Technical Communication
◎ What is Negative Resistance? How to test and determine?
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Negative impedance is a parameter to determine the stability of the oscillator circuit. If the negative impedance is too small, then the oscillator will be greatly affected by aging, temperature, and voltage changes.
The test circuit and procedure of negative impedance are as follows:
Test circuit:
Test procedure.
Connect the adjustable resistor in series with the quartz crystal to the circuit
Adjust Vr to make the circuit oscillate or deactivate
Test Vr when the circuit is just deactivated
Obtain the negative impedance value│-R│= R1+Vr?
R1: impedance of the crystal
Vr: Variable resistance
Recommended negative impedance value│-R│> 10*R1?
◎ Piezoelectric properties of quartz crystals
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The silicon dioxide molecule (SiO2) in a quartz material is electrically neutral with its electric dipoles balanced to each other under normal conditions. The silicon dioxide in (Figure 2, left) is a simplified picture in two dimensions. When we give positive and negative electric fields above the silicon atom and below the oxygen atom, respectively, the space system will repel each other in order to maintain the potential balance, forming a region of induced positive electric field below the oxygen atom and a region of induced negative electric field above the silicon atom. In the opposite case, when we give a negative and a positive electric field above the silicon atom and below the oxygen atom respectively, the two oxygen atoms will approach each other, creating an induced negative electric field below the oxygen atom and an induced positive electric field above the silicon atom. (Figure 2). However, when the horizontal position of the oxygen atom changes, the other oxygen atom in the vicinity will repel or attract the other atom, forcing the oxygen atom back to its original spatial position. Thus, the forces of the electric field and the forces between the atoms interact with each other, and the change in the electric field and the horizontal deformation form a state of interaction. This interaction results in a vibrational state with minimal energy consumption in the quartz material, which maintains a resonant frequency with the electric field, provided that the electric field is continuously supplied with energy. The amplitude of the oxygen atoms under this piezoelectric effect is related to the intensity of the electric field and the vector angle of the electric field on the silica. In practical applications, the electric field is generated by the metal electrodes plated on the quartz chip, and the vector angle of the electric field to the silica is determined by the cutting angle of the quartz crystal rod.
◎ Quartz Crystals and Frequency Control Components
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Quartz is a monocrystalline structure consisting of silicon dioxide (SiO2), a combination of silicon and oxygen atoms, in a hexagonal crystal system with a 32-point group (Figure 1). The monocrystalline quartz crystal structure is characterized by a piezoelectric effect, whereby when pressure is applied to the crystal in certain directions, an electrical potential is generated perpendicular to the direction of the applied force. In contrast, when an electric field is applied to the quartz crystal in some axes, deformation or vibration will occur in other directions. This piezoelectric effect of single crystalline quartz materials is used in the design and application of crystal oscillators to exploit its resonant frequency characteristics and to exploit its accuracy as a reference for various types of frequency signals. Because of the high material Q of quartz crystals, most frequency control components, such as resonators and oscillators, are based on quartz materials. Depending on their piezoelectric vibration properties, quartz-based frequency control components can be classified into bulk wave and surface acoustic wave components. Body wave vibration components such as quartz crystal resonators, quartz crystal filters and quartz crystal oscillators, and surface wave vibration components such as surface wave filters and surface wave resonators. When a quartz crystal is machined and ground to a specific size by a specific cutting method, it is commonly known as a quartz wafer (or quartz blank). The quartz wafer is placed in a vacuum environment, coated with electrodes, and then encapsulated with a conductive material on a metal or ceramic base, and becomes what is commonly known as a quartz crystal resonator. By using the low impedance and wave overlap characteristics of quartz resonators, a quartz crystal filter can be made with two adjacent electrodes. By adding different electronic oscillation circuits to quartz resonators, quartz oscillators with different characteristics can be made. For example, quartz frequency oscillator (CXO), voltage controlled crystal oscillator (VCXO), temperature compensated crystal oscillator (TCXO), thermostatic bath Oven Controlled Crystal Oscillator (OCXO)...etc. Surface acoustic wave resonance is the opposite of body wave resonance. Surface oscillation waves generated by coating the surface of a quartz crystal with an inter-digital-transducer (IDT) can produce a SAW Resonator or SAW Filter with short wavelength (high frequency) resonance.
◎ What is the Drive Level(DL)? How to determine the excitation power?
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Excitation power is the power consumed by the crystal during operation
There are two calculation methods to determine it
6.1. Using a high-frequency current probe to test the operating current of the crystal and then calculate it as follows
DL= I^2 * RL
RL= RR(crystal impedance)*(1+C0/CL)^2,
C0: Electrostatic capacitance
CL: Load capacitance
The crystal current is measured with a current probe, which is a relatively simple method.
6.2. The amplitude of the oscillation signal is measured with a high resistance voltage probe and calculated as follows:
DL=P=(V^2)*R/(Z^2)
Where V=(Vppout+Vppin)/2*2^0.5
Z=(R^2+X^2)^0.5
R=R1*(1+C0/CL)^2
X=1/ωCL
◎ What is the tri-state of a crystal oscillator?
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The output of the oscillator is controlled by a tri-state control, when the control is low, the output will be high resistance, when the tri-state is high, the output will have frequency and waveform output.
◎ What is load capacitance? A resonator has a nominal load (CL) of 20pF, how do I calculate the actual load capacitance used in a parallel resonant circuit?
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CL stands for Load Capacitance. Load capacitance is defined as the sum of all the equivalent capacitances between the two pins of the crystal in an oscillator circuit. In general, the IC manufacturer will give the recommended capacitance of the crystal in the specification.
The following equation can be used to approximate the required capacitance as shown in the figure:
CL = ((C1 x C2) / (C1 + C2)) + Cs
Cs is the stray capacitance of the circuit, usually 1~5pF, when CL is 20pF, the value of C1 and C2 is about 30~39pF.
◎ What are the uses of quartz crystal products?
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Crystal resonators are used in computer/interface card/telephone/pager/remote control/GPS/audio/video products
Oscillator
◎ What is PPM? How to calculate PPM value?
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PPM is parts per million. PPM is the ratio of the actual frequency to the nominal frequency.
For example, if the nominal frequency is 20.000000 MHz and the actual frequency is 20.000210 MHz, then the frequency deviation ppm=(20.000210-20)/20*1000000=10.5ppm.
◎ What are XO, VCXO, TCXO, OCXO?
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XO is an ordinary crystal oscillator without temperature compensation or voltage control to fine tune the output frequency, the temperature and frequency characteristics are mainly caused by the crystal itself.
VCXO is a voltage controlled crystal oscillator that has a pin that can be connected to an external voltage to fine-tune the output frequency.
TCXO is a crystal oscillator with internal temperature compensation circuit, which has good stability of output frequency when the operating temperature changes.
