RF filters are critical component in every RF system to perform filtering functions. There are two general group of filters, the passives and the actives. In most cases, these filters are passive filters. The passivity defines the linear characteristic of the filter where the filters exhibit fixed passband and stopband characteristics.
Reconfigurable Passive Filters
In some cases, it is deemed to be important and highly desirable for a filter to be made flexible or reconfigurable in terms of its center frequency, bandwidth or attenuation profile to serve different applications simultaneously for a given physical filter structure. For passive filters, reconfigurability is harder to achieve as the design is “fixed” in place.
Therefore, to achieve reconfigurability with passive filters, it can be conventionally achieved by having switched filter bank where each filter is having different center frequency as shown in the figure below. The switching of passband frequency is achieved by the external switches connected to the input and output of the filter bank.
However, because it is in essence multiple filters in one device, it is easily recognizable that the main disadvantage of this configuration is the sizing.
Reconfigurable Active Filters
Since passive filters faced a design bottleneck for achieving reconfigurability and flexibility, active filters are look at to solve the problem.
The picture below shows an example of an electronically tunable filter realized in planar structure. The tunability is achieved through varactor diodes which act as the variable capacitance. The junction capacitance value of the varactor diode varies as a function of biasing voltage. In other words, the filtering center frequency, bandwidth and attenuation profile is directly controlled and switched based on the input voltage that is fed into the filter.
To give an example, observe the picture below of the tunable filter on how the center frequencies is tuned from 1GHz to 1.75GHz. With such realization, a single filter will be able to do the job. From the physical point of view, this filter has the advantage of being small size and able to achieve the tunability function by utilizing active devices such as varactor diodes or switches.
The incorporation of such devices that introduces biasing voltages into filters are classified as active filter, where the filter characteristic is no longer linear.
Performance of Reconfigurable Active Filters
Now, let’s investigate the performance of a varactor tuned filter with varied input power levels as shown in the figure below. It can be observed that the filter no longer behaves as a filter by increasing the power level input to the filter!
What happened to this filter?
The truth is the filter suffers from non-linearity effect where power saturation effects come into play. But assuming that the filter is setup in a controlled environment, it works perfectly flexible and this power saturation effects will not affect it too much.
But the reality is always cruel. An antenna of RF transceiver receives all wireless signals with unknown power level as long as they are within the band of interest. How can one make sure that the input signal power does not saturate the active device of the tunable filter?
In order to resolve this, massive research had been conducted including better varactor or tuning devices and novel tuning method or technique. The research is also extended to not only making the center frequency tunable but also the passband bandwidth and attenuation level at the stopband.
But are these the only parameters that gauge the performance of tunable filter? The answer is no! Other parameters such as the linearity, tuning time and tuning range are also important for some specific application such as electronic warfare.
If you find this topic interesting, let us know in the comments. More interesting ideas will be discussed in the next article with regards to the latest technology in tunable filter realization.
Originally published at http://filpal.wordpress.com on April 3, 2022.
