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Quartz Crystal Training Seminar
QUARTZ CRYSTAL OSCILLATOR TRAINING SEMINAR Nov, 2000 Presented by: Jim Socki, Crystal Engineering Session 1 Introduction & Class Organization How Quartz Crystals Work: An Intuitive Approach, Part 1 - Crystal Structure, Unit Cell - Piezo Electric Effect - Resonance - Some properties of quartz Session 2 How Quartz Crystals Work: An Intuitive Approach, Part 2 Modes of Vibration The electrical and mechanical models for crystals (motional parameters) Different Crystal “Cuts” -AT, BT -X, Y -SC -Tuning Fork X-Rays Resonance Frequency of different crystal cuts - Fundamental Freq - Overtone Freq Temperature Behavior of different crystal cuts The Inverted Mesa Session 3 How Quartz Crystals Work: An Intuitive Approach, Part 3 A practical crystal design
What is a crystal?
• A class of materials arranged in a definite, geometric pattern in three dimensions (table salt and sugar are common examples)
• Quartz Crystal is silicon and oxygen arranged in a crystalline structure (SiO2).
• SiO2 is also found abundantly in nature in a non-crystal structure (“amorphous”) as sand.
A method for predicting the behavior of
a crystal: The unit cell
+ Represents silicon atom
- Represents oxygen atom
Not actually correct, but this method allowsa good understanding of quartz crystals
The unit cell of crystal silicon dioxide
-
+
+
-
+
-
-
+
+
-
+
-
-
+
+
-
+
-
+
-
A pushing force:(aka: compression)
A pulling force:(aka: tension)
+-
The Piezoelectric Effect
Crystal
Current Meter = 0
+ - + - + -
+ - + - + -Charges canceleach other, sono current flow
Crystal material at rest: No forces applied,so net current flow is 0
The Piezoelectric Effect
Crystal
Current Meter deflects in + direction
- - - - -
+ + + + +
Due to properties of symmetry,charges are net + on one side & net - on the opposite side: crystal getsthinner and longer
Crystal material with forces appliedin direction of arrows………..
Force
The Piezoelectric Effect
Crystal
Current Meter deflects in - direction
+ + + +
- - - - -
…. Changes the direction of current flow, and the crystal getsshorter and fatter.
Changing the direction of theapplied force………..
Force
The electromechanical nature of piezoelectric material
• In general, if you deform a piezo crystal by applying a force, you will get charge separation: Think of a simple battery.
• Taking it one step further, what would happen to the crystal if you applied an electrical force that results in the exact same current flow from the proceeding circuit?
The electromechanical effect
Crystal
…. With the switch open, the crystal material is now at rest again:the positive charges cancel the negative charges.
Now, replace the current meter with a power source capableof supplying the same current indicated by the meter….
+ - + - + -
+ - + - + -
power source
switch
charges cancel
The electromechanical effect
Crystal
…. and, the crystal should get shorter and fatter.
When the switch is closed, and you apply the exact amount of power to get the same current that resulted when you squeezedthe crystal, the crystal should deform by the same amount!!
power source (battery)
+ side
- side
+ + + +
- - - - -
The electromechanical effect
Crystal
…. the crystal should get longer and skinnier.
What will happen if you switched the battery around??
power source (battery)
- side
+ side
- - - - -
+ + + + +
Summary of the Piezoelectric & Electromechanical Effect
• A deformation of the crystal structure (eg: squeezing it) will result in an electrical current.
• Changing the direction of deformation (eg: pulling it) will reverse the direction of the current.
• If the crystal structure is placed into an electrical field, it will deform by an amount proportional to the strength of the field.
• If the same structure is placed into an electrical field with the direction of the field reversed, the deformation will be opposite.
The piezoelectric & electro-mechanical effects by themselves do not fully describe how crystals we build operate: You need
to understand the concept of……..
RESONANCE
Q: What do the following events have in common?
1. The destruction in 1940 of the Tacoma Narrows Bridge.
2. Ella Fitzgerald shattering a glass with only a slight amplification of her voice.
3. A tuned electric circuit operating at its series frequency.
4. A quartz crystal operating at its series frequency.
