Filters based on yttriumiron-garnet (YIG) technology provide passbands or rejection bands that can be tuned across wide microwave frequency ranges. One of the companies that has succeeded in shrinking the size of the technology in recent years, Micro Lambda Wireless (www.microlambdawireless.com), in the form of its compact cubeshaped filters, has now packed two filter channels into these miniature 1.4-in. cube-shaped packages. The company's MLFRD series of dualchannel YIG band-reject filters each provides two tunable rejection bands with coverage within the overall 2-to-18- GHz band. Individual coverage includes 2 to 6 GHz, 2 to 8 GHz, 6 to 18 GHz, and 8 to 18 GHz, making these bandreject filters well suited for removal of unwanted signals in electronic-warfare (EW) and electronic-countermeasures (ECM) receivers, especially where high alternate-channel rejection is needed.
The MLFRD series (see figure) of dual-channel YIG band-reject filters includes the models MLFDR-0206 (2 to 6 GHz), MLFRD-0208 (2 to 8 GHz), MLFRD-0618 (6 to 18 GHz), and MLFRD-0818 (8 to 18 GHz), each housed in a 1.4-in. cube and available with an analog or digital driver (see table). The two lower-frequency models provide 40-dB rejection bands of at least 5 MHz wide and maximum of 20 MHz wide. The two higherfrequency filters offer 40-dB rejection bands of at least 15 MHz wide and a maximum of 35 MHz wide. The maximum 3-dB rejection band for the two lower-frequency filters is 120 MHz while the maximum 3-dB rejection band for the two higher-frequency filters is 100 MHz.
For applications such as militarygrade receivers, where it is essential to maintain consistent performance for both band-reject channels, the filters are specified for maximum channel-to-channel tracking of 3 MHz for the two lower-frequency models (MLFRD-0206 and MLFRD-0208) and maximum channel-to-channel tracking of 5 MHz for the two higher- frequency filters (MLFRD-0618 and MLFRD-0818). The channel-tochannel isolation is at least 50 dB for all four filters.
In spite of the high signal rejection provided by the MLFRD series dual-channel YIG band-reject filters, they exhibit low insertion loss for other signals in the band, reaching a maximum of 2 dB for all models. The maximum VSWR for all models is also low, at 2.0:1. The filters are designed to handle input levels as high as +28 dBm without damage, and have a minimum limiting level of +5 dBm for the two lower-frequency models and +10 dBm for the two higher-frequency models. The maximum spurious level for each filter is 4 dB.
The filters benefit from the high-quality- factor (high-Q) properties of YIG spheres, which can be tuned within an electromagnetic (EM) field by means of an applied current. The tuning sensitivity for all four filters is typically 20 MHz/ mA, with worst-case tuning linearity of 3 MHz for the two lower-frequency filters and 5 MHz for the two higherfrequency filters. The tuning hysteresis is typically 5 MHz for the two lowerfrequency filters and typically 10 MHz for the two higher-frequency filters. With full tuning, the two lower-frequency filters provide a total passband range of DC to 8 GHz while the two higherfrequency filters have a passband range of DC to 18 GHz.
The MLFRD series YIG band-reject filters are specified for operating case temperatures of 0 to +65C. Over that temperature range, the maximum frequency drift with changing temperature is 10 MHz for the two lower-frequency models and 20 MHz for the two higherfrequency models. The filters require heater voltage of +28 VDC for temperature stability, drawing heater current of 300 mA during surge operation and 100 mA during steady-state operation (at +25C). For applications in severe environments, all four filter models can be supplied in versions with extended operating operating temperature ranges. In addition to application in EW and ECM receivers, such filters have been used in broadband communications receivers as well as in sensitivity deep-space (astronomy) receiver applications. Their excellent channel-to-channel frequency tracking and high isolation makes them suitable for a number of uses. Micro Lambda Wireless, Inc., 46515 Landing Parkway, Fremont, CA 94538; (510) 770-9221; Fax: (510) 770-9213; E-mail: firstname.lastname@example.org, Internet: www.microlambdawireless.com.