Sunday, August 2, 2015

A temproary diversion from the Antenna installation --- Homebrew Crystal Filters

A Diversion from the Antenna Installation

My life seems to be consumed with installing the new two element beam; but it appears that the goal of having an new antenna may take a bit longer than thought (or desired). There has to be a workable solution that does not involve spending $3000 on a small tower. I have received some new inputs that will require a bit more noodling so the antenna project is now in the "needing more evaluation" stage.
So we don't lose your interest in this blog I want to share some tribal knowledge about building homebrew crystal filters. There is much information about the actual selection and characterization of homebrew filters and I will not cover that aspect. My bent is once you have the filter crystals and the selected capacitors how do you translate that into hardware.
In a recent project of mine called the LBS - II, I decided to use a 5.185 MHz homebrew crystal filter principally because I misplaced my normal stash of 4.9152 MHz crystals which happens to be my most favorite IF frequency behind 9.0 MHz. Because the LBS - II was (by design) to utilize Surface Mount parts to make the radio more compact I needed a way to install the crystals in a small footprint. Some noodle time came up with a layout that works and in fact is ideal if one elects to use the 9.0 MHz SSB Filter Kit Model #351 sold by INRAD. Here are the specifics:
  • The footprint involves five pad areas of 0.2 inches wide by 0.4 inches long. Thus the overall size is 1 inch long by 0.4 inches wide.
  • All of the filter capacitors are soldered along one edge of the assembly (this makes adjustments very easy to accomplish).
  • The design provides a good ground plane area under the crystals which is very important to achieve the stop band attenuation
  • The board can be made by etching, milling or ugly style with a hacksaw and xacto knife
  • The board area being small is ideal for retrofitting into existing designs
  • An all important final step is to connect all of the crystal cans to ground which is another factor in the stop band attenuation.

Drum Roll Please

Layout for the 9.0 MHz INRAD Crystal Filter

The 5.185 MHz  filter installed on the LBS-II. The Ground Wires have not been added at this point.

Stay Tuned de N6QW