Homebrew 455khz Ceramic IF Filter

So today after reading some more of SolderSmoke – Global Adventures in Wireless Electronics” and feeling somewhat enthusiastic about advancing some projects i went out to my work bench and soldered up a test rig used to determine the motional parameters of crystals and in my case, ceramics.


Circuit went together rather uneventfully, built in what i call my Ugly Manhattan style more slum lord millionaire than trump towers LOL. But it works and it does the job it was designed to do. And thats all that matters. Schematic can be seen in the above picture, but there is a better one to follow.


So i plugged in the first ceramic and took a measurement with the switch open and as expected, the 455khz resonator gave a nice readout of 455.68khz. About what you would expect.


Next we hit the switch, or in my case, pushed the bit of solder wire onto the groundplane with a pen and got the second reading. 448.12khz, now with these 2 frequencies in hand i could plug them into some software and get the values for Lm and Fo which are needed to design the filter.


With the two values i plugged them into Ladder Crystal Filter Design and got the values for Lm = 128.6mh and Fo = 447.2085khz which i then plugged into Dishal to design the filter as i like its interface that little bit better.


Immediately i saw a huge issue, the centre freq is all wrong, its way to low, if this was going into a homebrew rig, that would not be so much of an issue, just change the frequency of the LO. But, dropping this into my ICYAWOOD is not going to work, the IF there is right on 455khz, and so that is also where i need my centre frequency. Not sure what to do at this point. I do have some resonators that are closer to 458khz with their resonate frequency, they might have a better chance, but im thinking the low Q of these ceramics is the real issue and even if i come up a few khz, that is still not going to get me closer to 455khz. Back to the drawing board for now.


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