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Industry News

A Low Profile Switch Filter Bank

May 17, 2004
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Switch filter banks or integrated filter assemblies have been primarily built as large high profile block type structures that consume a large portion of available design space. This has primarily been driven by the physical limitations of the RF filters, switches and control circuitry. The Lorch Microwave model 4IFA-1600/22-300-MP utilizes a host of new technology to provide the design engineer with a four channel integrated filter assembly that features a low profile mechanical outline. The switch bank contains four RF band pass filters and MMIC switches contained in a high reliability package built to operate over a standard mil-spec environment.


The mechanical configuration starts out with a low profile package that is 1.6" L x 1.6" W x 0.3" H. The package contains four mounting pads located on the outside corners with No. 2 helicoil inserts to assure a good RF ground. Figure 1 shows the filter bank’s complete mechanical outline. The RF and DC connections are realized by soldering 0.018 mil pins to a customer supplied RF trace. The gull wing pin configuration provides stress relief between the switch bank and circuit board without damage to either device during environmental conditions such as thermal shock and vibration. The housing is machined out of aluminum to maintain a lightweight package that can be laser welded for an airtight seal. Standard finish on this device is silver plating with other finishes such as nickel, gold, tin or chromate available.

Fig. 1 Mechanical outline.

The RF filter portion of the model 4IFA switch filter bank features four band pass filters. The filters of this particular unit are all 0.01 ripple Chebychev designs; however, other transfer functions such as Gaussian, Bessel, Butterworth, linear phase and elliptical are available. In order to meet the extremely low profile package requirements, high Q low profile capacitors are utilized with RF coils that have been optimized to provide maximum Q with a limited coil diameter and wire gauge. The specifications for all four channels are summarized in Table 1.

The switch circuitry consists of a low cost non-reflective SP4T switch in a 16-lead QSOP package. Two switches are used to complete the circuit with one located at the input and one at the output. The typical frequency range for RF switches is DC to 4 GHz. The RF switches provide a minimum of 30 dB isolation per switch that combines for greater than 60 dB total isolation input to output. Each switch has approximately 0.8 dB insertion loss. The combined total of the input and output switches and the filter loss provide the 7.5 dB maximum loss specified. The switches also operate with a +5 V supply that eliminates the need for any negative voltage that was common on older technology designs. The control logic consists of true TTL logic. A 2:4 decoder has been integrated on the switches requiring only two control lines to select each channel. The overall switching speed on this bank is 200 ns maximum, driven by the delay in the narrow band filter. Typical IP3 numbers are 40 dBm, with +20 dBm as the maximum input power. To integrate the switches and filters a standard FR-4 PC board has been designed to internally combine all the components and switches and make the necessary connections to the outside environment.

The model 4IFA-1600/22-300-MP low profile switch filter bank has been designed for military applications where space is at a premium. The unit meets all standard Mil-Std-202 environmental requirements such as random vibration, sine vibration, thermal shock and a standard operating temperature of –55° to +85°C. Figures 2, 3, 4 and 5 show the electrical performance of the final unit. Additional information and other specifications not shown may be obtained by contacting the company.

Fig. 2 Channel 1 performance.

Fig. 3 Channel 2 performance.

Fig. 4 Channel 3 performance.

Fig. 5 Channel 4 performance.

Conclusion

A low profile switch filter bank is now available that provides a surface-mount package with a 0.300 maximum height. The model 4IFA switch filter bank offers big performance in a small package.

Lorch Microwave,
Salisbury, MD
(410) 860-5100,
www.lorch.com.

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