SIFilter (Stepped Impedance 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
FElement	First filter component (automatic, capacitive, or inductive)	None	Automatic
Zo	Desired input/output impedance	Ohm	50
ZL	Characteristic impedance of low impedance sections	Ohm	25
ZH	Characteristic impedance of high impedance sections	Ohm	100
Delta	Length added to filter sections for tuning performance	mil	0

Notes

1. A stepped impedance filter provides a lowpass frequency response between the input and output ports. The design is realized using alternating wide and narrow microstrip lines.
2. The stepped impedance filter uses wide microstrip lines to approximate shunt capacitors and narrow lines to approximate series inductors 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. The parameter FElement specifies whether the first stub is inductive or capacitive. If Automatic is chosen, the first component is inductive if Zo/ZL > ZH/Zo and capacitive otherwise.
5. 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.
6. Because this filter design strategy is approximate, the resulting stopband attenuation can not satisfy the specification. Choosing more sections than that computed by the design 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. All filter sections 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, see: Matthaei, Young and Jones, Microwave Filters, Impedance-Matching Networks, and Coupling Structures, Artech House, 1980, pp. 365-374.

Example

A stepped impedance lowpass filter was designed for a maximally flat response with a 3 dB attenuation at the passband edge frequency of 4 GHz. Choosing FElement as Automatic results in a capacitive first component. Tuning using the Optimization Assistant yielded a value of Delta = 27.446 mil.