Bird Technologies 4391A - Manual (Page 15)

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Chapter 2 Theory of Operation
Description of Operation
Figure 2 is a block diagram of the major functional parts of the Model 4391A RF
Wattmeter. The Microcomputer integrated circuit shown, controls all the other
portions of the instrument, which fall into two major groups.
Figure 2 Circuit Block Diagram
Keyboard, Range Switches, and Display Group
The keyboard and range switches serve only to pass information to the computer.
The display, of course, returns information from the computer to the operator. The
display, which is comprised of four seven-segment LED digits, is strobed digit by
digit left to right at a rate of approximately one digit per millisecond. This serves to
conserve battery power and drive circuitry while providing scanning for the col-
umns of the keyboard. Each time a digit is strobed, the corresponding column of
the keyboard is read and if a key is pressed, the computer puts the code for that
key into a memory cell. The nine mode keys select which parameter is to be mea-
sured. The three modifier keys simply modify the way in which the result is dis-
played. The range selector switches identify to the computer the nominal full scale
values of the elements used. They have no effect on input sensitivity, which is
determined by the elements.