A Story Of Two Crystal Filters

While waiting for parts and boards to arrive for the main things I am working on, I thought I would turn my attention to the age old problem of crystal filters. Its not like you can easily buy filters off the shelf these days and even finding suitable crystals can be a problem. The low profile half cut crystals can be hit and miss and full size crystals can be expensive. Hayward wrote a few papers not long back on using 9Mhz crystals in simplified designs and pretty much if you buy 9Mhz 30pf crystals you should be able to duplicate his work with minimum effort. Problem is,  9Mhz crystals are non existent in Australia even in the low profile cases and they are $1 or more each in quantity on mouser. Screw that Jacko.

I have had mixed results making home brew filters, but this one looks the business. Other than being 4K wide, it looks like a filter should look. Fixing the width is just a matter of changing the capacitor values. The second filter is 600Hz wide for CW, thats just about where I like it, but its got -10dB loss, not sure the issue just yet but my gut feeling is that its one of the crystal is off from the others. I did measure them all, but you know, failure is always an option. I am going to suck all the crystals out and start with a fresh batch of 8 and see if that is the problem and then change the caps on the SSB filter to narrow it up some. I hope to have 2 very usable filters.

Typically i fix things until they are broken, but hey this actually looks alright. I need to narrow up the caps some, but now its not all loss and no stop band. -60dB rejection is more and good enough for the kinds of girls I go out with.

Facebooktwitterredditpinterestlinkedin