Lowpass Filters using Series Stubs. Jack R Bratherton, Roger D Pollard, Stavros Iezekiel, Michael J Thornton" Microwave and Terahertz Technology Group University of Leeds LEEDS.LS2 9JT email:
[email protected] *Filtronic Components Limited Airedale House, Royal London Industrial Estate, Churlestown,Shipley, West Yorkshire.BDI 7 7SW
Abstract. A number of short circuited series stubs have been measured. Based on parameters derived from these measurements, a range of lowpass filters have been constructed which show excellent agreement in cut-off frequencies but unexpected behaviour in their stopbands. Simulated results for filters with ideal stubs are presented with no observance of these peculiarities. It is proposed that these peculiarities are due to resonant coupling between the stubs and evidence to support this theory
is presented.
Introduction. Series stubs have been used for a number of years as reactive elements [ 13 in printed filters, with work recently carried out on coplanar waveguide [2] and microshield [3,4]. The medium chosen for this work is a soft substrate microshield line with the dimensions given in Figure 1. This line exhibits single mode performance up to 46GHz with low loss and dispersion ( Figure 2 ).
Figure 1 Dimensions of soft substrate microshield line.
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cteristics of micros The series stubs in this work were to be used as a high perfomance alternative to stepped
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sections, with lowpass filters constructed using short circuited series stubs as inductive elements and low impedance sections used to realise shunt capacitances. Two sets of filters were constructed each containing circuits designed to cut-off at 4, 8, 16 and 32 GHz, both sets being based on 5 section Tchebyshev prototypes.
Results were predicted for ideal filters of both types ( Figure 3 ), with the series stub filters showing
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improved roll-off and wider stopbands. These results have been obtained using impedance data obtained from Hewlett Packard’s HFSS [5] for the stepped impedance sections and measurements for the series stubs which were then used in conjunction with Super Compact [6] without modelling the discontinuity parasitics involved. One might expect the results from such an analysis to be accurate for filters operating at low frequencies, or alternatively for those with high aspect ratios ( i.e. sections much longer than they
are wide.)
Measured performances. The cut-off frequencies for the stepped impedance filters were not accurately predicted. However, this was due to undercutting in the etching process [1,7] changing the characteristic impedances and effective dielectric constants of the filter sections. The roll off frequencies are all around 90 percent of those predicted with the higher frequency filters showing other deviations probably due to parasitics. The stopband shapes are as expected ( Figure 4 ). 0
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Figure 4 Measured performance of 4GHz filters. In contrast the cut-off frequencies of the stub filters were very close to those predicted, verifying the measurements of the stubs. The roll off is also well predicted but in the stopbands of the filters there is always a large spike ( Figures 4’5’6 ).
In the author’s opinion this is almost certainly due to an unwanted coupling affecting the stubs at resonance, i.e. when the stubs are a quarter of a wavelength long. So far the author has been unable to reproduce these performance deviations modelling the circuits with parasitic lumped capacitors and inductors using Super Compact.
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A 6mm short circuit stub measured on its own shows rejection at lOGHz implying that the effective dielectric constant of the stubs is approximately 1.5 and fitting the measurements to a model suggests a stub characteristic impedance of 1400hms. These derived parameters are verified by the frequency accuracy of the filters.
The centre stub of the 4GHz filter is 7.17mm long giving a resonant frequency around 8.3GHz which is where this filter exhibits a spike ( Figure 4 ). The 8GHz filter exhibits a spike around l6.8GHz corresponding to the 3.59"
centre stub ( Figure 5 ). In order to verify this coupling theory filters were
made, to roll off at 46Hz, one with the centre stub replaced by a high impedance section and one with the outer stubs replaced. The filter with no centre stub had no passband spike around 8.3GHz ( Figure 7 ) and the filter with only a centre stub exhibited this spike ( Figure 8 ). Any spikes due to the outer stubs would fall nearer the second passband so their affects would be less visible.
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Figure 7 Measured performance and circuit pattern of filter with no centre stub.
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Figure 8 Measured performance and circuit pattern of filter with centre stub only. Conclusion. A set of series stub filters have been presented. These exhibit far better performance than stepped impedance filters designed from the same Tchebyshev prototype, except for an unwanted spike in the
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stopband. This spike has been shown to be due to an unwanted coupling effect when the stubs are at quarter wave resonance.
This work has been generously supported by Filtronic Components Limited.
[l] Wadell, Brian C., Transmission Line Design Handbook, Artech House
[2] N I Dib, L P B Katehi, G E Ponchak, R N Simons, “Theoretical and Experimental Characterisation of Coplanar Waveguide Discontinuities for Filter Applications.”, IEEE Transactions on Microwave Theory and Techniques, Vol MTT-39, NO. 5, pp. 873-882, May 1991.
[3] T M Weller, L P B Katehi, G M Rebeiz, “A 250-GHz Microshield Bandpass Filter.”, IEEE Microwave and Guided Wave Letters, Vol5, No. 5, pp. 153-155 May 1995.
[4] T M Weller, L P B Katehi, G M Rebeiz, “High Performance Microshield Line Components.”, IEEE Transactions on Microwave Theory and Techniques, Vol MTT-43, No. 3, pp. 534-543, March 1995 [5]High Frequency Structure Simulator (HFSS), Hewlett Packard Company, Santa Rosa, CA.
[6] Super-compact PC, Microwave Harmonica Inc. [7] V Rizzoli, “Highly Efficient calculation of Shielded Microstrip Structures in the F’resence of Undercutting.”, IEEE Transactions on Microwave Theory and Techniques, Vol M’IT-27, No. 2, pp. 150157, February 1979
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