455khz IF Filters Part 3

So i have these LTW455IT ceramic bandpass filters in my parts box, 455kHz Bandwidth +/-2kHz at 6dB with 6db insertion loss. So i thought i would plot out its response to see how it looks, and to be honest it is very close to spec, but a little bit high in freq, centre should be 455 + or – 1khz, mine showed a centre of 457khz, 2khz higher, now i did not impedance match to 2K ohms, but i probably should have. (BLUE Graph Line)

murata-ceramic-1

filterplot3

 

murata-ceramic-0

So i thought to myself, if one is good, two must be better and then plotted out the response (RED GRAPH) The added capacitance pulled the freq down closer to 455khz. Each filter is 6th order so this essentially made a 12th order filter, I kind of expected the skirts to steepen, but they did not, I do not know why, but i would like to find out.

murata-ceramic-2

Measured with the signal generator on the input and scope on the output with the grounds connected together, isolated from the filter grounds. I did connect the filter grounds together but was getting strange readings, i think you have to use dc blocking caps on the output if you use the inbuilt ground pins.

Anyway, it was a little bit of fun, that did not amount to much other than one of these filters would make a nice narrow SSB filter for a tranceiver that has a 3khz or larger front end bandpass.

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