Crystal Ladder Filter Circuit

All the capacitors are the same value. For single sideband use 39pf. For CW use 100pf for a bandwidth of 700 to 1000Hz. Use 300pf for a bandwidth of 500 to 700Hz. The crystals used are 3.457MHz available from BG Micro and other parts catalog businesses.

This design comes from some articles in QST on the design of crystal ladder filters.

The first one is "Ladder Crystal Filter Design", by J. A. Hardcastle, November 1980, QST, Page 20. This article was a little scary with all the formulas and such, and he only explored one filter using 8.454MHz crystals. His outlines and formulas weren't that difficult to follow, but the article didn't exactly make you want to rush to build one.

The second one is "A Unified Approach to the Design of Crystal Ladder Filters", by Wes Hayward, May 1982, QST, Page 21. This was the first article that hinted at the fact that regular computer crystals or color burst crystals could be used in a very effective homebrew crystal filter. Be aware that Fig. 3 in the article doesn't include the values for the toroids. The feedback for that appeared in the July 1987 issue of QST, Page 41. Circuits needed to evaluate crystals were given, however, he included the statement "None of the filter circuits in this paper is suitable for exact duplication."

The classic article of all time, that in my opinion, bust open the doors to easy and inexpensive homebrew receivers, was the article "Designing and Building Simple Crystal Filters," by Wes Hayward, QST, July 1987, P 24. This article showed how using the same capacitance for each capacitor in a crystal ladder filter gave very good results, very inexpensively, and easily adjusted bandwidth. The article inspired me to build several filters with crystals on hand, and they all worked great! In the past, the crystal filter was always a difficult item because of the expense and critical matching requirements, and now it was the easiest thing to build for the homebrew receiver.

I never used any of the evaluation circuits that were recommended for matching the crystals, yet I was always able to build a filter that worked very well in the receivers I built. His very simple formula for adjusting the bandwidth by changing the value of the capacitors worked with every filter.

Doug DeMaw wrote an article in the January 1990 issue of QST, page 21, "A Tester for Crystal F, Q and R", that improved on the circuits that were in Wes Hayward's "Unified Crystal...." article, and gave all the instructions for picking and matching the crystals that would work best with Hayward's "Designing and Building Simple Crystal Filters".