FIRST, terminology explained:
1. Insertion loss: the numerical ratio of direct transmission of
signal source to the load impedance of power (P0) and insert the
filter transmission of power to the load impedance (P1). Usually
decibels (dB) for units of measure, expressed as IL = 10 lg(P0 / P1).
2. Passing band fluctuations: Differentials between band attenuation
of the maximum peak and the minimum Valley value.
3. Bandwidth: relative attenuation is less than or equal to a
provisionequivalent to the width of the frequency value
(such as 1 dB, 2dB, 3dB, 6dB, etc.).
4. Stopping band attenuation: that the rejection of the minimum
5. The width of resistance: the relative attenuation more than or
equal to the value of a certain provisions of the bandwidth
(such as 40 dB, 50dB, 60dB, 80dB).
6. Matching impedance: termination impedance value matching in
filter required of technical conditions.
SECOND, Application Guide
According to its structure, Quartz crystal filter is divided
into integrated single-chip filter and separate filter. Integrated
filter structure is simple, small size, low price, but its frequency
and bandwidth have restrictions, separation filter can
compensateintegrated filter for the shortcomings, which will achieve
the frequency and bandwidth to expand.As the development of
digital communications technology, some questions has brought about,
for example, the crystal filter’s group delay characteristics and
inter modulation distortion indicators, but separate filter can
resolve these more easily.
1. Impedance matching: when the excellent performance of the filter
circuit mismatch with its termination impedance, the
filtering properties will deteriorate and cause increased
volatility band, and the insertion loss would increase. When the
circuit impedance below the filter characteristic impedance, the
Centre frequencies will be lower, on the other hand shift. Filter
tests or use shall conform to the following schematic :Function
of "Signal + Level Meter" is completed by the Network Analyzer.
Ri, R0: Instrument resistance, General 50 Ω R1 - filter
external impedance at input end, impedance value is matching
impedance minus 50 Ω. R2 - filter external impedance at output
end, impedance value is matching impedance minus 50 Ω.The matching
impedance in filter conditions sometimes have capacitance
requirements, it should be link like in the map.
2. a reasonable measurement of level; as requirements of the
Crystal incentive level, the filter core element is still crystal,
in the absence of incentive-level requirements, generally we elect
0 dB as input level.
3. a good shielding: as good shielding to filter’s input and output
end, sources of energy can not be directly coupled to the load side.
To the VHF the filter or above ,the connection of the equipment
end should be match the principles of coaxial line. Filters should
be earthed as large area as possible, also we must isolate input
and output end, to ensure the filter’s stop band attenuation.