When making connections between circuits, components, or a combination of both, the matching of the impedance must be considered. For a crystal filter, if the appropriate value is not used the characteristics provided in the product's specification cannot be obtained.
To ensure the impedance is matched correctly, the peripheral components must also have the appropriate values.
See as the most common test circuits are expressed by a RC matching circuit, Murata implements said circuits to express the important points.
If you contact Murata, we will propose the optimal impedance matching circuitry.
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What will happen if a resistive component (R) in an input/output changes from the state where the resistance is matched?
When the resistance becomes lower than the matched value, the center frequency shifts to the negative spectrum. When the inverse occurs and the resistance becomes higher, the center frequency shifts to the positive spectrum.
For this reason, it is necessary to select a crystal filter with the correct impedance resistance with little variation (±10% is not acceptable).
What will happen if a capacitive component (C) in an input/output changes from the state where the capacitance is matched?
When the capacitance becomes lower, the waveform expands. Both the pass bandwidth and attenuation bandwidth expand, and the distortion at the apex disappears. (Low ripple)
When the capacitance becomes higher, the waveform narrows. Both the pass bandwidth and attenuation bandwidth become narrower, and the apex distorts. (High ripple)
For this reason, it is necessary to select a crystal filter with the correct impedance capacitance with little variation (±1.0 pF is not acceptable).
Next, the role of interstage capacitance (Cc) in a 4 pole filter is explained.
When the waveform and center frequency is shift in a 4 pole filter, the value of the interstage capacitance is adjusted to balance the waveform.
In this example, the interstage capacitance is increased by 2.0 pF at each stage to adjust the value from Cc=7.5 pF → 9.5 pF → 11.5 pF, matching the desired waveform with an interstage capacitance of 11.5 pF.
As described above, it is necessary to select a product with the correct interstage capacitance (Cc) with little variation.
The software 'SimSurfing' simulates the characteristics of Murata products.