Frequency Measurements; Dwell And Duty Cycle Measurements; Microcomputer Control; Peripherals To U1 - Fluke 78 Service Manual

2-15. Frequency Measurements

A voltage comparator is used for both signal detection in frequency mode and threshold
detection in continuity mode. In frequency mode, digital pulses from the voltage
comparator are routed to the counter. Pressing the range push button while in frequency
mode causes a range change in the primary function (ac or dc volts) that may change the
sensitivity.

2-16. Dwell and Duty Cycle Measurements

Dwell is a duty cycle measurement of the input waveform taken in the 40 V dc range. A
voltage comparator and frequency counter are used. Information from the selected
number of cylinders is combined with the duty cycle to compute dwell (the number of
degrees of distributor rotation that the points are closed).
The duty cycle function is performed in the 4 V dc range. Changing the trigger slope in
Duty Cycle (momentary button press) also changes the trigger slope for Dwell. The
meter returns to the default slope (-) when entering the sleep mode or when turned off.

2-17. Microcomputer Control

A microcomputer, integrated within U1, controls the various instrument functions and
drives the display. The position of the rotary switch, S1, is decoded by the
microcomputer from the three inputs F0, F1, and F2. All function modes, input ranging,
signal routing, active filter enable, a/d timing, and mode are controlled by the
microcomputer.

2-18. Peripherals to U1

In addition to input overload protection and input signal conditioning circuits, other
devices peripheral to U1 are needed to support the meter's features. The ac converter,
active filter, and a/d converter circuits require off-chip resistors and capacitors. Digital
drive and level-shifting circuits are needed for the beeper drive. A voltage reference is
generated separately from U1, and some discrete resistors and transistors support the
power supply.

2-19. AC Buffer

The ac buffer drives the converter and the frequency comparator and provides a driven
guard voltage.

2-20. AC converter

The averaging ac converter uses components R14, R18, R19, C4, C6, and C7. This ac
converter is a full-wave rectifying converter with a differential output, and it is gain
selected to give a dc output equal to the rms value for a sine wave input. Filtering is
provided by C4 and C7.

2-21. Active Filter

The active filter uses components R21, R22, C9, and C10. The active filter is a second
order low-pass filter with two poles at 5.9 Hz in normal mode. It filters input signal noise
and ac ripple from the ac converter, yielding stable a/d converter readings. The
microcomputer can disable the filter completely or enable the filter fast response mode
by shorting R21 and R22 with internal IC switches.

2-22. A/D Converter

Precision resistor network Z1-8, 9, and 10 connects to the three a/d buffer/integrator
range resistors. Z1-8 connects to 190 kJ for the 1-volt (read) range. Z1-9 connects to
Theory of Operation
Circuit Descriptions
2
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