Tunable Wideband Planar Filters for KAT-7

Theunis BeukmanSupervisor: Dr. Riana H. Geschke

November 2011Tune-all Wideband Planar Filters for KAT-7

1OutlineIntroductionMotivationSpecificationsDevelopment of a new type of filterPrototype design & resultsConclusion

The outline of my presentation is as follows:The Introduction consists of a motivation and specifications for this workA solution for the specifications will be given, with the development of a new type of filterAfter which the design and results will be presentedAnd the presentation is ended with some conclusions

2MotivationTolerances in the fabrication processesWaveguide filters: fine-tune with tuning screwsMicrostrip filters: fine-tune with electronic tuning elementsNeed for a new filter synthesis consisting of:Wide bandwidthTunable in frequency and bandwidthImplementable in microstrip

High accuracy is required for radio telescopes. Therefore we want to be able to fine-tune the filter after fabrication.In waveguide the response is normally fine-tuned with tuning screws,However, in microstrip, electronic tuning elements such as varactor diodes are used.In this work an extensive review of current filter literature is done. The conclusion was that there exists no filter that posses all of the following characteristics:Wide BWTunable in f0 and BWAnd implementable in microstrip technology

3Specifications for KAT-7 FilterTune-all response (i.e. tunable in f0 & BW)FBWripple = 49% (1.2 - 1.95 GHz) LA < 1 dBLR > 15 dBs21 20dB:0.89 to 1.1 GHz2.1 to 2.5 GHz

Although the KAT-7 telescopes have already been build, the front-end filter specifications are used for this design, due to the difficulty of tuning such a filter response. We require tuning in both frequency & BW of the response. This is also called a tune-all responseThe filter must have a wide BW of 49% in the L-band.A flat passband is required, as well assharp attenuation slopes4Ring-ResonatorResonance where circumference is n (n=1,2,3)Two possible field distributions at resonancePremise is to perturb modes with tuning elements

The development of this new type of filter, starts with a simple ring-resonator.A ring resonates at the frequency where it has a circumference of a wavelength, or multiples thereof.In this structure 2 possible field distributions may be supported.Therefore the premise for this work is to perturb these degenerate modes with tuning elements, in order to obtain a tunable filter response.

5Development of a New Tune-all Filtering-Section Step 1:

A new filtering-section is developed, consisting of a ring that is coupled directly to the 50 terminals in this manner, which produces a notch at resonance.Now when introducing a disturbance to the symmetry of the ring, the degenerate modes are excited.

This is illustrated here, where a variable capacitor causes the lower mode to shift in position to lower frequencies.Note that 2 transmission poles are then formed between the zeros, leading to a 2nd order passband.Therefore with this network the lower cut-off frequency may effectively be tuned.6Development of a New Tune-all Filtering-Section Step 2:

A series capacitor perturbs the ring in such a manner that the upper transmission pole is tuned to higher frequencies, as shown here.7Proposed Filtering-Section

Therefore, by adding these 2 capacitors, a bandpass response is obtained that is tunable in both frequency and BW

8Design of Complete FilterCascade filtering-sectionsIncreases selectivityDecreases return lossNon-tunable matchingCapacitor values chosen from design graphs according to the desired specificationsDesign biasing network for the 2 different varactorsOptimise response with closed form microstrip models in MWODetermine final layout with EM solver

These filtering-sections are now cascaded together to form a complete filter network. This increases the selectivity while the return loss decreases.It is also decided to directly couple the network at the 50ohm terminals, in order to avoid an increase in losses and network complexity. The implication of this is that as the response is tuned, the return loss will deteriorate. The capacitor values are chosen from design graphs according to the desired specs. Note that all the sections are the same and thus only 2 types of varactors are required. The biasing network is designed for these 2 different varactors.The filter response is optimised with closed form models in Microwave Office according to design rules and graphs. The advantage of this approach is that full wave simulations are minimal due to the complete circuit models.Only the final layout is determined using an EM solver.

9Number of Cascaded Sections1st prototype consist of 4 cascaded sections2nd prototype consist of 6 cascaded sections

The responses of the TEM-circuits for 4 and 6 cascaded sections respectively, are shown hereThe specifications are indicated with the grey areas.It is clear that a trade-off exists between the selectivity and return loss. Therefore it was decided to design 2 filtersThe 1st prototype consists of 4 sections and is designed in order to maintain the return loss above 15 dB across the tuning range, because a mismatch occurs at the terminals as the passband is tuned.The 2nd prototype consists of 6 sections and is designed to achieve high selectivity.10Biasing Network

The biasing network of these filters is a very important part of the design and careful consideration is required. The filter network with the DC biasing network is illustrated here:..(explain layout)The 1st thing to note is that high impedance resistors can be used as the RF chokes, because the varactors has very low power dissipation.The series capacitors are biased in pairs as shown here, while the shunt capacitors are independently biased.11Sensitivity of Varactor DiodesInfluence of losses on passband:

Influence of parasitic inductances on cut-off:

The shape of the passband is determined by the series resistances of the varactors. For instance with high loss in the series varactor (C1) the response has poor roll-off at the upper frequencies, as illustrated by the red line.Therefore the ratio between the losses of the varactors has to be considered in order to obtain a flat passband.

The parasitic inductances of the varactors have great influence on the cut-off frequencies, as illustrated here. Thus these affects also need to be considered for the design.12Prototype 1: 4 Cascaded SectionsBoard layout structured with laser Vias constructed with through-hole plattingPhysical size: 0.54 g 1.07 g

The fabrication of the 1st prototype, consisting of 4 sections, is shown here.The board is structured with a laser and the vias made with through-hole plating.The physical dimensions are also given here.13Prototype 1: Centred ResponsesFine-tuning the measured response:f0 = 1.53 GHz & FBW = 49%

As stated before, the aim is to be able to fine-tune the filter response after fabrication.In this graph the fine-tuned response is plotted against the designed response.A large frequency deviation was found with the fabricated results.This is substantially caused by the parasitic inductances which seem to be due to very poor component tolerances. The affect is therefore that the return loss is unfortunately lower than 15 dB.14Prototype 1: BW-TuningSimulated: BW = 17.5%Measured: BW = 24.1% (lower f0 & poor LR)

Here, BW-tuning is illustrated for a fixed centre frequency.Due to the deviations in the parasitic components, the fabrications tuning is done at lower frequencies, hence, the poorer return loss.17.5% BW tuning is obtained with the simulated filter and 24.1% with the fabrication.15Prototype 1: Frequency-TuningSimulated: f0 = 5%Measured: f0 = 6.1% (lower f0 & poor LR)

Frequency-tuning is illustrated here, for a fixed FBW.5% f0-tuning is obtained with the simulated filter and 6.1% with the fabrication.

16Prototype 2: 6 Cascaded SectionsBoard layout structured with laser Vias constructed with through-hole plattingPhysical size: 0.47 g 1.69 g

The fabrication of the 2nd prototype, consisting of 6 sections, is shown here.Obviously this filter is larger than the previous

17Prototype 2: Centred ResponsesFine-tuning the measured response:f0 = 1.53 GHz & FBW = 49%

Again deviation in the parasitic inductances were found, and thus the return loss is very poor for the fine-tuned response18Prototype 2: BW-TuningSimulated: BW = 18.8%Measured: BW = 19.8% (lower f0)

Here the BW-tuning is 18.8% for the simulation and 19.8% for the fabrication, but with the return loss closer to 10 dB19Prototype 2: Frequency-TuningSimulated: f0 = 8.5%Measured: f0 = 8.5% (lower f0)

And the f0-tuning for both the simulation and fabrication is 8.5%

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Compare Prototype FiltersThe obtained specifications for both filters are summarised here.The only thing to take from this table is that, although the lower attenuation slope of the 6 section filter is out with a mere 10 MHz, all the KAT-7 specifications are indeed met.21Advantages and DisadvantagesAdvantages:WidebandTunable in f0 and BWImplementable in microstripLow lossesHigh selectivity

Disadvantages:Sensitive to parasitic componentsPhysically largePoor out-of-band rejectionThe advantages of this new type of filter is that:..

The disadvantages are that the filter is:Sensitive to parasitic components. ->Therefore the components should be measured and characterised before implementationPhysically large, however, ->the size can be reduced by meandering the linesPoor out-of-band rejection is obtained(next slide)

22RecommendationsProblem: Poor out-of-band rejection in prototype 2

To demonstrate the poor out-of-band rejection, the TEM-circuit response of the 6 cascaded sections are shown here.The required suppression is indeed achieved, however, it is not desirable to have a spurious passband at DC.23RecommendationsSolution: Cascade a wider BPF with prototype 2

[2] M. Sanchez-Soriano, E. Bronchalo, and G. Torregrosa-Penalva, Compact uwb bandpass filter based on signal interference techniques, Microwave and Wireless Components Letters, IEEE, vol. 19, no. 11, pp. 692 - 694, November 2009.

The advantage of this type of filter is, however, that a wider BPF (based in signal-interference techniques) can be cascaded to it without changing the main design. The result is 20 dB suppression around the passband as shown here.24ConclusionFollowing the literature review, there exists a need for a tune-all wideband filter synthesisA new filter design, based on perturbed ring-resonators, was proposed for KAT-7 specificationsThe theory was confirmed with fabricated filtersThis type of filter can also be applied to other wideband specifications such as that of MeerKAT(just read it)

25AcknowledgementsSKA project for scholarshipSonnet Software for the academic licenseApplied Wave Research for the academic license Wessel Crouwkamp and Wynand van Eeden, for their help with all the fabricationsI would just like to acknowledge the following organisations and people for their contribution to this work.26