Saturday, March 15, 2008

Ceramic Filter Units

Some time ago I ordered a package of 500 KHz ceramic filters from Dans Small Parts and Kits , and at the same time also ordered two of his grab bag kits. When the order arrived I found that among other things, the grab bags contained a great many 500 KHz filters (Dan must have thought I really needed 500 KHz filters!).

A hand full of these filters was sent to Ian K3IMW, and together we have been playing with them to get a better feel for their characteristics as both filters and as crystal replacements in oscillator circuits:
  • The resonant impedance of 500 KHz ceramic resonators seems quite low.

  • These devices do not require very much feedback drive when used in oscillator circuits. In fact, if you provide too much drive they tend to oscillate on overtone frequencies...sometimes multiple frequencies at the same time.

  • With minimal drive in oscillator circuits these resonators seem to have stability characteristics that approach that of more traditional crystal resonators.

  • By putting 2 ot 3 of these units in parallel in a VXO arrangement it is possible to pull the operating frequency by 30 or more KHz.

  • With 4 of these resonators in a ladder filter configuration, it required 0.002 mfd of capacitance to make up a respectable 3.2 KHz wide IF filter. By adding even more capacitance we were able to make much narrower filters for CW operation. Values of 0.004 mfd resulted in quite nice sounding filters of around 1.4 KHz bandwidth.
As a result of these tests, it now seems that the 500 KHz ceramic resonators will play a significant role in design of my 500 KHz DC receiver building blocks.