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Calculating Number of Sections
The following curves show the stopband frequencies normalized to the 3
dB bandwidth for filters with 2 to 8 sections. A ratio of stopband frequency
to 3 dB bandwidth is used.
The curve given on the right shows an asymmetric frequency response resulting from the circuit used. Other schematics may be utilized to yield different attenuation characteristics (i.e. steeper on the high frequency side of the passband and shallower on the low side).
Please note that the frequency response curves shown are based on a low ripple Chebyshev Transfer Function. Exact performance is related directly to the unloaded, component selection and package size. If you have a critical parameter please contact the factory so full compliance may be assured.
Example:
A BP-Series filter has a center frequency of 600 MHz and a 3 dB bandwidth
of 120 MHz. Use the curve for 7-50% bandwidth filters. A stopband attenuation
of 30 dB is required at 360 MHz and 50 dB is required at 960 MHz.
The
percentage bandwidth is 20%, calculated as follows:
For the first stopband
requirement:
Number of 3 dB bandwidths
from center frequency =
From the 7-50% bandwidth
attenuation curve, we find that a minimum of 3 sections is required.
The second stopband
requirement is:
Number of 3 dB bandwidths
from center frequency =
From the 7-50% bandwidth
attenuation curve, we find that 5 sections minimum are required.
The greater number
of sections must be used to insure full specification compliance; therefore,
a 5 section should be used.
Insertion Loss Calculation
Knowing the number of sections, center frequency and bandwidth of the filter, insertion loss may be calculated using the following formula:
Example: 6BP8 - 725/145-S
1. Percentage BW = 145/725 x 100 = 20%
2. LC from table = 6.8
3. Number of Section (from P/N) = 6
4.
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