The Evolution of MMIC Filters for Modern Wideband Receivers

Switchable filter networks and tunable filters have been integral system functions in wideband RF & Microwave systems for decades. As the spectrum utilization continues to grow across countless applications, including EW, UAVs, radar, tactical radio, cellular communications, Wi-Fi, IoT and many others, the need for compact microwave filters to provide frequency selectivity and interference control continues to expand.

Traditional filter construction techniques have relied on waveguide and coaxial cavity mediums or suspended substrates to achieve low insertion loss. PIN diode switches have been popular for switched filter banks due to their desirable properties for achieving low loss and high linearity. Although the best performance can often be achieved employing these conventional methods, some clear drawbacks include larger size, weight, high cost and complex control circuitry, 

MMIC devices have proliferated wideband transceiver chains over the last several decades with enormous benefits to systems designers with high-performance functionality from MMIC Amplifiers, frequency converters, Switches, attenuators and several other functions provided enhanced performance, smaller sizes and simplified construction techniques with surface mount packaging now available to beyond 40 GHz. On the other hand, MMIC Filters have experienced much slower adoption in modern transceivers due to concerns about the realizable performance of MMIC filters when compared to their hybrid counterparts such as cavity filters, printed circuits filters, SAW filters and lumped element types. 

The previously mentioned assumption of MMIC filter limitations has been challenged by several research papers and new products entering the marketplace with interesting performance characteristics along with size, cost and flexibility advantages. To help to illustrate the state of art of MMIC filters, here we present several examples of actual MMIC Filters including MMIC switched filter banks, a representative tunable band pass filter, and MMIC low pass filters with adjustable cutoff frequencies.

MMIC-based Wideband Tunable Receivers

Switched filter networks are a crucial function of wideband EW receivers with the essential performance characteristics of frequency selectivity, channel isolation, insertion loss, power handling and high dynamic range. The combination of the switch and filter loss must not limit the required receiver sensitivity. The presence of high-level interference must not desensitize the receiver’s ability to lock onto the desired signal. 

Most of the circuit functions have been readily available as COTS from leading MMIC suppliers for the past few decades apart from the filters until recently. The constraints imposed on equipment size, manufacturing techniques and costs spurred renewed interest in exploring the MMIC technology as a practical method to realize the switchable filters. The performance improvements of MMICs including amplifier dynamic range and MMIC switch parameters, more favourable trade-offs for implementation of MMIC filters have emerged.  Some of the MMIC filter trends emerging into the marketplace include switchable MMIC filters, tunable MMIC filters and switchable MMIC tuned filters. Each of these filter topologies are explored below.

MMIC-Based Switchable Filter Bank

MMIC-based Switchable filters with configuration in the figures below have been designed and are now available in the marketplace.  A commonly used topology for implementing bandpass filters in the MMIC medium is the cascade on a lowpass filter with a highpass filter. The popularity of this configuration is based on the ability to fabricate high-quality MIM Capacitors with small size, tight tolerances and a wide range of capacitance values. Capacitors can be implemented using semiconductor junctions and enable tunability. Several options are available in the MMIC medium for implementing inductors including spiral inductors and electrically short transmission lines. Another significant benefit of the MMIC medium is the ability to implement multiple-throw switches and this helps to reduce interconnect losses while blending in the transition design between the two components. 

While the MMIC implementation of the filter will have higher insertion loss than the conventional construction medium such as printed circuit filters and cavity filters, modern MMIC offers excellent options to circumvent this limitation. As an example, consider adding a bypass LNA to the front end of the receiver to obtain lower NF for the weak signal condition.

Some of the important considerations for specifying a switchable MMIC filter are summarized in the table below.

The performance for an actual 2 to 18 GHz switchable filter ATEK656N5 is shown in the figure below. The filter design consists of 6 filters with sub-octave overlapping bandwidths covering the 2 to 18 GHz band. The performance characteristics include the input and output SP6T switches all integrated into a single MMIC chip housed in a 5 x 5mm SMT package. The design achieves excellent linearity of 33 dBm input IP3 making it compatible with state-of-the-art front-end MMIC LNAs.

ATEK656N5 Insertion Loss

A common application for wideband tunable filters arises from the issue of harmonics and sub-harmonic generation in commercially available PLL-VCO synthesizer products. Commercially available products that typically cover bands from 100 to 8000 MHz often produce harmonics and subharmonics as high as 15 dBc below the desired synthesized carrier and this cannot be tolerated in many systems. A switchable bank can be used to suppress the undesired harmonics while conveniently selecting a pertinent filter associated with the desired carrier frequency.

An example of a commercially available switchable filter bank that can accomplish the desired task is shown in Figure 3. The MMIC filter consists of 7 distinct bandpass filters on a single MMIC chip with an input and output SP7T Switch for selection purposes. Each filter passband is less than an octave bandwidth thereby enhancing the ability to reject 2^nd order harmonic and providing an additional benefit of IP2 enhancement. The typical insertion loss is 7 dB and this includes the loss of the inputs and output switches.

MMIC Based Tunable Filters

The ability to adjust the center frequency of a bandpass filter or the cutoff frequency of a low-pass and high-pass filter enables practical flexibility in implementing microwave systems designs. The ability to adjust filtering for different interference environments in wireless systems or reconfigure filter stopbands to account for unforeseen spurious signals from nonlinearities or to simplify product family variants by adjusting channel frequency can save considerable time and reduce costly design cycles. Practical MMIC tunable filters are becoming available in the marketplace and this trend is expected to expand as more industry practitioners and researchers take notice of the progress.

The ability to implement precision series and shunt MIM and semiconductor junction capacitors in compact form makes capacitor-based lumped element high-pass and low-pass filter attractive topologies for MMIC designers. Bandpass filters can be configured by cascading low-pass and high-pass filters. Center frequencies and cutoff frequencies can be adjusted by voltage control of capacitive semiconductor junction.  

The frequency response of a 1 to 3 GHz tunable bandpass filter is shown in the below figure.

An example of a digitally tunable low pass filter is shown in the figure below. The 4-bit control interface allows for the filter cutoff frequency adjustment from 0.35 to 1.7 GHz. The low-pass filter insertion loss is 2 dB and 40 dB stop band rejection. The filter is housed in a 4 x 4 mm with 50-ohm matched ports. The MMIC low-pass filter has a very good linearity with 40 dBm input IP3 making it practical for a wide variety of applications.

The MMIC tunable low-pass filter can be extended in frequency with adjustable cutoff frequencies well over an octave. The MMIC low pass filter shown in the figure has a passband adjustable from 1 to 2.5 GHz configurable using a 4-bit digital interface. The filter delivers 50 dB of rejection beyond 8 GHz and has excellent linearity with 45 dBm IP3.

ATEK offers several MMIC filter configurations

• Switchable filterbanks
• Digitally controlled tunable filters
• Analog-controlled tunable filters
• Custom designs. To address customer-specific frequency plans and multi-function MMIC integration preferences including the addition of amplifiers and control functions.

To address the challenging SWaP requirements, higher integration can be achieved by including amplifier blocks to switchable filterbanks. ATEK671 is a switchable filter bank from 2 to 18 GHz with 5 filters. Each band pass filter is cascaded with an LNA to improve the NF, and an additional selectable path includes a wideband 2-18 GHz LNA without a filter. All these functions are realized in less than 12 x 12 mm area.

ATEK671 Block Diagram

2-18 GHz Switchable Filterbank and LNA bank, Insertion Loss Response

Some filters from ATEK are listed below.

ATEK also offers products used in conjunction with filters to realize wideband system designs. An abbreviation of MMIC amplifiers, switches and attenuators is given below.

2 - 18 GHz Switch Tuned Receiver using MMIC Filters

8 GHz Wideband Switch Tuned Receiver using MMIC Filters