Crystals for IF Filters

Home

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 200Hz, 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 all of 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 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 or smaller holders appear to have characteristics that produce significantly narrower maximum pass band widths 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 with a slower attenuation curve on the low frequency side of the pass band which becomes worse as the bandwidth is increased for a given filter. This can be improved by using more sections but never completely removed. The maximum bandwidth is what is possible for each type of crystal or ceramic resonator is shown in the table and users must ensure that the overall response is satisfactory for their specific requirements.

A crystal used on its third or fifth overtone will show a significantly higher motional LC ratio and a correspondingly reduced maximum bandwidth in a ladder filter compared to the same crystal used on its fundamental frequency.

Where a greater bandwidth is required than can be achieved with ladder filters then half or full lattice filters must be used with crystals having the required frequency characteristics.

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.

An alternative site that provides a downloadable version of the Dishal filter calculator as a Windows executable is located here. This calculator also allows for a higher number of filter sections.

NB: I have used these techniques to design and construct ladder filters up to 37.5MHz with fundamental mode crystals - see an example here. However, short connections and extra care is required to measure the crystal parameters at the higher frequencies otherwise the filter frequency responses will be incorrect.

Keys: