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FAST ELECTRONICALLY TUNABLE FILTER BANK Miguel S. Ruip´ erez, Daniel Amor, Jorge Gonz´ alez and Jos´ e I. Alonso * Indra Sistemas S.A. Ctra. de Loeches 9, 28850 Torrej´ on de Ardoz, Madrid, Spain. Telephone: +34-91-626-8529. E-mail: [email protected], [email protected], [email protected] * Dpto. Se˜ nales, Sistemas y Radiocomunicaciones. ETSI Telecomunicaci´ on. Universidad Polit´ ecnica de Madrid. Ciudad Universitaria s/n, 28040 Madrid, Spain. Telephone: +34-91-336-7358. E-mail: [email protected]. Abstract— This article shows a new electronically tunable filter bank. The main characteristics are: wide frequency range, 6 to 18GHz, compact size and fast response within the temperature range of -40 C to 75 C. To achieve these objectives the filter bank is built using HMIC and MMIC technology. The purpose of the system is to act as a fast tunable system in communications, avionics and EW systems. Index Terms— Tunable circuits and devices, Tunable filters, Electronic Warfare, Radar. I. I NTRODUCTION T HE analysis of the spectrum occupied by communica- tions and radar signals requires the use of equipment capable to handle signals in an agile way, in order to be able to process the whole objective spectrum. One of the structures capable of achieving this kind of operation consists in a switched filter bank. Most manufactures [1] [2] [3] approach this problem using a switched fixed filter bank. This solution has as main drawback the strong relationship between the number of bands built and the bandwidth of these bands. The bandwidths needed for processing the signals may lead to a large number of channels, thus increasing size and weight. This size increment may be acceptable for fixed ground stations, but mobile and avionics applications have strong mechanical limitations, requiring compact size and lightweight equipment. To overcome this, a tunable switched filter bank is designed and built. In this system the spectrum is divided in a series of discrete bands, each one containing a contin- uously tunable filter which is responsible for subchannel selection. This tunable behavior boosts system performance due to the capability of the filters to be tuned to a specific desired frequency and avoiding adjacent jamming signals that could overdrive the receiver. This way of operation reduces the size of the unit, which, along with the use of hermetic connectors and laser welding, makes this system highly suitable for Radar and EW applications. Fig. 1. Switched filter bank block diagram II. SYSTEM DESCRIPTION The main goal of the design process consists in achiev- ing a broadband operation keeping a moderate bandwidth suitable for further processing. Figure 1 shows the block diagram proposed for this system. The SP5T switch and tunable filter structures will be the key elements, they will be responsible for achieving the proposed frequency response and channel isolation. The architecture showed uses a two step band selection. The two steps required are: 1) Switched behavior, selection of a discrete band, 2) Tunable behavior, continuous selection of a subband inside the previous band The filter bank designed includes central frequency interleaving between adjacent bands to obtain smooth band switching and reduce the possible variations due to com- ponents tolerances, production process and environmental conditions. The system includes two independent filtering channels in order to allow its use in applications that require simultaneous transmision/reception. These channels are symmetrical, requiring half the available space each. Table I shows the electrical, mechanical and environ- mental specifications. In the following sections we will describe the different elements shown in the block diagram. Each element has been measured before integration in order to test its cor- rect performance. This process eliminates failures due to faulty components in the early stages of the manufacturing process.

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Page 1: [IEEE MILCOM 2005 - 2005 IEEE Military Communications Conference - Atlantic City, NJ, USA (17-20 Oct. 2005)] MILCOM 2005 - 2005 IEEE Military Communications Conference - Fast Electronically

FAST ELECTRONICALLY TUNABLE FILTER BANK

Miguel S. Ruiperez, Daniel Amor, Jorge Gonzalez and Jose I. Alonso∗Indra Sistemas S.A. Ctra. de Loeches 9, 28850 Torrejon de Ardoz, Madrid, Spain. Telephone: +34-91-626-8529.

E-mail: [email protected], [email protected], [email protected]∗ Dpto. Senales, Sistemas y Radiocomunicaciones. ETSI Telecomunicacion.

Universidad Politecnica de Madrid. Ciudad Universitaria s/n, 28040 Madrid, Spain. Telephone: +34-91-336-7358.E-mail: [email protected].

Abstract— This article shows a new electronically tunablefilter bank. The main characteristics are: wide frequencyrange, 6 to 18GHz, compact size and fast response withinthe temperature range of -40◦C to 75◦C. To achieve theseobjectives the filter bank is built using HMIC and MMICtechnology.

The purpose of the system is to act as a fast tunable systemin communications, avionics and EW systems.

Index Terms— Tunable circuits and devices, Tunable filters,Electronic Warfare, Radar.

I. INTRODUCTION

THE analysis of the spectrum occupied by communica-tions and radar signals requires the use of equipment

capable to handle signals in an agile way, in order tobe able to process the whole objective spectrum. One ofthe structures capable of achieving this kind of operationconsists in a switched filter bank.

Most manufactures [1] [2] [3] approach this problemusing a switched fixed filter bank. This solution has asmain drawback the strong relationship between the numberof bands built and the bandwidth of these bands. Thebandwidths needed for processing the signals may leadto a large number of channels, thus increasing size andweight.

This size increment may be acceptable for fixedground stations, but mobile and avionics applications havestrong mechanical limitations, requiring compact size andlightweight equipment.

To overcome this, a tunable switched filter bank isdesigned and built. In this system the spectrum is dividedin a series of discrete bands, each one containing a contin-uously tunable filter which is responsible for subchannelselection.

This tunable behavior boosts system performance dueto the capability of the filters to be tuned to a specificdesired frequency and avoiding adjacent jamming signalsthat could overdrive the receiver. This way of operationreduces the size of the unit, which, along with the use ofhermetic connectors and laser welding, makes this systemhighly suitable for Radar and EW applications.

Fig. 1. Switched filter bank block diagram

II. SYSTEM DESCRIPTION

The main goal of the design process consists in achiev-ing a broadband operation keeping a moderate bandwidthsuitable for further processing.

Figure 1 shows the block diagram proposed for thissystem. The SP5T switch and tunable filter structures willbe the key elements, they will be responsible for achievingthe proposed frequency response and channel isolation.

The architecture showed uses a two step band selection.The two steps required are:

1) Switched behavior, selection of a discrete band,2) Tunable behavior, continuous selection of a subband

inside the previous band

The filter bank designed includes central frequencyinterleaving between adjacent bands to obtain smooth bandswitching and reduce the possible variations due to com-ponents tolerances, production process and environmentalconditions.

The system includes two independent filtering channelsin order to allow its use in applications that requiresimultaneous transmision/reception. These channels aresymmetrical, requiring half the available space each.

Table I shows the electrical, mechanical and environ-mental specifications.

In the following sections we will describe the differentelements shown in the block diagram. Each element hasbeen measured before integration in order to test its cor-rect performance. This process eliminates failures due tofaulty components in the early stages of the manufacturingprocess.

Page 2: [IEEE MILCOM 2005 - 2005 IEEE Military Communications Conference - Atlantic City, NJ, USA (17-20 Oct. 2005)] MILCOM 2005 - 2005 IEEE Military Communications Conference - Fast Electronically

TABLE IPROPOSED SPECIFICATIONS

gain 5 dB

band 6 - 18GHz

instantaneous BW 1GHz nominal

switching time < 200ns

selectivity 40dB

tuning voltage 0-5.5V

power consumption <10W

temperature -40◦C to 75◦C

dimensions 120 x 110 x 15mm

weight 290 gr.

Fig. 2. SP5T diode placement

A. SP5T switch

The whole frequency range has been divided into 5discrete bands in order to reduce the band that each filterhas to tune.

This switch is designed to provide an isolation betterthan 45dB, insertion loss better than 3dB, input matchbetter than 10dB and broadband operation.

The isolation required is achieved using a series/parallelconfiguration. The switch is built using 5mil Duroid sub-strate. Figure 2 shows the placement of the diodes inthe five branches. Using 3 diodes per branch, althoughincreases switch insertion losses, provides the requiredisolation. The loss obtained in the filter is better than 2.4dBat 18GHz, and input VSWR is better than 2:1. 3dB fixedattenuator is placed before the SP5T in the final assemblyto increase input match.

B. Tunable filter structure

The schematic adopted for the realization of the filterstructure consists in fixed filter + tunable filter + LNA +tunable filter, as seen in figure 3. This structure allowshigher selectivity and reduced noise figure than using ahigher order tunable filter followed by an amplifier, due tothe losses present in coupled line filters.

Fig. 3. Filter chain

Fig. 4. LNA assembly

The use of two filters helps reduce the order required toachieve the required rejection. The effects associated withelements tolerances require less adjustments than whendealing with high order filters.

Tunable filters consist in microstrip combline filtersloaded with a FET transistor in cold configuration used asvariable capacitor. These filters have been realized usingthe design process proposed in [6]. The use of fixed filterseliminates the first spurious response at 3f0, preventing theoverlapping of this spurious response with the passband ofupper frequency bands.

The LNA included in the filtering stages helps reducethe impact of filter losses in the system performance andprovides isolation between tunable passband filters.

This amplifier requires a negative voltage to providethe right current operation point. This voltage is providedthrough a negative regulator connected to a configurableresistor array. Each amplifier has its own array, so individ-ual adjustments are possible.

The filtering chain is shown in figure 5. This figureshows the combline filter assembly using 10mils Alumina.The figure shows the FET transistors attached to theresonators used to tune the response. By using these activeelements we provide a fast response with a tuning voltagerequirements as low as 0-5V.

Figure 6 shows the mechanical assembly of the five filterchains.

C. Active amplitude equalization

As this kind of filters show an increasing gain withtuning voltage [6], amplitude equalization is desired inorder to reduce the gain ripple inside the passband.

In this case the equalization is achieved using a commer-cial MMIC voltage controlled attenuator. This attenuator isdriven to provide up to 5dB extra attenuation in the upperfrequencies of the passband.

Page 3: [IEEE MILCOM 2005 - 2005 IEEE Military Communications Conference - Atlantic City, NJ, USA (17-20 Oct. 2005)] MILCOM 2005 - 2005 IEEE Military Communications Conference - Fast Electronically

Fig. 5. Combline filter

Fig. 6. Tunable filter assembly

D. Low frequency electronics

In order to reduce the space required for the operation ofthe module, control electronics have been built using sur-face mount components and monolithic ICs when possible.Control electronics are mainly placed in the back side ofthe module, reducing RF leakage through low frequencypaths.

The system has the following control signals: Tuningvoltage (analog), subband selection (TTL) and Powersupply (analog). These signals power the system and letthe user choose the frequency band desired. To adapt thosecontrol signals to the system, the following circuits havebeen designed:

• Switch driver using broadband components to allowhigh speed switching.

• Amplitude equalization.• Voltage regulators.Special care has been taken when designing these elec-

tronics to provide fast operation, mainly when driving theSP5T switch or tuning each subband. The SP5T driver

Fig. 7. Low frequency electronics

is seen in figure 7. This driver has a final dimension of13.1 x 8.5mm. The layout of the board has been optimizedin order to reduce the size and keep parasitics as low aspossible.

An extra output signal is generated in a temperaturesensor, included for monitoring purposes. This sensor andthe signal conditioning electronics provide a linear outputvoltage with temperature, ranging from 0.07V @ -40◦C(-40◦F) to 2.37V @75◦C (167◦F).

III. MANUFACTURABILITY

Manufacturability has been one of the key elements inthe design stage. This has required an extensive modelingof the tuning elements, tight control of the manufacturingprocess and the use of automatic assembly procedures.

Test JIGs are used to check each RF stage individuallybefore integration and the module contains enough adjust-ment points to provide the right operation point for eachindividual element, reducing the impact of component andproduction parameter dispersion.

This special care in the design process yield highrepeatability and reduced assembly and adjustment times,making the unit suitable to be produced in medium to largequantities.

To guarantee that the module is suitable for operationunder military portable and avionics conditions, every unitundergoes screening procedures and tests in accordancewith MIL-PRF-38534D and MIL-STD-883E.

The whole systems is tested using automated testbenches, guaranteeing that each unit accomplish the spec-ifications after manufacturing and screening tests. Thisautomated test reduces the time required to check itscorrect performance.

IV. MEASURED PERFORMANCE.

Figure 9 shows the bandpass response of the system.The comparison between the measured results of thefiltering chains outside the filter bank and once assembled

Page 4: [IEEE MILCOM 2005 - 2005 IEEE Military Communications Conference - Atlantic City, NJ, USA (17-20 Oct. 2005)] MILCOM 2005 - 2005 IEEE Military Communications Conference - Fast Electronically

Fig. 8. Tunable filter bank top view

Fig. 9. System response

shows no great differences mainly due to the good match-ing obtained in both SP5T. Table II shows a summary ofthe results obtained..

The use of the active amplitude equalization has re-duced the difference between the gain in the upper andthe lower passbands inside every filter chain. The gainvariation results in 7dB in the worst case, compared tothe 13dB shown in the filter chains. The five channelshave experienced a mean improvement of 4dB. An externalequalizer or variable gain amplifier may be used to flattenthe response, but not to degrade the speed of the system

TABLE IIOVERALL ELECTRICAL CHARACTERISTICS

Band Mean BW (MHz) Midband S21

6-9GHz 800 4

9-12GHz 800 5

12-15GHz 1000 5

15-18GHz 1100 7

Fig. 10. System response

these elements must provide a fast response.The speed of the module has been assessed under two

conditions:• The switching time between different bands is below

40ns• The tuning speed inside any given subband is below

150nsFigure 10 shows a typical response of the module when

tuned to a signal within a given subband. this figures givean overall switching time lower than 200ns in the case thata band and subband change is required.

V. CONCLUSION

The system designed achieves the objectives of provid-ing a tunable filtering structure with a fast response timeand reduced size.

The structure built acts as an efficient RF tunable filter-ing unit covering a band in excess of 6 to 18GHz, includingtwo independent transmission / reception channels.

Accordance to MIL screening procedures guaranteesreliability in portable and avionics applications.

The electrical and mechanical design allows quickassembly and adjustment, keeping the weight low. Theinterface provided and the hermeticity of the unit allowsan easy integration into portable and avionics radar/EWunits.

REFERENCES

[1] Elirsa Electronic Systems Ltd., http://www.mw-elisra.com/[2] Chelton Control Systems, http://www.cheltoncs.com[3] Filtronics, http://www.filcs.com[4] D. Porcino and W. Hirt, ”Ultra-Wideband Radio Technology: Poten-

tial and Challenges Ahead”, IEEE Communications Magazine , vol.41, pp. 66-74, Jul 2003.

[5] Ralph H. Halladay, Anthony M. Pavio, Steven D. Bingham andAnthony Kikel, ”A Monolithic Channelized Preselector for EWReceiver Applications”,Microwave Symposium Digest, 1988., IEEEMTT-S International, vol. 2, pp. 573-576, May 1988.

[6] German Torregrosa-Penalva, Gustavo Lopez-Risueno, Jose I. Alonso,”A Simple Method to Design Wideband Electronically TunableCombline Filters”,IEEE Transactions on Microwave Theory andTechniques , vol. 50, issue 1, pp. 172–177, Jan 2002.