Page 140 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Useful Electronic Circuits and Construction Techniques to Get You Going

Useful Electronic Circuits and Construction Techniques to Get You Going 133

includes switchable elements to optimize internal circuitry for low-frequency
clock crystals or higher frequency resonators. No extra components are needed
except for power supply decoupling, placed as close as possible to the chip. The
only potential difficulty with this chip is the very high impedance at which the
inputs work. Even low levels of leakage around the crystal connection pins will
stop the circuit oscillating. This includes PCB leakage currents, so it is impor-
tant that the board is carefully cleaned of flux residues and preferably guarded
as recommended in the application guidelines. If the 32kHz output is too fast
for the application, division stages can easily be added in CMOS D-type logic as
shown. The 74HC74 contains two dividers, getting you down to 8kHz. The same
components, connected differently, can provide 90° phase shifted outputs I and
Q. Using CMOS logic, this module will operate any conventional LED using a
single 3.3V lithium cell and an FET or transistor buffer. Another useful IC is
the 74HC4060, which combines a free inverting gate between pins 10 and 11
which can be used as an oscillator, and 14 stages of division, with most of the
division stages made available on pins (Fig. 6.9). With some tweaking of the
biasing components, this will work with a great variety of two-terminal res-
onators. The divider chain will get a 32kHz clock crystal down to 2Hz.
Ceramic resonators offer another approach to timing. For example, 455kHz
resonators are available from Murata and others, which are economical and
easy to drive. The HA7210 can again be used, or you can make your own circuit
using simple gates. The Murata design notes are very helpful, and recommend
a particular kind of logic gate. This is the CD4069UB series (Fig. 6.10). These
gates are “unbuffered,” which gives a reduced open loop gain compared with
buffered gates, reducing the likelihood of oscillation at a harmonic of the
design oscillation frequency. Sometimes it can be difficult to get any of these

+5V

4096 Vcc
8192 1024 100nF
16 256
16384
64 512 Crystal
resonator,
32 RST
etc. 2x
128 CLK 15pF
16 In
GND Out 10M 220k
74HC4060
CD4060
Figure 6.9 The 74HC7060 provides a free gate for
exciting crystal resonators, together with a long
divider chain for selection of lower frequencies.

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.