Crystal Ladder filter


Dan Watson writes:

Here’s a little board for making a four pole crystal ladder filter.
This type of filter has a very high Q and narrow bandwidth, allowing you to select a specific frequency from your signal. Because of the narrow bandwidth, selection of the capacitors and tuning of the filter is important to achieve the desired response. You will want to test your filter after designing and assembling it to verify performance.

Project info at The Sync Channel blog.

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  1. Properly designed for best selectivity, this type of crystal ladder filter will have an input and output impedance of a at least a couple of hundred ohms to even higher. That requires input/output matching via transformers and/or attenuators. These matching elements are missing from this design. A four crystal ladder would be borderline OK for a relatively narrow CW filter, say 500Hz or so, nothing broader unless more crystals are added. Simply buying crystals and dropping them on a board like this is asking for bad results. Each crystal’s lumped element motional parameters should be measured in a specialized test jig and matched. A decent filter like this will use perhaps four crystals out of forty or fifty in a batch all bought at one time. It is best practice to try and reduce parasitics between the crystals by soldering a wire across the tops of all the crystal cans, this is especially true for the taller (and preferable) HC49/U crystal packages. Finally, the Author uses short HC49/US crystal packages. It is widely accepted that the crystal elements in the taller HC49/U cans usually have higher Q and tighter motional parameters in batches. Simply put, while this filter may work in minimally demanding applications like a toy CW direct conversion receiver, a redo is in order. My opinion only…

    1. Correction:

      “…It is best practice to try and reduce parasitics between the crystals by soldering a GROUNDED wire across the tops of all the crystal cans”

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