What Is a Tunable Bandpass Filter?
A Tunable Bandpass Filter is a type of tunable filter that exhibits significant attenuation outside of the passband of the filter, and contains a resonant or other adjustable element that allows for the center frequency and other filter characteristics to be adjusted. Tunable bandpass filters differ from fixed bandpass filters in that there are one or more elements, often resonant elements, that can be adjusted, either electrically or mechanically. The adjustment of key elements of a filter can significantly change the filter response of a filter beyond just changing the center frequency. Though, adjustment of the center frequency is the most common use of a tunable bandpass filter.
There are many applications that channels spread over a relatively wide bandwidth. One approach to isolating those channels is to use a filter bank, which is typically a bank of fixed bandpass filters specifically designed to pass only a small given range of frequencies in each filter bank channel. Prior to filtering, the input signal must be divided to each channel of the filter bank, and hence the signal energy entering each filter bank channel is attenuated according to the depth of division and associated losses.
Key Tunable Bandpass Filter Specifications
- Frequency range [Hz]
- Nominal Bandwidth [Hz or % of frequency range]
- 30 dB/3 dB ratio [ratio]
- 50 dB/3 dB ratio [ratio]
- Tuning accuracy [% of frequency range]
- VSWR [ratio]
- Insertion loss [-dB]
- Impedance [Ohms]
- Input power maximum [dB]
- Operating temperature range [degrees C/F]
Another solution to dealing with multiple frequency channels is to have a separate receiver or complete receiver signal path including antenna for each channel. As using filter banks or separate receivers isn’t viable for many applications, an alternative is to use a tunable bandpass filter that can cover the desired spectrum and exhibits the desired filter characteristics to match the frequency channel plan of the receiver. The caveat here being that a tunable bandpass filter is only able to receive a single channel at a time, where filter banks and multiple receivers are able to receive multiple channels at the same time, albeit at the cost of complexity, size, efficiency, and possibly cost.
Tunable bandpass filters can be actively electrically adjusted or tuned with mechanical actuators, such as knobs, slides, or switches. For portable communication and sensing systems, it is likely that the tunable bandpass filters used in these designs will be electrically controlled, as these are more compact and with some added circuitry and systems, be software programmable. Mechanically tuned bandpass filters are often used in laboratory and testing environments where it is key to isolate signals within a given frequency band.
It is important to note that with most tunable filters, the filter characteristics change as a function of the tunable filter element. This can impact the center frequency, which is usually what is desired, but may also impact other filter characteristics. Key filter characteristics that may be affected by the tunable filter element adjustment are insertion loss, bandwidth, and out-of-band rejection.