Tools for designing crystal ladder filters【links】
There are various bandpass filter design tools with crystal oscillators. Here, I will introduce online tools that is easy to use. Although it is a simple procedure, it is possible to design a sufficiently practical filters by using 1 to 3.
A design example with these tools (center frequency ~ 10MHz, 6 elements, bandwidth ~ 2.7kHz) is shown in below picture.
1. Calculation of crystal oscillators
When designing a ladder filter, it is necessary to obtain the equivalent circuit parameters of the crystal oscillator. Use the tool below to derive the parameters from the experimental results.
e.g. mass product crystal oscillator (10.000MHz)
fs = 9.997733 MHz
fp = 10.013933 Hz
Cs = 9.965 fF
2. Calculation of filter parameters
Calculate the bandpass filter parameters and input/output impedance using the eparameters derived in 1.
※Screen shot of the linked page
3. Calculation of matching circuits
Input / output impedance matching is important to obtain the characteristics as designed.
Impedance conversion using a transformer is good but it is more convenient to employ L-C circuits because it can be designed flexibly.
※Screen shot of the linked page
