Microwave Planar Varactor Tuned Bandpass Filters: Historical Overview
---
paper_title: Active GaAs MMIC band-pass filters with automatic frequency tuning and insertion loss control

paper_content:

This paper presents the design and measured performance of active tunable band-pass MMIC filters and matched MMIC control circuits, both fabricated using a standard 1-/spl mu/m GaAs MESFET foundry process. The control circuits generate filter frequency-tuning and Q-control bias voltages by using a dual-loop master-slave control scheme. Separate 3-section filters cover 1.5-2.0 and 1.96-2.64-GHz bands with 3-dB bandwidths of 86/spl plusmn/6 and 126/spl plusmn/1.0 MHz, respectively. The control circuits automatically maintain the filter insertion loss to within /spl plusmn/0.5 dB over these tuning ranges, and regulate the center frequency and insertion loss to within better than /spl plusmn/1.2 MHz and /spl plusmn/0.3 dB over a temperature range of -50/spl deg/C to +75/spl deg/C. >
---
paper_title: Some Design Considerations and Realizations of Iris-Coupled YIG-Tuned Filters in the 12-40 GHz Region

paper_content:

YIG filters covering the frequency ranges 12 to 18, 18 to 26, and 26 to 40 GHz have been designed and developed. The filters in the 18- to 40-GHz range were of three sections and achieved 80-MHz bandwidth and greater than 40 dB of higher order mode rejection. The filter in the 12- to 18-GHz region was of four sections utilizing staggered spheres in order to achieve a larger bandwidth filter with a minimum magnet gap. The bandwidth of this filter was 50 to 60 MHz with greater than 55 dB of higher order mode rejection. Useful information for the design of these filters was obtained with a special test fixture built to measure mutual coupling between spheres as a function of the angle between the line joining the sphere centers and the dc magnetic field. This data is reported for various iris sizes, shapes, and sphere spacings and indicates large differences in mutual coupling as the angle is varied from 0 to 90/spl deg/. The effect of various types of discontinuities and iris sizes and shapes on higher order mode excitation and coupling are discussed as well as constructional problems and the design of high field magnets from vanadium permendur.
---
paper_title: High-Q tuned active bandpass filter for wireless application

paper_content:

The paper presents a novel high-Q microwave tuned active bandpass filter using varactor diodes for tuning and a BJT to compensate for losses. The experimental filter with stepped resonator circuits and special coupling elements have been fabricated and tested demonstrating a good agreement with the circuit model predictions. Tuning range of 400 MHz for the active filter is achieved with the centre frequency of 1.6 GHz. A 3 dB bandwidth of 50 MHz is obtained and passband gain is typically 0.5-1.0 dB in the whole tuning range.
---
paper_title: Tunable active bandpass filters using three-terminal MESFET varactors

paper_content:

The concept of three-terminal MESFET varactors is presented. Based on this concept, two types of tunable active bandpass filters working at the X-band were developed. The first circuit used two MESFETs, of which one was used as a MESFET varactor and the other was used to generate a negative resistance. The second circuit used only one MESFET, which had both the functions of the two MESFETs in the first circuit. The experimental results showed the possibility of using the MESFET in the active mode to generate both a negative resistance and a variable reactance. >
---
paper_title: A varactor-tuned active microwave bandpass filter

paper_content:

Traditionally, microwave filters have been implemented as passive networks of waveguide, transmission-line, or discrete lumped elements. More recently, microwave active filters have also been realized. A microwave high Q-active bandpass filter is presented. using a varactor diode for tuning, and an active component giving a substantial increase in Q of the resonant sections of filter. High-Q filter elements offer low-filter insertion loss and also yield sharp passband corners, which cannot be achieved by lossy filter and amplifier combination. >
---
paper_title: Improvements in losses and size of frequency tunable coplanar filter structures using MMIC negative resistance chips for multistandard mobile communication systems

paper_content:

A new band-pass filter design technique is presented that uses coplanar passive interdigitated filters assisted by an MMIC active component which performs a negative resistance with an associated imaginary part in order to compensate for the losses of the passive filter and to reduce significantly the length of the resonators. A tunable version of the filter is also presented.
---
paper_title: Microwave active filters based on coupled negative resistance method

paper_content:

A novel coupled negative resistance method for building a microwave active bandpass filter is introduced. Based on this method, four microstrip line end-coupled filters were built. Two are fixed-frequency one-pole and two-pole filters, and two are tunable one-pole and two-pole filters. In order to broaden the bandwidth of the end-coupled filter, a modified end-coupled structure is proposed. Using the modified structure, an active filter with a bandwidth up to 7.5% was built. All of the filters show significant passband performance improvement. Specifically, the passband bandwidth was broadened by a factor of 5 to 20. >
---
paper_title: Varactor-tuned active bandpass filter with low noise performance

paper_content:

An active tunable bandpass filter based upon the negative resistance method using a microstrip combline network is presented. The negative resistance is realised using a common-drain series feedback structure. 0.4 octave tuning with significant improvement in passband performance was achieved and the noise performance was comparable with its passive counterpart.
---
paper_title: Tuning a bandpass filter by optical control of a negative-resistance circuit

paper_content:

A novel tunable active bandpass filter is presented in X-band. Basically, this is an end-coupled half-wavelength microstrip bandpass filter coupled by a high-Q value negative-resistance circuit, which is optically tunable. In this filter, one MESFET works both as a tuning element and negative resistance. To increase the tunability of the active MESFET circuit compared with the half-wavelength resonator, the port of the negative resistance is connected to the open stub through one side line in the quarter-wavelength coupler. Tuning range of 123 MHz is achieved by optical tuning. This optically tunable range of the active bandpass filter exceeds 1% of the center frequency.
---
paper_title: Active MMIC filters with negative resistance compensation

paper_content:

Two tunable filters are presented for 3.8 and 3.4 GHz, based on active negative resistance loss compensation. The emphasis has been placed on the noise performance of the filters. The second and fourth-order filters have noise figures of 19 and 11 dB, respectively, being near the calculated theoretical minimum.
---
paper_title: MMIC tunable bandpass filter using a ring resonator with loss compensation

paper_content:

Ring resonators incorporating negative resistance circuits for loss compensation are studied for use as high Q tunable MMIC resonators. The design of a 13 GHz MMIC bandpass filter together with a background theoretical analysis, are presented. Simulation results indicate unloaded Q-factors of 1000 over 1 GHz tuning bandwidth.
---
paper_title: Magnetically tunable rectangular waveguide E-plane integrated circuit filters

paper_content:

A rigorous field theory method is described for the computer-aided design (CAD) of magnetically tunable E-plane metal-insert filters, where the waveguide sections are symmetrically loaded with ferrite slabs, and for large-gap finlines filters on a ferrite substrate. The design method is based on field expansion in suitably normalized eigenmodes which yields directly the modal scattering matrix of all discontinuities. The theory includes both higher-order mode interaction and the finite thickness of the metal inserts. Optimized data are given for magnetically tunable Ku-band metal insert and finline filter examples. The metal-insert type achieves a tuning range of its operating midband from about 14.1 to 15.7 GHz. The theory is verified by measurements. >
---
paper_title: Coplanar MMIC active bandpass filters using negative resistance circuits

paper_content:

A second order active bandpass filter for a fixed centre frequency and a tunable one have been developed in a coplanar line technique using negative resistance circuits. These filters operate in the 1.8 GHz band and have a 3 dB bandwidth of 110 MHz. The tuning range of the centre frequency is 200 MHz. Each filter is fabricated on a GaAs substrate and occupies an area of 1.5/spl times/1.6 mm/sup 2/ on the wafer including all biasing elements.<<ETX>>
---
paper_title: MMIC active bandpass filters using varactor-tuned negative resistance elements

paper_content:

This paper describes techniques for realizing microwave active filters using single transistor active resonators in a negative resistance configuration, The negative resistance topologies for both bipolar (AlGaAs/GaAs HBT) and FET (MESFET or HEMT) devices are studied and compared. The essence of the technique is that the input reactance of the transistor circuit resonates with an external capacitor or inductor, whilst the negative resistance is used to compensate for the losses in the resonator. It is shown that the FET device is ideally suited for this application as it can have a varactor-controlled negative resistance component. Three-stage and two-stage monolithic varactor-tuned bandpass filters have been demonstrated using this technique. The measured response of the three-stage filter exhibits a 120 MHz 3 dB-bandwidth centered on 2.3 GHz, 0 dB insertion loss with only /spl plusmn/0.1 dB ripple in the pass-band, up to 100 dB of stop-band attenuation at low frequencies, and over 50 dB of rejection up to 6 GHz. The two-stage filter exhibits a 400 MHz 3 dB-bandwidth centered on 4.7 GHz, with tunable insertion gain and only /spl plusmn/0.1 dB ripple in the pass-band.
---
paper_title: Magnetically tunable oscillators and filters

paper_content:

Magnetically tunable filters and oscillators can be built throughout the 500-MHz-110-GHz range using a number of ferrite materials and varied geometries. The authors survey current trends and state-of-the-art results for oscillators and filters using YIG spheres/sub ,/ YIG films, and hexagonal ferrites as the magnetically tunable elements. >
---
paper_title: Active varactor tunable bandpass filter

paper_content:

A microstrip active varactor tunable bandpass filter is presented. The filter is based upon the negative resistance method using a microstrip combline network. A 1.8-GHz, two-pole device with a bandwidth of 2% was constructed. 0.6 octave tuning with significant improvement in passband performance was achieved compared with its passive counterpart. >
---
paper_title: Stepped-Ferrite Tunable Evanescent Filters

paper_content:

A new technique is described for the design of magnetically tunable filters. With this approach, the resonant sections tune at the same rate enabling maintenance of response shape as center frequency is varied. The filters are minimum-phase structures, realizable with as many poles as required. The design is capable of handling much greater power than single-crystal YIG designs, although the tuning bandwidth is much less.
---
paper_title: Magnetically tunable multisection bandpass filters in ferrite-loaded evanescent waveguide

paper_content:

A description is given of a novel type of bandpass filter constructed in ferrite-loaded waveguide and using the evanescent-mode principle of operation. These filters may be multisection, and their operating band of frequencies may be tuned by a single-knob control of the d.c. magnetic field on the ferrite. Performance of a 3-section bandpass filter is given.
---
paper_title: A new varactor-tuned microstrip ring bandpass filter with harmonic suppression

paper_content:

A new bandpass filter that is composed of two stage microstrip ring resonators with the center frequency of 5.775 GHz and the bandwidth of 100 MHz has been presented in this paper. For the suppression of unnecessary harmonics, lowpass filters are integrated into a microstrip ring resonator. Harmonic suppression ratios of about 35 dB are obtained. Also, a varactor-tuned microstrip ring bandpass filter with harmonic suppression is studied. Insertion loss of 6 dB and tuning bandwidth of 450 MHz are obtained for this circuit.
---
paper_title: Varactor-tuned combline bandpass filter using step-impedance microstrip lines

paper_content:

In this paper, a varactor-tuned combline bandpass filter using step-impedance microstrip lines is considered so that the absolute passband bandwidth can be maintained nearly constant within the tuning range. The difference between the odd- and even-mode characteristics of the coupled microstrip line makes it difficult to design a tunable bandpass filter with minimum degradation in passband performance. By using step-impedance microstrip lines, couplings between resonators can be controlled so that the constant bandwidth requirement could be satisfied with reasonable design parameter values. Lumped inductors are used for input and output coupling networks. Design equations are derived, and experimental results are compared with theoretical ones based on these equations.
---
paper_title: Varactor tuned microstrip ring resonators

paper_content:

Varactor tuned microstrip ring resonators have been developed with more than 30 percent tuning bandwidth using packaged hyperabrupt junction varactor diodes. The experimental tuning bandwidth was found to be in very close agreement with the predicted value obtained using analysis based on transmission line modelling. The attractiveness of the approach presented lies in its simplicity, the tuned resonator could find applications in microwave oscillator circuits and frequency agile communication systems.
---
paper_title: A design of tunable UWB filters

paper_content:

A novel design method is proposed for UWB filters using a ring resonator structure with a stub. A five stage UWB filter with 86.6% relative bandwidth is fabricated based on a design method which shows an excellent performance, i.e. low insertion loss of 0.58 dB and flat group delay of 1.6 nsec. We also extend the design method to the one for a wide range tunable filter. Simulated results show that more than 40% tunable range can be obtained with a single capacitor for each ring resonator.
---
paper_title: Electronically tunable and switchable filters using microstrip ring resonator circuits

paper_content:

Two microstrip ring resonator circuits are described: an electrically switchable filter and a tunable switchable filter, both using p-i-n diodes. In the first circuit, a microstrip ring resonator loaded with two p-i-n diodes is developed as a switchable filter. By replacing one p-i-n diode with a varactor diode, the switchable filter is made electronically tunable. Over 20-dB isolation with 9% tuning bandwidth has been demonstrated. Equivalent circuits have been developed for both circuits. The analysis was based on the transmission-line model including the effects of diode parasitics, coupling gaps, dispersion, and mounting gap capacitance. The experimental results agree very well with the theoretical calculation. >
---
paper_title: Electronically Tunable Microwave Bandpass Filters

paper_content:

Combline filters with novel input and output coupling networks which enable broad-band tuning to be achieved with minimum degradation in passband performance are discussed. Explicit design formulas for these filters are presented. Computer analysis of varactor tuned combine bandpass filters including the small signal varactor equivalent circuit is presented enabling filter performance to be easily evaluated. The design and experimental performance of a varactor tuned combline filter, realized in suspended substrate stripline is described. This filter toned from 3.2 GHz to 4.9 GHz exhibited low passband insertion loss and its performance was is close agreement with theoretical expectations.
---
paper_title: Varactor Tuned Bandpass Filters Using Microstrip-Line Ring Resonators

paper_content:

This paper describes the fundamental characteristics and experimental results of newly developed tunable bandpass filters using rnicrostrip-line ring resonators. The experimental filter has a steeper attenuation slope and less circuit instability than conventional filters, and it seems to be a suitable filter for MICs or MMICs.
---
paper_title: Electronically tunable ring resonator microstrip and suspended-substrate filters

paper_content:

Electronically tunable ring resonator microstrip and suspended substrate microwave filters with high tuning speed are presented. The filters are realised using two coupled ring resonators with GaAs varactors as the tuning elements. Computer optimised circuits are used at the input and the output to obtain better than 10 dB return loss across the tuning band. Tuning ranges of 1.075-2.25 GHz for the microstrip filter and 1.35-2.075 GHz for the narrower suspended substrate filter are predicted by computer simulation. >
---
paper_title: An electronically tunable microstrip bandpass filter using thin-film Barium-Strontium-Titanate (BST) varactors

paper_content:

A tunable third-order combline bandpass filter using thin-film barium-strontium-titanate varactors and fabricated on a sapphire substrate is reported. Application of 0-200-V bias varied the center frequency of the filter from 2.44 to 2.88 GHz (16% tuning) while achieving a 1-dB bandwidth of 400 MHz. The insertion loss varied from 5.1 dB at zero bias to 3.3 dB at full bias, while the return loss exceeded 13 dB over the range. The third-order intercept of the filter was found to be 41 dBm.
---
paper_title: Figure of merit of tunable ferroelectric planar filters

paper_content:

Different tunable microstrip resonators are suggested to be usedin tunablemicrowave filters. Ferroelectric capacitors are chosenas the tunable components of theresonators A novelcriterion for description oftheperformance ofthetunable filters is suggested.
---
paper_title: Tunable combline filter with continuous control of center frequency and bandwidth

paper_content:

A new combline filter structure with a continuous tunability for both the center frequency and bandwidth is presented in this paper. The passband-width tunability is achieved by placing variable coupling reducers between the filter resonators. The coupling reducers, operating as bandwidth control subnetworks, are designed as detuned resonators made up of a line segment ending in a variable capacitor. The proposed filter structure is experimentally validated with the design, construction in suspended stripline technology, and characterization of a low-cost filter prototype for terrestrial digital video broadcasting receivers operating in the UHF band (470-862 MHz). Other relevant factors, such as the intermodulation distortion produced by the varactors used to control the bandwidth electronically or the power-handling performance of the constructed filter, are also discussed. The reconfigurable filter module described in this paper is very suitable for the design of flexible multifunction receiver subsystems simultaneously supporting signals with a different bandwidth.
---
paper_title: Varactor-tuned hairpin bandpass filter with an attenuation pole

paper_content:

In this paper, a varactor-tuned hairpin bandpass filter using a tapped open stub is studied. In this configuration the filter's center frequency as well as the bandwidth can be controlled. The tapped open stub introduces the attenuation pole which can be located near the passband by adjusting the stub length. Varactors loaded at the end of hairpin resonator and the tapped open stub enable us to control the center frequency with constant bandwidth as well as to have a large tuning bandwidth. Based on the detailed analysis, the varactor-tuned hairpin bandpass filter is designed, in which the tuning bandwidth reaches 15 % of the center frequency at 2 GHz.
---
paper_title: A broadband tunable combline filter using active devices

paper_content:

Electronically tunable microwave filters with fast tuning speed can be realized using varactor tuned combline microstrip structures. Even when GaAs varactors are employed, the resonator Q is limited by the varactor losses and the filter bandwidth has to be more than 8-10%. The authors present a solution to the problem. The varactor losses are compensated by coupling a negative resistor in the tuned circuit. This resistor is realized using an active device. Computer simulation results for a filter with 3% bandwidth are presented. Performance results with and without the negative resistance are presented to bring out improvement due to negative resistance. >
---
paper_title: Design of tunable ferroelectric filters with a constant fractional band width

paper_content:

The main principle of design of tunable ferroelectric filters with a constant fractional band width in the tuning range are discussed. Lumped ferroelectric capacitors are used as the tunable components of the microstrip filter. The limitation of the filter performance is determined by the commutation quality factor of the ferroelectric varactor. The influence of the loss factor of the ferroelectric capacitor is analyzed. The 3-pole 1% fractional band pass filter with a 3 band width tuning range without a degradation of the characteristics is designed.
---
paper_title: Tunable planar combline filter with multiple source/load coupling

paper_content:

In this paper a topology of planar combline filter that provide additional transmission zeros is present. The filter uses multiple coupling between source/load and the resonators to generate up to three transmission zeros. The coupling takes place by means of coupled lines of the source and the load to the resonators. In order to demonstrate the viability of topology a microstrip filter has been constructed. This very selective structure provides responses for the adjacent channels reject in systems that analog and digital channels shared (DVB and analog TV).
---
paper_title: Theoretical and experimental investigation of novel varactor-tuned switchable microstrip ring resonator circuits

paper_content:

A microstrip ring resonator circuit loaded with two p-i-n diodes has been developed for use as a switchable filter. By replacing one p-i-n diode with a varactor diode, the switchable filter can be made electronically tunable. Isolation exceeding 20 dB with 9% tuning bandwidth was demonstrated. An analysis based on transmission line theory was used to model both circuits. The analysis includes the effects of diode parasitics, coupling gaps, dispersion, and mounting-gap capacitance. The experimental results agree very well with the theoretical calculation. >
---
paper_title: A simple method to design wide-band electronically tunable combline filters

paper_content:

A new systematic approach for designing wide- band tunable combline filters is presented. New results on tunable combline filter theory are proposed and explicit de- sign formulas, to obtain the filter design parameters from specifications, are included. These design parameters are: center frequency, resonator electrical length, instantaneous bandwidth and tuning capacitance. The proposed design technique is used to construct an X-band wideband mi- crostrip tunable filter from 8.0 GHz to 12.0 GHz with com- mercial GaAs FETs as tuning elements. Parasitic effects and simulation problems are also discussed.
---
paper_title: Narrow-band, fixed-tuned, and tunable bandpass filters with zig-zag hairpin-comb resonators

paper_content:

"Hairpin-comb" filters have been previously shown to have special properties that are advantageous for the design of compact, narrow-band, and bandpass microstrip filters. Herein, a new "zig-zag" form of hairpin-comb filter is introduced, which is shown to have additional important advantages for designing compact narrow-band filters. Examples with computed responses and the measured results from high-temperature superconductor trial designs are presented. The considerable flexibility available in the design of bandpass filters of this sort is shown to be helpful in the design of tunable bandpass filters having nearly constant bandwidth and passband shape as they are tuned. Measured results for tuning over nearly an octave range are presented.
---
paper_title: Active GaAs MMIC band-pass filters with automatic frequency tuning and insertion loss control

paper_content:

This paper presents the design and measured performance of active tunable band-pass MMIC filters and matched MMIC control circuits, both fabricated using a standard 1-/spl mu/m GaAs MESFET foundry process. The control circuits generate filter frequency-tuning and Q-control bias voltages by using a dual-loop master-slave control scheme. Separate 3-section filters cover 1.5-2.0 and 1.96-2.64-GHz bands with 3-dB bandwidths of 86/spl plusmn/6 and 126/spl plusmn/1.0 MHz, respectively. The control circuits automatically maintain the filter insertion loss to within /spl plusmn/0.5 dB over these tuning ranges, and regulate the center frequency and insertion loss to within better than /spl plusmn/1.2 MHz and /spl plusmn/0.3 dB over a temperature range of -50/spl deg/C to +75/spl deg/C. >
---
paper_title: High-Q tuned active bandpass filter for wireless application

paper_content:

The paper presents a novel high-Q microwave tuned active bandpass filter using varactor diodes for tuning and a BJT to compensate for losses. The experimental filter with stepped resonator circuits and special coupling elements have been fabricated and tested demonstrating a good agreement with the circuit model predictions. Tuning range of 400 MHz for the active filter is achieved with the centre frequency of 1.6 GHz. A 3 dB bandwidth of 50 MHz is obtained and passband gain is typically 0.5-1.0 dB in the whole tuning range.
---
paper_title: Tunable active bandpass filters using three-terminal MESFET varactors

paper_content:

The concept of three-terminal MESFET varactors is presented. Based on this concept, two types of tunable active bandpass filters working at the X-band were developed. The first circuit used two MESFETs, of which one was used as a MESFET varactor and the other was used to generate a negative resistance. The second circuit used only one MESFET, which had both the functions of the two MESFETs in the first circuit. The experimental results showed the possibility of using the MESFET in the active mode to generate both a negative resistance and a variable reactance. >
---
paper_title: A varactor-tuned active microwave bandpass filter

paper_content:

Traditionally, microwave filters have been implemented as passive networks of waveguide, transmission-line, or discrete lumped elements. More recently, microwave active filters have also been realized. A microwave high Q-active bandpass filter is presented. using a varactor diode for tuning, and an active component giving a substantial increase in Q of the resonant sections of filter. High-Q filter elements offer low-filter insertion loss and also yield sharp passband corners, which cannot be achieved by lossy filter and amplifier combination. >
---
paper_title: Improvements in losses and size of frequency tunable coplanar filter structures using MMIC negative resistance chips for multistandard mobile communication systems

paper_content:

A new band-pass filter design technique is presented that uses coplanar passive interdigitated filters assisted by an MMIC active component which performs a negative resistance with an associated imaginary part in order to compensate for the losses of the passive filter and to reduce significantly the length of the resonators. A tunable version of the filter is also presented.
---
paper_title: Microwave active filters based on coupled negative resistance method

paper_content:

A novel coupled negative resistance method for building a microwave active bandpass filter is introduced. Based on this method, four microstrip line end-coupled filters were built. Two are fixed-frequency one-pole and two-pole filters, and two are tunable one-pole and two-pole filters. In order to broaden the bandwidth of the end-coupled filter, a modified end-coupled structure is proposed. Using the modified structure, an active filter with a bandwidth up to 7.5% was built. All of the filters show significant passband performance improvement. Specifically, the passband bandwidth was broadened by a factor of 5 to 20. >
---
paper_title: Varactor-tuned active bandpass filter with low noise performance

paper_content:

An active tunable bandpass filter based upon the negative resistance method using a microstrip combline network is presented. The negative resistance is realised using a common-drain series feedback structure. 0.4 octave tuning with significant improvement in passband performance was achieved and the noise performance was comparable with its passive counterpart.
---
paper_title: Tuning a bandpass filter by optical control of a negative-resistance circuit

paper_content:

A novel tunable active bandpass filter is presented in X-band. Basically, this is an end-coupled half-wavelength microstrip bandpass filter coupled by a high-Q value negative-resistance circuit, which is optically tunable. In this filter, one MESFET works both as a tuning element and negative resistance. To increase the tunability of the active MESFET circuit compared with the half-wavelength resonator, the port of the negative resistance is connected to the open stub through one side line in the quarter-wavelength coupler. Tuning range of 123 MHz is achieved by optical tuning. This optically tunable range of the active bandpass filter exceeds 1% of the center frequency.
---
paper_title: Active MMIC filters with negative resistance compensation

paper_content:

Two tunable filters are presented for 3.8 and 3.4 GHz, based on active negative resistance loss compensation. The emphasis has been placed on the noise performance of the filters. The second and fourth-order filters have noise figures of 19 and 11 dB, respectively, being near the calculated theoretical minimum.
---
paper_title: MMIC tunable bandpass filter using a ring resonator with loss compensation

paper_content:

Ring resonators incorporating negative resistance circuits for loss compensation are studied for use as high Q tunable MMIC resonators. The design of a 13 GHz MMIC bandpass filter together with a background theoretical analysis, are presented. Simulation results indicate unloaded Q-factors of 1000 over 1 GHz tuning bandwidth.
---
paper_title: Coplanar MMIC active bandpass filters using negative resistance circuits

paper_content:

A second order active bandpass filter for a fixed centre frequency and a tunable one have been developed in a coplanar line technique using negative resistance circuits. These filters operate in the 1.8 GHz band and have a 3 dB bandwidth of 110 MHz. The tuning range of the centre frequency is 200 MHz. Each filter is fabricated on a GaAs substrate and occupies an area of 1.5/spl times/1.6 mm/sup 2/ on the wafer including all biasing elements.<<ETX>>
---
paper_title: MMIC active bandpass filters using varactor-tuned negative resistance elements

paper_content:

This paper describes techniques for realizing microwave active filters using single transistor active resonators in a negative resistance configuration, The negative resistance topologies for both bipolar (AlGaAs/GaAs HBT) and FET (MESFET or HEMT) devices are studied and compared. The essence of the technique is that the input reactance of the transistor circuit resonates with an external capacitor or inductor, whilst the negative resistance is used to compensate for the losses in the resonator. It is shown that the FET device is ideally suited for this application as it can have a varactor-controlled negative resistance component. Three-stage and two-stage monolithic varactor-tuned bandpass filters have been demonstrated using this technique. The measured response of the three-stage filter exhibits a 120 MHz 3 dB-bandwidth centered on 2.3 GHz, 0 dB insertion loss with only /spl plusmn/0.1 dB ripple in the pass-band, up to 100 dB of stop-band attenuation at low frequencies, and over 50 dB of rejection up to 6 GHz. The two-stage filter exhibits a 400 MHz 3 dB-bandwidth centered on 4.7 GHz, with tunable insertion gain and only /spl plusmn/0.1 dB ripple in the pass-band.
---
paper_title: Active varactor tunable bandpass filter

paper_content:

A microstrip active varactor tunable bandpass filter is presented. The filter is based upon the negative resistance method using a microstrip combline network. A 1.8-GHz, two-pole device with a bandwidth of 2% was constructed. 0.6 octave tuning with significant improvement in passband performance was achieved compared with its passive counterpart. >
---

Title: Microwave Planar Varactor Tuned Bandpass Filters: Historical Overview

Section 1: Introduction
Description 1: Introduce the necessity for reconfigurable RF bandpass filters in next-generation mobile communication systems and provide an overview of varactor tuned bandpass filters, including their configurations and challenges.

Section 2: Design of passive tunable bandpass filters
Description 2: Discuss the characteristics and design principles of typical passive tunable filters employing various configurations like ring resonators, combline, interdigital, and hairpin structures.

Section 3: Design of active tunable filters
Description 3: Outline the principles and configurations of active tunable filters, focusing on compensating for insertion losses using negative resistance and highlighting the stability and noise considerations.

Section 4: Conclusions
Description 4: Summarize the key points discussed in the paper, emphasizing the importance of reconfigurability, miniaturization, and high performance in tunable bandpass filters for mobile communication systems.