Let’s Get Technical
Figure 4. This digital frequency divider circuit conditions the output signal from the fiber oscillator, converts it to
TTL logic levels, divides the signal frequency by 8,192, and produces an audible tone on the speaker.
for the base of transistor Q1. Q1 is
base-biased by the 47K resistor so
that it is always on unless something
pulls the base lower than 0.7 volts.
When Q1 is on, the collector
output sits at a logic zero level (
essentially, all of the + 5 V supply voltage
drops across the 1K collector resistor).
Although open for DC, the 0.01 µF
capacitor acts almost like a short
when operating at the high frequency
rate of the ring oscillator. So, the
capacitor allows the Fosc signal to
yank the base of Q1 low once for
When the base is taken low, the
transistor shuts off, allowing the
collector resistor to pull the output
signal up to a logic one level. When
the Fosc signal changes and Q1 turns
on again, the output at the collector
will go to a logic zero again, providing
the falling edge required by the
74LS393 CLK input. The 393’s are
cascaded to form a 13-bit counter.
Since 213 equals 8,192, we need that
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many pulses to get a single pulse
output to the speaker circuit. The
audio transformer is used to provide
a high impedance in the collector of
Q2, so that the 8 Ω speaker does not
yank on the + 5 V supply too much.
So — over the course of this fiber
series — we have seen that a beam of
light can be used in interesting and
useful ways. We should be aware of
this because, right now, light is the
fastest thing that we’ve got. Even with
the Dense Wave Division Multiplexing
techniques now used to place
multiple 2.4 Gbps optical carriers on
one fiber, we have barely tapped the
communication speeds made
possible with light. Perhaps we lack
the necessary understanding of
physics and nature to reach the
full potential, but it is worth the effort
to try. NV
About the Author
NUTS & VOLTS
James Antonakos is a professor in the
Departments of Electrical Engineering
Technology and Computer Studies at
Broome Community College. He has over
27 years of experience designing digital and
analog circuitry and developing software.
He is also the author of numerous
textbooks on microprocessors, programming,
and microcomputer systems. You may
reach him at antonakos_j@sunybroome.
edu or visit his website at www.suny