Microwave and RF filters are of immeasurable importance to wireless equipment because they are always located between the input and output stages of the RF and microwave active circuits. Microstrip planar filters are preferred for integration in low-power transceiver systems because they possess the properties of compact size, flexible layout, low cost, and easy fabrication whereas coaxial cavity filter is for lower loss and high power applications such as in base stations. Filters based on single mode resonator are widely used as bandpass filter in modern microwave systems due to their simple structure and ease of fabrication. The single mode resonator provides only one resonance per unit structure. This implies that to design a 4th order filter, 4 single mode resonators are required. The electrical length of the resonator is typically 1800 with only one resonance exists per unit structure.
The most common single mode resonator filers are end-coupled resonator filter, parallel coupled line filter, ring resonator filter, interdigital filter, and combline filter. The simplest of these are the end-coupled and parallel coupled line filter. Today lets talk about about a few of these filters.
End-Coupled Microstrip Bandpass Filter
The end-coupled microstrip bandpass filter is comprised of a series of capacitively coupled transmission line resonator element with electrical length of a half-wavelength at the midband frequency of the filter.
The 3rd end-coupled microstrip bandpass filter designed layout is illustrated in Figure 1 where each open-end microstrip resonator is approximately a half guided wavelength long at the midband frequency of 1 GHz of the bandpass filter.
The coupling capacitances are realized by gaps between the two adjacent open ends. These capacitances act as admittance inverters which reflect high impedance levels to the ends of the resonators which causes the resonators to resonate.
One of the drawbacks is the filter’s spurious frequency which occurs at only two times the fundamental frequency due to a half guided wavelength of the resonator length. This results in poor harmonic suppression when used as output filters in oscillators or amplifiers.
Moreover, in between the passband and its corresponding neighboring passband, there is no finite transmission zero (TZ) and hence the signals which are in this unwanted region will not be fully suppressed.
Parallel Coupled Line Filter
Another single-mode filter is the parallel-coupled resonator filter which uses half-wavelength line resonators. The parallel coupled line bandpass filter has the layout in such a way that adjacent resonators are parallel to each other along one-half of their length. This is shown Figure 2.
This parallel arrangement gives relatively large coupling for a given spacing between resonators and, thus, this filter structure is particularly convenient for constructing filters having a wider bandwidth as compared to the structure for the end-coupled microstrip filters.
With this feature, the parallel-coupled line resonator filter has become a very attractive research topic to many researchers and the parallel-coupled configuration has been widely used for realization of microstrip bandpass filters.
Interdigital Filter and Combline Filter
The two filter types discussed above are designed in planar structure which cannot be employed in base-station applications where the required Q values are in the range of 3,000 to 5,000 at 1,800 MHz. a new technology is needed. Thus comes the, coaxial transverse electromagnetic (TEM) filters. The TEM filters are the best candidate in this application because they offer the lowest design cost with the Q value up to 2,000.
Coaxial TEM filters are typically Combline or Interdigital structures. The interdigital bandpass filter consists of an array of N TEM-Mode or quasi-TEM-mode transmission line resonators. Each of these line resonators has an electrical length of 900 at the midband frequency and is short-circuited at one end and open-circuited at the other end with alternative orientation.
The interdigital filter has the advantage of a broad stopband and a highly symmetrical frequency response. However, from a physical viewpoint it has certain disadvantage.
First, it is large in size: the resonators are quarter wavelength long and for narrow bandwidths they are physically well separated. Second, the tuning screws are on alternate opposite faces of the filter which make it very hard for final electrical alignment.
For its counterpart, the combline filter being first introduced by Matthaei in 1963 can overcome these disadvantages at the expense of only a slightly asymmetrical frequency response. Combline filters are broadly used as bandpass filters in modern microwave systems, at least for frequencies below 10 GHz due to their compactness and broad stopband bandwidth characteristic. The filter consists of an array of coupled transmission lines shorted to ground at the same ends and loaded with capacitive elements at opposite ends as illustrated in Figure 3.
The shunt lines behave as inductive elements and resonate with the capacitors at a frequency below their quarter-wavelength frequency. With the presence of the capacitors, the filter’s electrical length is less than 900 yielding the advantage of a small and compact filter structure. Moreover, the capacitor also gives the advantage of having the capability of spurious response suppression.
As a result, the filter has a wider stopband between the first passband (desired) and the second passband (unwanted). Moreover, the tuning screws are only at one side of the filter which makes it easy to fine tune to get the final electrical alignment. With these advantages, combline filter has become the most commonly used bandpass filters in many communication systems and other microwave applications, especially in base station application.
The miniaturization of RF and microwave communication equipment is in progress with the development of mobile and satellite communication systems. This technological trend requires compact bandpass filters. The development of multi-mode technology which is capable of reducing the number of resonators required for a prescribed filter degree makes it possible to achieve a small filter size and compact design.
Particularly, one example is the dual-mode resonator which can be utilized as a doubly tuned resonant circuit and, hence, the number of required filter degree is reduced to half. Another example is the triple-mode and quadruple-mode resonator which can reduce the number of required degree to one third and to one fourth respectively. One may expect to have higher complexity and time consuming design for multi-mode as compare to single mode filter.