GW Instek GRF-3300 Series Student Manual - Manual (Page 13)

16 Microstrip Line Filters
3 321
16-3. Coupled line Band Pass Filter
Coupled line filter is a good choice if we want to design a band pass filter in
microstrip form which requires bandwidth about less than 20%. Wider bandwidth filter
generally require tightly coupled lines, which are hard to fabricate.
16-3-1. Design Example
We will design a coupled line filter with 0.5dB ripple, 2.4GHz center frequency,
15dB attenuation at 2GHz, and 10% bandwidth. The input and output impedance are both
50
:. The PCB material is FR4 with dielectric constant of 4.2 and thickness of 1.2mm.
16-3-2. Solution
First, we need to decide the order of the filter.
67 . 2 1 67 . 3
2
4 . 2
4 . 2
2
1 . 0
1 1
1 0
0
  ¸
¹
·
¨
©
§

¸¸¹
·
¨¨©
§

'

w
w
w
w
wc
w
' is the bandwidth ration and is equal to 0.1.
Referring to Figure 16-6, since 67 . 2 1

c
w
w
and attenuation is 15dB at 2GHz,
the order (N) becomes 2. From Table 16-1, we find the normalized values of g1 to g3 as
follows:
x g1=1.4029
x g2=0.7071
x g3=1.9841