Filters 101

Radio systems cannot operate without filters.  Marconi and Hertz were fortunate in that when they performed their experiments they had the entire spectrum to themselves.  The experiments proved a point but from a spectrum efficiency viewpoint they were a disaster.  They would have wiped out any other users of the radio spectrum for miles around.

It is the filter that ensures many radio systems can harmoniously share the radio spectrum within the same geographic area.  In duplexers, it is the embedded filters that allow the same antenna to be used for both transmit and receive purposes.

The filter performs the functions asked of it at a trade off. That trade off is in the form of power loss.  Loss wastes transmit power and reduces receiver sensitivity.  The more demanding the challenges set for the filter, the higher the loss will be.

As modern radio systems attempt to maximise the value the radio spectrum the need for ever more sophisticated filters is showing no signs of reducing.  Quite the contrary in fact: demands for low insertion loss and high isolation are as loud as ever.  But now there is an extra constraint: demand for small cells, active antennas and spectrum release means that filters need to be smaller and lighter than before, at a low cost.  Transceivers are being made ever smaller and lighter; filters are being expected to do likewise.

The size and weight constraint is particularly applicable to small cells and active antenna systems.  Fully integrated active antenna systems need many duplexers (quite possibly into double figures) within the antenna housing.  When it is considered that the entire system (effectively the radio elements of an entire base station) has to be lifted many metres into the air the imperative to reduce size and weight becomes clear.

Size and performance demands have always worked against each other. Typical duplexers could be in excess of two litres in volume and weigh more than a kilogram.  Reducing size can be achieved either by reducing the number of integral resonators, thus reducing stop band attenuation, or by reducing the size of each resonator, thus increasing insertion loss.

In general, there is a trend for filters to reduce in size as the type of resonators change from air cavity through dielectric-filled cavity to transmission line resonators of coaxial and microstrip varieties.  However, the corollary of this is an increase in insertion loss.  xCube technology is clearly an outlier in this trend, delivering much lower loss than would be expected from a filter of its size.