RS Microwave Technical Corner

January 1997

Compact High-Power Notch Filters

Bandstop or “notch” filters are important constituents of modern systems.  Applications include cellular communIcation, radar, countermeasures, wireless...the entire roster of today’s congested-spectrum signal applications.  These filters are the complement of bandpass filters and are used to selectively reject a narrow band of frequencies while minimally affecting the rest of the system bandwidth.

Most designs proceed from a lumped or distributed prototype, transform into a commensurate line realization using parallel or direct-coupled lines, for TEM implementations.  These circuits use quarter-wavelength lines, in machined or etched configurations, for high-Q performance.  However, unless the lines are dielectrically loaded, the sizes can be a problem.  Dielectrically loaded lines display reduced unloaded Q and spurious responses due to inhomogeneity. Figure (1) BELOW depicts a high-Q, high-power filter with rejection bands at 1030 and 1090 MHz and with a passband extending from DC to about 2 GHz.

FIGURE 1

61351       The 61351 is designed for Class 2H (1250 Watt) operation on shipboard. It is MIL Qualified and is                     nomenclatured as F-1628/URC-107(v)7.        

Operating Frequency:	967.5 - 1207.5 MHz
Passband Transmit Insertion Loss:	1.65 dB Max (0.7 dB Typical)
Receive Passband Insertion Loss:	0.65 dB Max (0.3 dB Typical)
VSWR:	2.0:1 Max (1.3:1 Typical)
Passband Group Delay:	100 Nanosec Max
Reject Bands:	1024.5 - 1040.5, 1083.5 - 1093.5 MHz
Rejection:	54 dBc
Minimum Size:	40.90 x 7.50 x 2.16” (1039 x 90.5 x 55 mm)

We have developed a circuit (Ref. [1]) which uses capacitively coupled 50 ohm coaxial sections to provide similar rejection, insertion loss and power handling characteristics with a significant reduction in size.  The key to implementation is the use of an adjustable series coupling capacitor (Pat. App. For) which allows for easy tuning to a variety of notch bandwidths.  The design is quite modular, and dimensions are easily predictable.  To add another pole means simply adding a known increment of length.  Figure (2) depicts one of the many such compact, coaxial notch filters developed using this configuration.

Originally developed for the high-power JTIDS/MIDS spread-spectrum military communication data link, these filters may also be combined with narrow band dielectric resonator circuits to provide very high Q transmission zeros for rejecting close-in interfering signals in wireless communication application, in both cellular and PCS systems.  These notch designs can also be combined with diode or mechanical switches and broad-band bandpass filters for harmonic reduction.  Figure (3) portrays one such application.  The filters may also be “embedded” in a bandpass structure, adding a supplementary real-frequency transmission zero. Figure (4) shows such an application.  

Ref [1]:    R. V. Snyder, “A Compact, High-Power Notch Filter with Adjustable F0 and Bandwidth”, IEEE Trans. on MTT, Vol. 42, No. 7, July, 1994, pp. 1397-1403.