The bandwidth of an IF filter in a communications receiver/transceiver depends on the type of signal that is to be received - Signel Sideband (SSB) requires a typical bandwidth of 2.4KHz, Morse (CW) about 1KHz, Amplitude Modulation (AM) about 6KHz and Frequency Modulation (FM) between 7.5 to 15KHz depending on the required channel spacing/ maximum deviation. A multimode rig will therefore require sufficient filters to cater for the required operational modes.
If filters are to be home made then the following table will give constructors an idea of what is possible using both quartz crystals and ceramic resonators in 8 section ladder filters:
This table comes with a health warning:- mostly just one example of each type was measured so the information is presented as typical of what is possible. The reader should make their own detailed measurements during the design phase once they have chosen their preferred technology.
Modern crystals in the 2-3MHz range in HC18/U (miniature) holders appear to have characteristics that produce significantly narrower pass bands than their older HC6/U equivalents. There is an obvious inverse relationship between the LC ratio of the equivalent motional components Ls and Cs of the crystal and the maximum bandwidth that can be achieved with a ladder filter - particularly clear with the crystals in the 3MHz range. The ratio of Cp to Cs also has a considerable effect on the achievable bandwidth.
Ladder filters have an asymmetric frequency response (slower attenuation curve on the low frequency side of the pass band) which becomes worse as the bandwidth is increased for a given filter. The maximum bandwidth is what is possible for each type of crystal or ceramic resonator is shown in the table but users must ensure that the overall response is satisfactory for their specific requirements.
Where a greater bandwidth is required than can be achieved with ladder filters then half or full lattice filters must be used.
Crystal parameters and maximum bandwidth figures were obtained using the calculators on two adjacent pages on the Giangrandi web site. These calculators allow the user to enter four measured crystal parameters and determine the component values and frequency responses for filters using from two to eight sections - a great time-saver.