The easiest way of visualizing this is through the motion of a
pendulum. An oscillation is the change in state from an extreme
state (A) to the other extreme state (A) and back again. This is
similar to completing a cycle.
Slide 3
An oscillator is quite simply a device designed to maintain the
rate of oscillation with a high degree of precision. This is
property is rated Q a Q factor (Quality factor).
Slide 4
Frequency is the rate of oscillations per unit of time. For
watchmaking purposes, we use the unit Hz (pronounced hertz) to
denote this rate. A hertz is a measure of frequency defined as the
number of events happening per second. It is also an SI unit. For
example, a drummer hitting a drum 5 times per second can be said to
have hit the drum at a rate of 5 Hz.
Slide 5
A crystal oscillator is an electronic circuit that uses the
mechanical resonance of a vibrating crystal of piezoelectric
material to control an electrical signal to a very precise
frequency.
Slide 6
The piezoelectric effect is the property of a material to
generate a voltage when flexed and flex when a voltage is placed
across it. Here's a quick animation showing how piezoelectricity
occurs. It's somewhat simplified, but it gives you the basic
idea:
Slide 7
1.Normally, the charges in a piezoelectric crystal are exactly
balanced, even if they're not symmetrically arranged. 2.The effects
of the charges exactly cancel out, leaving no net charge on the
crystal faces.
Slide 8
3.If you squeeze the crystal (massively exaggerated in this
picture!), you force the charges out of balance. 4.Now the effects
of the charges no longer cancel one another out and net positive
and negative charges appear on opposite crystal faces.
Slide 9
Quartz crystal has the property of piezoelectricity. It is used
as an oscillator due to its naturally high Q factor. Quartz
crystals are manufactured to operate in a wide range of frequency,
however the standard frequency used in watchmaking is 32,768Hz. (2
15 x1Hz)
Slide 10
What is a Hertz? What is its symbol? Give the definition of an
oscillation. Give the definition of frequency. What does the term
Piezoelectric refer to? What frequency does a typical wristwatch
quartz oscillator operate at?
Slide 11
Though the physical shape of the quartz crystal can vary
depending upon its purpose and required properties, we typically
see the XY cut used in watchmaking. It resembles a tuning fork.
This shape provides not only increased stability and ease of
manufacture, but the shock resistance necessary for an instrument
which is subjected to daily use on a wrist.
Slide 12
Slide 13
Slide 14
The precision of a quartz crystals frequency is affected by
such things as temperature change, ageing, pressure change and
shock. Lets explore each area: Temperature Ageing Pressure Change
Shock
Slide 15
The temperature characteristic of quartz oscillators varies
from shape to shape as does its size. There are vibrational modes
possible in quartz that vary less than one second per month over a
temperature of 0 to 50C, however quartz oscillators having this
broad of a temperature range are much larger than other types of
quartz oscillators. Wearing a watch on your arm provides a
temperature- controlling effect that allows a smaller quartz
oscillator to be used but one that also has a narrower temperature
range.
Slide 16
The following is a graph showing how the change in frequency is
effected by the ambient temperature. The curve is placed allowing a
range of temperatures to average out against the line at 0.
Slide 17
Like all bodies in motion, quartz oscillators age. This occurs
during the entire life of the watch. The longer it operates, the
more it will deviate from its original frequency. As it ages, it
will speed up. By the time a quartz watch is one year old, the
error due to ageing problems is negligible.
Slide 18
For Example: If a quartz watch changes by one second per month
due to ageing during the first month of operation, then it will
also change by one second per month during the following two
months, and one second per month the next four months, etc. This
logarithmic type of change is shown in the graph, and it is
normally from one second per month to four seconds per month change
in timekeeping per decade change in time.
Slide 19
This change is shown in the graph, and it is normally from one
second per month to four seconds per month change in timekeeping
per decade change in time.
Slide 20
Quartz does not suffer from significant gravitational effects
but it is sensitive to the ambient pressure. For this reason,
quartz crystals are mounted in a hermetically sealed can, either
under a complete vacuum or under a partial vacuum with helium gas
in the can. One of the failure modes of quartz crystals is for a
slow leak of air into the can to occur. This will cause a 20 to 30
second per day loss by a watch. The curve of pressure versus
timekeeping for the quartz is below.
Slide 21
Dropping a quartz watch causes changes in timekeeping accuracy
just as it does in watches having a balance wheel. There are two
kinds of damage that can occur at a given shock level. In one type
the quartz is disturbed but not permanently damaged. The other type
of damage results in a permanent change of the quartz, and it will
not recover to its original timekeeping accuracy without the
trimmer being readjusted. The curves are shown at a 4,000 G shock,
somewhat more severe than a 3-foot drop to a hardwood floor.
Slide 22
What effect does Temperature have on the quartz oscillator?
What effect does Ageing have on the quartz oscillator? What effect
does Pressure have on the quartz oscillator? What effect does Shock
have on the quartz oscillator?
Slide 23
Even with these effects changing the frequency of the quartz
oscillator, one must keep things in prospective when considering
accuracy. Due to the high frequency and relatively small errors
non- permanently damaging effects produce, the quartz oscillator in
a cheap $2.95 movement is still approximately 30 times more
accurate than that of a $5000 mechanical watch. The quartz
oscillator has an accuracy to within 5 seconds per month, as
opposed to 5 seconds per day on a fine mechanical movement.
