Designing A Crystal Ladder Bandpass Filter

Most hobbyists use crystals as an external clock signal for a microcontroller. A less common use would be to make a bandpass filter (BPF) for an RF signal. [Dan Watson] explains his crystal ladder design on his blog and links to several sources for understanding the theory and creating your own crystal ladder band pass filter. If you want a set of these purple PCBs you can order them straight from the purple fab.

[Dan]’s schematic
One of the sources that [Dan] cites is [Larry Benko]’s personal site which is primarily dedicated to amateur radio projects. Which you can find much more in-depth information regarding the design of a xtal BPF. [Larry] goes into detail about the software he uses and some of the applications of crystal ladder filters.

BPF designed by [Larry]
BPF designed by [Larry]
The process includes measuring individual xtals to determine which ones will work together for your target frequency. [Larry] also walks you through the software simulation process using LTSpice. If you aren’t familiar with Spice simulation you can get caught up by checking out the series of Spice articles by our very own [Al Williams].

Thanks to Dangerous Prototypes for the tip.

Hack Your Crystal’s Frequency

[Drone] tipped us off about [Joachim]’s efforts to alter a crystal’s frequency. Through a process called penning, a crystal’s resonant frequency is lowered by painting the crystal with an indelible ink marker. Our curiosity piqued, we went off  and found more information about penning crystals. It turns out this technique has been around for nearly as long as there have been amateur radio operators. Outside of your local oscillator, and radio jammer, how might you best use a hacked crystal?