SLFilter (Stub Lowpass Filter)

Symbol

Parameters

Name	Description	Unit	Default
Subst	Microstrip substrate name	None	MSub1
Fp	Frequency at passband edge	GHz	1
Ap	Passband edge attenuation (or ripple for Chebyshev)	dB	3
Fs	Frequency at stopband edge	GHz	1.2
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
StubType	Type of stubs (commensurate or variable length)	None	Variable Length Stub
FElement	First filter component (automatic, capacitive, or inductive)	None	Automatic
Zo	Desired input/output impedance	Ohm	50
ZS	Characteristic impedance of stubs	Ohm	50
ZH	Characteristic impedance of connecting sections	Ohm	90
Delta	Length added to stubs for tuning performance	mil	0

Notes

1. A stub lowpass filter provides a lowpass frequency response between the input and output ports. The design is realized using narrow lines to approximate series inductances and shunt open circuited stubs to realize shunt capacitances.
2. The stub lowpass filter uses narrow microstrip lines that approximate series inductors connecting open-circuited stubs that approximate shunt capacitors in order to provide a lowpass frequency response.
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. In order to obtain an impedance match, Chebyshev designs must use an odd number of components (N).
4. If the parameter StubType is set as Commensurate, all stubs will have equal lengths, and the stub line width is computed. The value of ZS is then ignored, and the resulting design can be difficult to realize. If StubType is set as Variable Length Stub, the stub line widths are determined from ZS and the stub lengths are computed from the specifications.
5. The parameter FElement specifies whether the first stub is inductive or capacitive. If Automatic is chosen, the first component is set to be capacitive since this tends to offer improvements in manufacturing.
6. 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. Because this filter design strategy is approximate, the resulting stopband attenuation may not satisfy the specification. Choosing more sections than that computed by the Design Assistant can improve the stopband performance.
7. The optimization minimizes the absolute difference between S21 and the specified passband edge attenuation (which equals the ripple for Chebyshev response) at the passband edge frequency by changing the stub lengths. All stubs are tuned by the same amount.
8. A SmartComponent subnetwork is empty until the Design Assistant is used to generate the design. Refer to Design Assistant.

For a more detailed discussion of this device, refer to: Matthaei, Young and Jones, Microwave Filters, Impedance-Matching Networks, and Coupling Structures, Artech House, 1980, pp. 375.

Example

A stub lowpass filter was designed for a maximally flat response with a 3 dB attenuation at the passband edge frequency of 1 GHz. A variable length StubType and automatic FElement resulted in 13 components for the design. Tuning using the Optimization Assistant yielded a value of Delta = 164.04 mil.