Fluke Biomedical 960SG-500 - Manual (Page 11)

Theory of Operation
4
4-1
Section 4
Theory of Operation
4.1 Frequency Generator
Refer to Drawing 960SG-100-13, Sheet 1; Signal Generator schematic in Appendix II while reading this topic. This
explanation is applicable to both signal generator circuit boards.
Crystal Y1, U7A, U7F, U7E, and associated circuitry provides the basic 2.000 MHZ frequency at an accuracy of 0.01%.
The frequency is fed to four section dividers, U4, U3, U2, and U1. The four ICs produce seven separate frequencies.
These frequencies are fed to U6 where they can be individually selected for generator output.
The first divider U4 is configured to divide the 2 Megahertz frequency by twelve. This produces a 167 KHz frequency.
The 167 KHz frequency from U4 is fed over to U6 and U3. U3 is a dual divider configured so that each section
provides a divide by ten function.
This produces two frequency outputs from U3, one 16.7 KHz and the other 1.67 KHz. The 1.67 KHz frequency is
supplied to U2 where it too is divided down in much the same manner as U3, to 167 Hz and 16.7 Hz frequencies. The
lower of the two frequencies from U2, 16.7 Hz, goes to U1 where it produces a 1.67 Hz and a 0.167 Hz frequency
output.
The dividers are all automatically reset to zero upon signal generator power-up.
4.2 Frequency Selection and Pulse Circuits
The frequency slide switch SW1 and IC U8 makeup a decimal to BCD (binary coded decimal) encoder. When U8’s
selected inputs are pulled low by slide switch SW1, a corresponding binary output is produced at the outputs. These
binary signals are inverted by U7B, U7C, and U7D and supplied to U6.
IC U6 is an eight channel analog input de-multiplexer. It supplies one of its analog inputs as an output to U5. The
seven frequencies from the divider circuit are also supplied to U6. The binary input to U6 determines the output
frequency selection as shown in Table 4-1. The output pulse frequency depends on the U6 code selection.
The output frequency selected from U6 is used to trigger U5, a one-shot multiplier. The one-shot multiplier circuit
produces an 800 ns wide pulse at a rate determined by the input frequency.