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Oscillation Circuit Evaluation Methods
What is "Frequency Deviation" (3) <Improvement Method>
A large frequency deviation (a large deviation in oscillation frequency on the positive or negative side) means a large deviation between the circuit load capacitance (the load capacitance of the circuit due to the capacitor capacitance of the oscillation circuit and the stray capacitance of the board) and the load capacitance of the crystal unit (the load capacitance described in the specification of the crystal unit).
There are two methods for improving the frequency deviation (approaching ±0ppm) as following. The improvement method is selected in consideration of the other characteristics of the oscillation circuit (negative resistance, drive level).
| １． | Method for bringing the circuit load capacitance closer to the crystal unit load capacitance In NDK, when the frequency deviation needs to be improved, basically the circuit load capacitance is changed without changing the current crystal unit load capacitance. |
| ２． | Method for bringing a crystal unit load capacitance closer to a circuit load capacitance This method is used when the frequency deviation becomes large if the specifications of negative resistance and drive level is to be satisfied, or when it is necessary to reduce the frequency deviation at the circuit constant specified by the customer. |
We will further explain the details of the two types introduced above.
１．Method for bringing the circuit load capacitance closer to the crystal unit load capacitance
Changing the capacitance of Cin and Cout in the circuit diagram above (changing the circuit load capacitance) brings the frequency closer to the crystal unit load capacitance. The following shows the relation of the frequency deviations when the capacitance of Cin and Cout is changed.
(*1) Graph measured under the condition: Cin = Cout.
The following items can be read from the graph above.
- When the frequency deviation tends to be negative, it approaches ±0ppm by reducing the capacitance of Cin and Cout.
- When the frequency deviation tends to be positive, it approaches ±0ppm by increasing the capacitance of Cin and Cout.
２．Method for bringing a crystal unit load capacitance closer to a circuit load capacitance
Changing the load capacitance specification of the crystal unit (changing the crystal unit load capacitance) brings the frequency closer to the circuit load capacitance. The following shows the relation of the frequency deviation when the crystal unit load capacitance is changed.
The following items can be read from the graph above.
- When the frequency deviation tends to be negative, it approaches ±0ppm by increasing the crystal unit load capacitance.
- When the frequency deviation tends to be positive, it approaches ±0ppm by reducing the crystal unit load capacitance.