RF filters are mainly used for frequency selection, allowing the required frequency signals to pass through and reflect unwanted interfering frequency signals, which is one of the indispensable key components of the RF system. Filters can be divided into low-pass, high-pass, band-pass, and band-stop filters according to the characteristics of frequency selection. Today we will get acquainted with tunable bandpass filters.
A tunable bandpass filter is a tunable filter that exhibits significant attenuation outside of the filter's passband and contains resonant or other tunable components that allow adjustment of the center frequency and other filter characteristics. A tunable bandpass filter differs from a fixed bandpass filter in that it will have one or more elements, usually resonant, that can be adjusted electrically or mechanically. Adjustments to the key components of a filter can significantly alter the filter response of the filter, not just the center frequency. However, adjusting the center frequency is the most common use of tunable bandpass filters.
5th Order Tuned Bandpass Filter, N-Type Female, 125MHz 250MHz, 5% Bandwidth
There are many applications where the channels are distributed over a relatively wide bandwidth. One way to isolate these channels is to use a filter bank, which is typically a fixed set of bandpass filters specifically designed to pass only a given small range of frequencies in each filter bank channel. Before filtering, the input signal must be divided into each channel of the filter bank, so the signal energy entering each filter bank channel is attenuated according to the depth of the division and the associated losses.
Tunable bandpass filter key parameters.
Frequency range [Hz].
Nominal bandwidth [Hz or % of frequency range].
30 dB 3 dB ratio [ratio].
50 dB 3 dB ratio [ratio].
Tuning accuracy [% frequency range].
VSWR [ratio].
Insertion loss [-db].
Impedance [ohms].
Maximum input power [dB].
Operating temperature range [degrees Celsius Fahrenheit].
Another solution for dealing with multi-frequency channels is to equip each channel with a separate receiver or a complete receiver signal path, including antennas. Since the use of filter banks or individual receivers is not feasible for many applications, an alternative is to use a tunable bandpass filter that can cover the desired spectrum and exhibit the desired filter characteristics to match the frequency channel plan of the receiver. It is important to note here that tunable bandpass filters can only receive a single channel at a time, where the filter bank and multiple receivers are capable of receiving multiple channels at the same time, albeit at the cost of complexity, size, efficiency, and possible cost.
Tunable bandpass filters can be actively electrically adjusted or tuned by means of mechanical actuators such as knobs, sliders, or switches. For portable communication and sensing systems, the tunable bandpass filters used in these designs are likely to be electronically controlled, as they are more compact and have additional circuitry and systems that can be programmed in software. Mechanically tuned bandpass filters are commonly used in laboratory and test environments where isolating signals within a given frequency band is key.
5th Order Tuned Bandpass Filter, N-Type Female, 15GHz 3GHz, 5% bandwidth
It is important to note that for most tunable filters, the filter characteristics vary as a function of the tunable filter elements. This can affect the center frequency, which is usually expected, but it can also affect other filter characteristics. The key filter characteristics that can be affected by the tuning of tunable filter elements are insertion loss, bandwidth, and out-of-band rejection.