SBFilter (Stub Bandpass Filter)

Symbol

Parameters

Name	Description	Unit	Default
Subst	Microstrip substrate name	None	MSub1
Fs1	Lower stopband edge frequency	GHz	0.5
Fp1	Lower passband edge frequency	GHz	1.3
Fp2	Upper passband edge frequency	GHz	2.7
Fs2	Lower stopband edge frequency	GHz	3.5
Ap	Passband edge attenuation (or ripple for Chebyshev)	dB	0.1
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	Chebyshev
StubConfig	Configuration of interior shunt stubs	None	Two Parallel Stubs
StubType	Shunt Stub Type	None	Short Circuit Quarter Wave
Zo	Desired input/output impedance	Ohm	50
D	Impedance control parameter (0 < D < 1)	None	1
Finf	Frequency of infinite attenuation (for open circuit stub type)	GHz	1.0
Delta	Length added to stubs for tuning performance	mil	0

Notes

1. A stub bandpass filter provides a bandpass frequency response between the input and output ports. The design uses shunt stubs connected by lengths of transmission line. If the specified passband response is too narrow, large differences in impedance values can result in a non-realizable configuration.
2. This design is typically practical for fractional bandwidths of 0.4 to 0.7 or higher. If the bandwidth is too narrow, the design will generally require large differences in impedances between the stubs and the connecting lines, producing an unrealizable configuration.
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. The parameter StubConfig specifies whether the interior stubs (all but those closest to the source and load) are implemented as a single stub or as two stubs in parallel. Choosing a single stub often produces narrower stub line widths, and therefore this parameter can impact the ability to manufacture.
5. The parameter D offers some control over the ratio between the stub impedances at the ends of the filter to those in the interior. In some cases where a MCROSS or MTEE width constraint violation is encountered, decreasing (or increasing) this value in the range 0 < D < 1 can remedy the problem.
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.
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 frequencies. All stub lengths are tuned by the same amount. Because only the stub 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 lengths of all lines.
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. 595-608.

Example

A stub bandpass filter was designed for a Chebyshev response with a 0.1 dB ripple. The passband edge frequencies are at 1.3GHz and 2.7 GHz respectively. The design uses two parallel stubs in the interior regions with D = 1. The design required 4 stubs. Tuning using the Optimization Assistant yielded a value of Delta = -19.316 mil.