Filter Structures Essay

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Filter Structures
K&L filters are available in bandpass, lowpass, bandreject and highpass designs. When specifying your filter needs, be sure to supply all pertinent passband and stopband information. To achieve the best results, additional specifications such as phase, group delay, power, size, environmental, and mechanical requirements should be supplied.
Lumped Element
The elements in the filter are lumped (i.e. concentrated over a small area). The inductors are coils of wire wound around cylindrical formers, and the capacitors are parallel plate chips or simpler portions of substrate material.
Combline
Combline filters replace the inductors in a lumped element filter with distributed inductors or lengths of transmission line leaving the capacitors lumped, although distributed capacitance is sometimes used . Advantages:
High "Q" factors can be obtained (3500)
Small size can be traded off with "Q"
Bandwidths from 3% to 50% can be obtained
Designs cover 500MHz to 26.5GHz.
Interdigital
Interdigital filters are entirely distributed networks consisting of an array of short circuit quarter wavelength lines.
Advantages:
High "Q" factors can be obtained (5500)
Small size can be traded off with "Q"
Bandwidths from 5% to 66% can be obtained
Designs cover 500MHz to 26GHz
Suspended Substrate Stripline (S.S.S.)
These filters are also entirely distributed consisting of both series and shunt transmission line sections.

Advantages:
Very selective devices are standard
Designs cover 100MHz to 40GHz

The tables below indicate the 0.5, 1.0, 1.5:1 VSWR and +5?phase bandwidths with regard to the normalized 3dB bandwidth.
Example:
Determined all the bandwidths below for filter model 5B121-500/T80-O/O. The insertion loss is:
(2)(5.5)
16
+0.2=0.9dB

From the tables:
The 0.5dB BW = (83%)(80) = 66MHz
The 1.0dB BW = (88%)(80) = 70MHz
The 1.5:1 VSWR BW = (85%)(80) = 68MHz
The +5?phase BW = (78%)(80) = 63MHz
0.5 dB Bandwidth vs. 3 dB Bandwidth
1.0 dB Bandwidth vs.
3dB Bandwidth

Insertion Loss
Number Sections

2
3
4-10
0.5 - 1.0dB
69%
82%
88%
1.0 - 1.5dB
65%
80%
85%
1.5 - 2.0dB
61%
78%
83%
2.0 - 2.5dB
58%
76%
81%
2.5 - 3.0dB
54%
74%
79%
3.0 - 4.0dB
50%
72%
77%
4.0 - 5.0dB
42%
68%
72%

1.5:1 VSWR Bandwidth vs. 3dB Bandwidth
+/- 5?Phase BW vs. 3 dB BW for Linear Phase Filters

Insertion Loss
Number of Sections

2
3
4 - 10
0.5 - 1.0dB
53%
70%
78%
1.0 - 1.5dB
48%
66%
74%
1.5 - 2.0dB
44%
62%
69%
2.0 - 2.5dB
39%
51%
65%
2.5 - 3.0dB
35%
53%
61%
3.0 - 4.0dB
30%
49%
56%
4.0 - 5.0dB
21%
40%
47%

3dB Bandwidth Tolerance
(Percent of f0)

Percent Bandwidth
Tolerance on Percent BW
1 - 4%
-0. + 0.5%
4.1 - 30%
-0. + 2%
30.1 - 60%
-0. + 4%
60.1 - 100%
-0. + 6%

Lumped Element Filter Structures
The structures shown are used as follows:
1. This is the classical "resonant ladder" used in high frequency wideband applications. The circuit is obtained by a lowpass to bandpass transformation. Its advantages are geometric symmetry and a small element value variation for broadband transformations.

2. The capacitively coupled "tank" circuit is an excellent structure for narrowband use. Over
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