Microwave Filters,Lumped - element LC filters , Planar filters, Coaxial filters, Cavity filters, Dielectric filters and Electro acoustic filters

Lumped - element LC filters

An LC tank circuit consisting of parallel or series inductors and capacitors is a best example of a simplest resonator structure. These LC tank circuits have the advantage of being very compact & small in size, but having the low quality factor of the resonators which leads to relatively poor performance. 

Figure: Commercial photograph of Lumped-element LC filters.

Lumped-Element LC filters have both an upper and lower frequency range. When the frequency gets very low such as kHz to Hz range, then the size of the inductors used in the tank circuit becomes prohibitively large. Very low frequency filters are often designed with crystals to 8overcome this problem. As the frequency gets higher, into the 600 MHz and higher range, the inductors in the tank circuit become too small to be practical.

 Planar filters

Micro-strip transmission lines (as well as CPW or strip-line) can also make good resonators and

filters which offer a better compromise in terms of size and performance with respect to lumped element filters. The process used to manufacture micro-strip circuits is very similar to the processes used to manufacture PCB and these filters have the advantage of largely being planar.

Precision planar filters are manufactured using a thin-film process. Higher Q factors can be obtained by using low dielectric materials for the substrate such as quartz or sapphire and lower resistance metals such as gold.

Figure : Commercial photograph of Planer filters

  Coaxial filters

Coaxial transmission lines provide higher quality factor than planar transmission lines and are thus used when higher performance is required. The coaxial resonators may make use of high-dielectric constant materials to reduce their overall size.

Figure: Commercial photograph of Coaxial filters

  Cavity filters

 

Still widely used in the 40 MHz to 960 MHz frequency range, well constructed cavity filters are capable of high selectivity even under power loads of at least a megawatt. Higher Q as well as increased performance stability at closely spaced (down to 75 kHz) frequencies can be achieved by increasing the internal volume of the filter cavities.

Physical length of conventional cavity filters can vary from over 82" in the 40 MHz range, down to under 11" in the 900 MHz range. 

Figure : Commercial photograph of Cavity filters.

  Dielectric filters

Pucks made of various dielectric materials can also be used to make resonators. As with the

coaxial resonators, high-dielectric constant materials may be used to reduce the overall size of the filter. With low-loss dielectric materials, these can offer significantly higher performance than the other technologies previously discussed.

Figure: Commercial photograph of Dielectric filters.

  Electro acoustic filters

Electro acoustic resonators based on piezoelectric materials can be used for filters. Since acoustic wavelength at a given frequency is several orders of magnitude shorter than the electrical  

wavelength, electro acoustic resonators are generally smaller than electromagnetic counterparts such as cavity resonators. A common example of an electro acoustic resonator is the quartz resonator which essentially is a cut of a piezoelectric quartz crystal clamped by a pair of electrodes. This technology is limited to some tens of megahertz. For microwave frequencies, thin film technologies such as surface acoustic wave (SAW) and bulk acoustic wave (BAW) have been used for filters.