Design and realization of narrow quartz crystal filter at 70MHz

The 70MHz narrow band quartz crystal filter has been developed. The filter features: center frequency 70MHz, 3dB band width ≥13.5KHz, ripple in pass band ≤0.5dB, stopband attenuation ≥50dB, rectangle coefficient ≤2.2, insertion loss ≤3dB, input and output impedance 50Ω, range of operation temperature -40°C~65°C. The temperature performance of crystal filter depends on crystal's cut type, therefore, according to the range of operation temperature, we used the round wafer of AT-cut 35°24'±2', the wafer diameter was 5.5mm, and the ones from temperature are within ±13ppm, which ensured the center frequency deviation less than 1KHz. In order to meet the stopband attenuation and spurious response attenuation ≥50dB, the electrode of the high-end frequency crystal and the low-end frequency crystal were designed respectively; We selected aluminum electrode as the electrode material, because the aluminum electrode film has the advantages of light weight and good conductivity; Owing to stable characteristics and good symmetry of differential bridge circuit structure's filter, and the impedance transformation which may be realized by resistance transformer, consequently, we selected the differential bridge circuit structure. At last, the result of the experiment proves the filter is feasible, and the parameters meet the need of requirement.