exam for Electronic Engineering
Jocelyn yang 
EE1301Session24-Clocks-Class.pdf
3/24/2018
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EE 1301: MODERN ELECTRONIC TECHNOLOGY
SESSION #24: CLOCKS
03/23/2018
Instructor: Joseph Cleveland, Ph.D. Email: [email protected]
Thought for the Day
The clock never stops,
Never stops,
Never waits.
We're growing old.
It's getting late.
Ben Folds
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Clock Technology
Shadow Clock: The Obelisk
• 3500 B.C. – Egyptians used moving shadow of obelisks
• Divided the day into two parts
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Shadow Clock: The Sundial
• ~1500 BCE in Egypt • Measured work hours • Problems
– Clouds and sun’s changing position
– Distance from equator
• Some history: Why does a modern clock rotate in a “clockwise” direction? Answer: Direction a shadow moves north of the equator
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Water Clocks (clepsydra)
~ 1500 BCE in Egypt • Used flow of water
from/into container • A float was lifted by the
rising water • A rack and pinion gear
turned the hour hand • Used
– For timing the speeches of orators
– As late as the 16th century, Galileo used a mercury clepsydra to time his experimental falling bodies
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Water Clocks
• Problem: water flow depended on pressure which decreased as container emptied
• Exponential decay in clock time – not very accurate
• Early versions did not have a minute hand – WHY?
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Clock Escapement Mechanism
• 3rd century BCE in Greece an escapement was added to water clock which transferred rotational energy to intermittent motions
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Hour Glass or Sand Clock
• Crafted by a Frankish monk, named Liutprand, in 8th century – He served at the cathedral
in Chartres, France.
• Timing uses – Preacher’s sermons
– Torture sessions
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Spring-Powered Clock
• Invented around 1500-1510 and used an escapement wheel
• Problem: the clock slowed down as the mainspring unwound.
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Galileo’s Pendulum
• A biographer says these observations came from timing the suspended lamp in the cathedral in Pisa, Italy as a young man during church services
• Galileo observed that the period of an oscillating pendulum is constant
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Pendulum Clocks
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Time to return to the starting point is the period T
Period of the swing
g = acceleration of gravity
.
If L = 1 meter
. ⁄
.
L
A naturally occurring time keeper!
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Pendulum Clocks …
• 1656: first pendulum clock • Rotation of escapement
wheel regulated by the "natural" period of oscillation of the pendulum – It had an error of less than 1
minute a day – Later refinements reduced
errors to less than 10 seconds a day.
– Ultimately it achieved an accuracy of a hundredth of a second a day.
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John Harrison’s Nautical Clock (H4)
• 1761 - The clock technology that won the “Longitude Prize”
• Now on display at the Royal Observatory in Greenwich, England
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Quartz Clocks and Watches
• The first quartz clock was built in 1927 by Warren Marrison and J. W. Horton at Bell Telephone Laboratories. – A quartz crystal will vibrate at a precise
natural frequency based on physical dimensions
• Modern quartz watches use precise frequency of 32,768 Hz and a 15-stage binary counter – A typical quartz clock will gain
or lose 15 seconds per 30 days
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length
constant
width
Modern Quartz Clocks …
• Modern quartz watches use precise frequency of 32,768 Hz
• A 15-bit binary counter clocked at this frequency produces an output pulse on the 32,768th count
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FF1 FF2 FF3 FF15
For all FFs Q so counting starts over.
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Atomic Clocks
• Six infrared lasers each gently push cesium atoms together into a ball which cools them to near absolute zero.
• Vertical lasers gently toss the ball upward.
• Under the influence of gravity, the ball then falls back down through a resonant cavity.
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Atomic Clocks …
• The microwave signal of the cavity alters the states of the cesium atoms which causes fluorescence.
• If the cavity is tuned to 9,192,631,770 cycles per second, the fluorescence is maximum.
• This "Natural frequency" defines the period of one second.
• The NIST cesium-clock is so accurate that it will neither gain nor lose a second in 20 million years!
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Modern Clocks: Summary
• Modern clocks rely on the inherent natural period or frequency for some property – Pendulum clocks
– Quartz clocks
– Atomic clocks
• Resonance of electronic state transition of cold Ce atoms
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Homework Question Guidance
Two NOT gates are inserted at the input terminals of a NAND gate, as shown in the figure. What single gate with inputs A,B and output Y is this arrangement equivalent to?
Approach: Set up the truth table
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X Y Z
0 0 0
0 1 1
1 0
1 1
X
Y
Z
0
0
1
1
1&1=1 NOT(1)=0 0
1 0
1 1&0=0 NOT(0)=1
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End of Session
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