The microwave frequency is in that portion of the electromagnetic spectrum where the wavelength is of the same order of magnitude as the characteristic size of the circuit carrying the electrical energies. The frequencies most often considered to be in this category lie between approximately 1 and 200 GHz. Microwave circuits usually contain distributed circuit elements. Circuits used at lower frequencies usually have lumped elements while circuits used at higher frequencies use optical techniques. As one can ascertain, the microwave frequency range has been applied widely in communications systems, radar systems and in various other applications. High performance filter are an integral part of microwave systems.
Parallel coupled microstrip filters are extensively used as band pass filters in such systems due to their small size and their relatively easy fabrication. Such filters can be designed with reasonable accuracy using the design information obtainable in the literature. Microstrip (MS) is used in circuits where discrete devices are bonded to the circuit, where easy access is needed for tuning, or a compact design is needed.
Since the electromagnetic fields lie partly in air and partly in the dielectric, obtaining solutions for the characteristic impedances and effective dielectric constant in MS is more complicated than it is for stripline. Furthermore, microstrip is only approximately a TEM transmission line, but unless the circuit to be used is for very broad bandwidth applications or it is physically many wavelengths long, dispersion will not be a problem. Thus the TEM approximation gives useful results in the design of microstrip circuits. Since microstrip is a non-homogenous medium, the even and odd mode phase velocities for a couple or pair of microstrip lines are unequal. The difference in the phase velocities results in the filter having an asymmetric passband response, deteriorates the upper stopband performance and moves the second passband (which is about twice the center frequency) towards the center frequency.
Certain bandpass filters which have been built on microstrip are referred to as parallel edge coupled filter devices. The prior art is replete with such devices. Reference is made to an article entitled "Broadbanding Microstrip Filters Using Capacitive Compensation" by Inder J. Bahl of ITT Gallium Arsenide Technology Center and published in Applied Microwave, August/September 1989, pp. 70-76. The paper describes a capacitor compensated parallel coupled microstrip filter design with a symmetrical passband and second passband above twice the filter center frequency. Each resonator, in a typical parallel edge coupled device, is a half wavelength long. The first quarter wavelength coupled to the previous resonator and the second quarter wavelength coupled to the following resonator. If this type of filter is realized in a TEM structure it could have an infinite rejection at twice the center frequency and a second passband at three times the center frequency which allows the passband to have functional bandwidths of 40% to 60%. However, as indicated above, microstrip is not a true TEM structure and the rejection at twice the center frequency is relatively poor because the coupled sections of the resonators have even and odd mode phase velocities that travel at different speeds. The even mode travels in the dielectric and the odd mode (the coupling fields between the conductors) travels in the air and dielectric which causes the odd mode to travel faster than the even mode.
Another reason why such filters are not used for broad bandwidths is because they require tight coupling and therefore the physical separation between resonators is extremely small and the dimensions are so critical that such filters have been relatively impractical to construct and manufacture.
As indicated in many prior art designs, the poor stopband rejection forces the microwave designer to employ a lowpass filter preceding the bandpass filter in many systems. The second passband of a bandpass filter, at twice the center frequency, also results in poor second harmonic suppression when used as output filters for oscillators and amplifiers. To overcome this problem bandpass filters using parallel coupled stepped impedance resonators have been implemented. See an article entitled "Bandpass Filters Using Parallel Coupled Strip Line Step Impedance Resonators" published in the IEEE Transactions on Microwave Theory and Techniques, Vol. NTT-28, No. 12, December 1980 by M. Makimoto and S. Yamashita, pp. 1413-1417. This article gives approximate design formulas for bandpass filters using parallel coupled stripline stepped impedance resonators (SIR). These are not microstrip devices but are stripline devices.
The prior art was also aware of techniques used to slow down the odd mode velocity in microstrip coupled line filters. See an article entitled "Improved Performance Parallel Coupled Microstrip Filters" by M. R. Moazzam, et al., published in Microwave Journal, November 1991, pp. 128-135. This article discusses techniques which are employed to improve the stopband performance of the microstrip parallel coupled line filters. The phase velocities of the two modes may be equalized or a longer path for odd mode energy may be provided; the odd mode phase velocity is higher than the even mode phase velocity. Some of the methods used by the prior art to improve stopband performance include over coupling the resonators, suspending the substrate, using parallel coupled step impedance resonators and using capacitors at the end of coupled sections. As indicated in the article, such techniques increase the cost of the original filter and are difficult to implement. The article describes a planar technique for phase velocity compensation whereby the odd mode length is extended by introducing wiggle to the coupled lines. The technique does not add cost to the system and employs wiggly lines to provide compensation of phase velocity difference in parallel coupled microstrip lines.
In view of the above, it is an object of the present invention to provide an improved microstrip filter apparatus eliminating many prior art problems.
It is a further object of the present invention to provide a microstrip structure that allows tight coupling and solves the even and odd mode phase velocity difference problem to enable the construction of broadband filters.