CLFilter (Coupled-Line Filter)

Symbol

Parameters

Name	Description	Unit	Default
Subst	Microstrip substrate name	None	MSub1
Fs1	Lower stopband edge frequency	GHz	1.8
Fp1	lower passband edge frequency	GHz	2
Fp2	upper passband edge frequency	GHz	2.2
Fs2	lower stopband edge frequency	GHz	2.4
Ap	passband edge attenuation (or ripple for Chebyshev)	dB	3
As	stopband edge attenuation	dB	20
N	number of filter sections (or 0 to compute N)	None	0
ResponseType	type of frequency response (maximally flat or Chebyshev)	None	Maximally Flat
Zo	desired input/output impedance	Ohm	50
CouplingType	Type of input/output coupling	None	Coupled Line Transformer Input
Delta	length added to coupled sections for tuning performance	mil	0

Notes

1. A coupled-line filter provides a bandpass frequency response between the input and output ports. N coupled-line sections produces an N-1 order filter response. Additional numbers of sections can be used to steepen the transition band roll off or widen the pass bandwidth.
2. Because of the heavy computational burden in determining the line parameters, a brief delay will occur for the design.
3. For a Chebyshev (equal ripple) frequency response, ripple levels greater than about 1 dB are not recommended. Exceeding this value will typically deform the shape of the passband characteristics.
4. If N is zero, the number of filter sections will be computed from the frequency/attenuation information. If N is non-zero, the design will use the frequency/attenuation parameters only for determining the design center frequency.
5. Using a Coupled Line Transformer Input CouplingType will use and extra coupled line section on the inputs and outputs to feed the device. Choosing Tapped Line Transformer Input will bring the feedline directly into the first resonator.
6. A SmartComponent subnetwork is empty until the Design Assistant is used to generate the design. Refer to Design Assistant.

1. For a more detailed discussion of this device, see D. M. Pozar, Microwave Engineering, 2nd Edition, John Wiley & Sons: New York, 1998, pp. 477-485.
2. The optimization minimizes the absolute difference between S21 in dB and the specified passband edge attenuation (which equals the ripple for Chebyshev response) at the passband edge frequencies. Because only the line lengths are changed, this tuning will typically center the response within the specified passband. More advanced shaping of the passband response can be accomplished by manually tuning the widths and spacings of the coupled filter sections.

Example

A coupled-line filter was designed for a maximally flat response with the 3 dB passband edge frequencies at 2 GHz and 2.4 GHz respectively. Coupled Line Transformer Inputs were used. The design required 4 coupled-line sections. Tuning using the Optimization Assistant yielded a value of Delta = -5.244 mil.