The spurline is a type of

radio-frequency Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the ...

and

microwave Microwave is a form of electromagnetic radiation with wavelengths shorter than other radio waves but longer than infrared waves. Its wavelength ranges from about one meter to one millimeter, corresponding to frequency, frequencies between 300&n ...

distributed element filter with

band-stop In signal processing, a band-stop filter or band-rejection filter is a filter that passes most frequencies unaltered, but attenuates those in a specific range to very low levels. It is the inverse of a ''band-pass filter''. A notch filter is ...

(notch) characteristics, most commonly used with

microstrip Microstrip is a type of electrical transmission line which can be fabricated with any technology where a conductor is separated from a ground plane by a dielectric layer known as ''substrate''. Microstrip lines are used to convey microwave-freq ...

transmission lines. Spurlines usually exhibit moderate to narrow-band rejection, at about 10% around the central frequency. Spurline filters are very convenient for dense

integrated circuits An integrated circuit (IC), also known as a microchip or simply chip, is a set of electronic circuits, consisting of various electronic components (such as transistors, resistors, and capacitors) and their interconnections. These components a ...

because of their inherently compact design and ease of integration: they occupy surface that corresponds only to a quarter-wavelength

transmission line In electrical engineering, a transmission line is a specialized cable or other structure designed to conduct electromagnetic waves in a contained manner. The term applies when the conductors are long enough that the wave nature of the transmis ...

Structure description

It consists of a normal

line breaking into a pair of smaller coupled lines that rejoin after a quarter-wavelength distance. Only one of the input ports of the coupled lines is connected to the feed microstrip, as shown in the figure below. The orange area of the illustration is the microstrip transmission line conductor and the gray color the exposed dielectric. Spurline microstrip

Where

\lambda_g

is the

wavelength In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats. In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...

corresponding to the central rejection

frequency Frequency is the number of occurrences of a repeating event per unit of time. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio ...

of the bandstop filter, measured - of course - in the microstrip line material. This is the most important parameter of the filter that sets the rejection band. The distance between the two coupled lines can be selected appropriately to fine-tune the filter. The smaller the distance, the narrower the stop-band in terms of rejection. Of course that is limited by the circuit-board printing resolution, and it is usually considered at about 10% of the input microstrip width. The gap between the input microstrip line and the one open-circuited line of the coupler has a negligible effect on the frequency response of the filter. Therefore, it is considered approximately equal to the distance of the two coupled lines.

Printed antennae

Spurlines can also be used in printed antennae such as the

planar inverted-F antenna An inverted-F antenna is a type of Antenna (radio), antenna used in wireless communication, mainly at ultrahigh frequency, UHF and microwave frequency, frequencies. It consists of a monopole antenna running parallel to a ground plane and ground ...

. The additional resonances can be designed to widen the antenna bandwidth or to create multiple bands, for instance, for a

tri-band In telecommunications, a multi-band device (including (2) dual-band, (3) tri-band, (4) quad-band and (5) penta-band devices) is a communication device (especially a mobile phone) that supports multiple radio frequency bands. All devices which ha ...

mobile phone.

History

A spurline filter was first proposed by Schiffman and Matthaei in

stripline In electronics, stripline is a transverse electromagnetic (TEM) transmission line medium invented by Robert M. Barrett of the Air Force Cambridge Research Centre in the 1950s. Stripline is the earliest form of planar transmission line. De ...

form in 1964. Bates adapted the design for microstrip in 1977. Nguyen and Hsieh improved the analysis for microstrip implementations in 1983.Cam Nguyen; Kai Chang, "Millimeter wave planar integrated circuit filters", ch. 2 in Kenneth Button (ed), ''Topics in Millimeter Wave Technology: Volume 1'', p. 177, Elsevier, 2012 .

References

* C. Nguyen and K. Chang, “On the analysis and design of spurline bandstop filters,” IEEE Trans. Microw. Theory Tech., vol. 33, no. 12, pp. 1416–1421, Dec. 1985.

Primary sources

* B. M. Schiffman; G. L. Matthaei
"Exact design of band-stop microwave filters"
''IEEE Transactions on Microwave Theory and Techniques'', vol. 12, iss. 1, pp. 6-15, 1964. * R. N. Bates
"Design of microstrip spur-line band-stop filters"
''IEEE Journal on Microwave Optics and Acoustics'', vol. 1, iss. 6, pp. 209-204, November 1977. * C. Nguyen; C. Hsieh
Millimeter wave printed circuit spurline filters"
''IEEE MTT-S International Microwave Symposium Digest'', pp. 98-100, 1983. Microwave technology Distributed element circuits {{tech-stub