Why should the crystal cases be grounded in a crystal ladder filter?

I saw this instruction in the Easy BITX manual “Use a small piece of wire to wire the four crystals to the nearby ground point to prevent “blow through” for the filter. Solder this after the crystals have been soldered in place.” As mounting the crystals for the ladder filter is my next step in homebrewing Easy BITX, I wanted to know why this potentially difficult step is needed. Soldering to the metallic case is likely to be a bit tough for me. Here is the result of my investigation into this matter.

Connecting the metal cans (cases) of the crystal resonators directly to a common ground point on the PCB is to eliminate unwanted stray capacitive coupling. I located a solder pad on the Easy BITX PCB near the crystals labelled as GND. This practice enhances the performance of a crystal filter by ensuring that only the desired signal frequencies pass through, preventing signal leakage.

What is “Blow Through”?

In a filter, “blow through” (or “leakage”) occurs when unintended, high-frequency signals bypass the filter components entirely—often through parasitic capacitance between components—instead of being filtered. If crystals are not grounded, their metal cases can act as small capacitors that allow signals in the stopband (unwanted signals) to “leak through” at higher, undesirable levels. A, “blow through” results in reduced stopband attenuation, meaning the filter doesn’t effectively block unwanted frequencies.

Why Grounding the Crystals Works

1. Reduces Parasitic Capacitance: Connecting the crystal case to ground creates a “shield” that interrupts the capacitance between input and output.
2. Improves Stopband Rejection: Grounding the cans can significantly improve the stopband attenuation, allowing the filter to perform better than 80 dB, which is often required for high-quality radio filtering.
3. Mechanical Stability: It can also reduce mechanical stress and vibrations that might otherwise affect the stability of the crystals.