RF filters, along with all filters have a variety of different specifications which relate to their performance.
Understanding what these filter specifications mean and how they relate to the performance parameters enables a them to be properly understood and the correct filters specified, bought or designed.
RF filter specifications summary
This naturally there are very many different RF filter specification that will be seen.
- Pass-band: The pass-band specification defines the region in which the signal passes through relatively un-attenuated. It is the band in a low pass filter, extends up to the cut-off frequency. For high pass filters it is designated as the band above which signal pass through, or for a band pass filter, it is the band between the two cut-off frequencies.
When looking at the pass-band specification, it is worth noting the level of attenuation between which the pass-band is taken. Normally the -3dB points are used, but for some filters, like crystal filters, especially, the -6dB points are often used.
- Cut-off frequency: This RF filter specification is normally taken to be the point at which the response of the filter has fallen to -3 dB relative to the in-band level. With certain filters, typically equi-ripple types such as the Chebyshev or inverse Chebyshev, the cut-off point has to be defined differently as in-band ripple levels can make determining a -3dB point more difficult. The cut-off frequency is often designated fc.
- Ripple band: Within the pass-band, the filter response may show variations in its response - ripples. The variation is known as the ripple band. Often RF filters that have a very sharp turn at the cut-off frequency have a higher level of in-band ripple.
- Transition band: Once the RF filter response has gone beyond the cut-off point, the response falls away in a region known as the transition band. It is the region between the pass-band and the stop-band. This region is also sometimes referred to as the "skirt."
- Stop-band: This is the band where the RF filter has reached its required out of band rejection. The stop-band rejection may be defined as a required number of decibels.
- Number of poles: A pole is a mathematical term. There is one pole for each capacitor or inductor in a filter.
- Roll-off: Each filter has an ultimate roll-off rate. It is governed by the number of poles in the filter. The ultimate roll-off is 6⋅n dB where n is the number of poles. Different types of filter may reach their ultimate roll off rate at different rates, but they all reach the same ultimate roll-off.
- Phase shift: The phase shift is another important factor for any RF filter design. It is accommodated into the overall response of the filter by considering the calculations for H(s) where s = jω. The phase response can be of importance to a waveform because the waveform shape will be distorted if the phase changes within the pass-band. A constant time delay corresponds to the phase shift increasing linearly with frequency. This gives rise to the term linear phase shift referred to in many RF filter designs.
- Impedance: Filters have a characteristic impedance in the same way that as an antenna feeder. For them to operate correctly the input and output must be properly matched.
Dependent upon the particular filter, there will be a variety of different specifications that may be used, but the parameters and specifications described above are the major ones that will be most commonly encountered.