEMORY (CPRAM). The POTENTIOMETER DAC CIRCUIT is able to produce 16 independently
adjustable dc voltages. These voltages are used for continuous controllable functions such as
Position, FOCUS and VARiable.
The CENTRAL PROCESSOR also has a number of analog inputs that are internally converted into
digital. These inputs are used for automatic calibration (YCAL, XCAL, MTBCAL and DTBCAL),
AUTOCURsor position, power fail and probe indication (via PROBE DETECTION block). An
important output is the "uP" output. It is via this output that the CONTROL CIRCUIT blocks found
throughout the block diagram are controlled.
The CENTRAL PROCESSOR communicates to the TEXT/CURSOR CONTROL which text and
cursors have to be displayed. This information is stored in the TEXT/CURSOR MEMORY. The
TEXT/CURSOR CONTROL has one output that switches (SW) the vertical, horizontal and
intensity/focusing sections between normal signal (trace) display and text display. This text display is
done in between the signal display in such a sequence that "holes" in the signal are almost invisible.
The YT output of the digital-to-analog converter VERTICAL DAC produces the analog signal that
determines the vertical position during text display. The XT output of the HORIZONTAL DAC does
the same for horizontal text display. The ZT output produces a signal that determines the intensity
during text display.
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