Performing measurements on a signal of interest generally involves being able to extract that signal from those around it, and this task requires filtering. For most engineers, adding a filter to a test setup first requires scouring the work area for an available filter and then trying to locate the documentation that identifies that particular filter's electrical characteristics (and hoping it has a wide-enough passband at the correct center frequency). For the lucky ones, there is the MLBF-series of tunable filters from Micro Lambda Wireless, which even look like test instruments. Leveraging the company's yttrium-iron-garnet (YIG) resonator technology, these bandpass and bandreject filters are available at frequencies through 50 gHz and 20 gHz, respectively, in a compact housing measuring 4 x 10 x 13 in. and weighing just 9 lbs. The housing includes a small display screen and tuning controls, along with ethernet and universal serial Bus (usB 2.0) interfaces for connection to a programmable automatic-test-equipment (ATE) setup. The two types of filters are available in enough different frequency ranges and performance levels to suit most needs.

The MLBF-series filters are smartly packaged in instrument-style cases that fit well into laboratory or general benchtop applications. The series consists of bandpass versions from 500 MHz to 50 GHz and band-reject models from 500 MHz to 20 GHz. These compact cases include one of the company's Yigtuned filters, a power supply, cooling fan, two-line, 16-digit display, alphanumeric keypad, rotary control, input and output connectors, and digital bus ports. Each filter includes ethernet and universal serial Bus (USB) interface ports to simplify connections to an ATE system. The display shows the current center-frequency setting on the top line, with a cursor that is positioned by the left and right arrow keys. When a new frequency is entered via the keypad, the numbers are shown on the second line of the filter's display, and the new setting is activated by pressing the MHz key on the keypad.

This is an extensive line of filters, with bandpass models available as narrow as 1 to 2 gHz and as wide as 3 to 50 GHz and band-stop filters as narrow as 0.5 to 2.0 GHz and as wide as 2 to 20 GHz. Bandpass filters are constructed with four, six, and seven stages, with tunable 3-dB bandwidths that range from as narrow as 15 MHz in the four-stage 0.5-to-4.0-GHz model MLBFP-40540 filter to 500 MHz in several six- and seven-stage 18- and 20-GHz filters. The insertion loss of these bandpass filters is in the range of 6.2 to 9.2 dB, depending upon the frequency range and number of stages.

The band-stop filters are designed with 10, 12, and 16 stages, with 3-dB notch bandwidths ranging from 100 MHz at tunable frequencies from 0.5 to 2.0 GHz to 150 MHz in many of the higher-frequency band-stop filters. the insertion loss of each of these bandstop filters ranges from 2.7 to 3.2 dB.

Connections to the filters are made by means of front-panel input and output connectors, with sma female connectors used in models operating to about 26.5 GHz and K connectors used in models operating above 26.5 GHz.

In both types of filters, a frequency can be tuned by means of moving a cursor under one of the display digits and entering a numeric value with the keypad. An entry is finalized by hitting the MHz key. When resolution finer than 1 MHz is needed, a decimal point can be shifted on the display screen. For even simpler operation, the rotary dial can be used for tuning filter center frequency, with clockwise motion increasing frequency and counterclockwise motion decreasing frequency. Of course, each of the filters can also be tuned remotely, by means of program control.

Most of these YIG filters can handle input power levels approaching 1 W (about +28 to +30 dBm) without damage, although they start to limit input signals higher than typically +5 or +10 dBm. In most cases, test signals that would follow the filter into a spectrum analyzer or vector network analyzer (VNA) would not exceed about +5 dBm without risking damage to the analyzer's front-end electronics.

As examples of the bandpass filters, model MLBFP-42026 operates from 2.0 to 26.5 GHz while model MLBFP- 78020 works from 8 to 20 GHz. The former is a four-stage filter while the latter is a seven-stage filter. The model MLBFP-42026 provides a 3-dB bandwidth of 35 MHz with passband insertion loss of 7.2 dB. It has an input limiting level of +10 dBm. Model MLBFP-78020 has a much wider 3-dB bandwidth of 500 MHz with 7.7-dB passband insertion loss. It has an input limiting level of +13 dBm.

As examples of the band-reject filters, model MLBFR-0502 operates from 0.5 to 2.0 GHz while model MLBFR-0220 runs from 2 to 20 GHz. The former is a 12-stage design while the latter is a 16-stage filter. The model MLBFR-0502 features a 3-dB bandwidth of 100 MHz and a focused 40-dB bandwidth of 5 MHz. It limits with input power levels of 0 dBm and has insertion loss of 3.2 dB. Model MLBFR- 0220 has 3-dB bandwidth of 150 MHz and 3-dB bandwidth of 10 MHz with insertion loss of 3.7 dB.

Each of the benchhtop YIG filters weighs 9 lbs. and is designed for operating temperatures from 0 to +60C. They are 10 in. wide by 4 in. high by 13 in. deep and conform to shock, vibration, and humidity levels per MIL-PRF-28800F for general instrumentation Class 3 requirements. The instrumentation filters are equipped with a standard Ethernet port and USB Mini-B connector for connection to a personal computer running the most popular operating systems from Microsoft, including Windows XP (32-b), Windows Vista (32- and 64-b), and Windows 7 (32- and 64-b versions). JB

Micro Lambda Wireless, Inc., 46515 Landing Parkway, Fremont, CA 94538; (510) 770-9221, FAX: (510) 770-9213, www.microlambdawireless.com.

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