9mhz IF Filter

Well i did not think i would get any bench time so soon, or feel enthusiastic for that matter, but here I am tinkering today and doing McRadio at the McBench. Anyway, i think i posted about this IF filter sometime back, I should go and look but I wont LOL, anyways its a Cohn Min Los design and what I did differently was not to use transformer matching, but rather to use LC impedance matching to get the 50ohm up to the 200 ish ohms of the filter.

Now i did also build one using transformers, the difference was, this one has less of a stop band but better shape and much more symmetrical, the other one had a better stop band but its shape was not very good at all. Greater attenuation would be nice and I think the reason for the lower stop band is the Q of the LC match, which is going to be rather low, somewhere lower than 10. After doing some reading, I thing by using a T or Pi match I can improve the Q a lot and that would improve the stop band. Its an experiment i plan to do sometime soon to see if it improves things.

This is the PCB board i laid out and had made, the xtals I used were just whatever 9mhz junk xtals I could stuff in there. No matching, nothing, just grab 8 xtals and stuff them in. The block of wood is the former I used to make the shielding can.

The shielding can is made from some 0.1mm brass sheet. I placed it over the former, clamped it in place with a g clamp then beat the crap out of it with a hammer and soldered it in place.

50db stop band is not all that great, but it should be usable. Pass band is a little lumpy, but should also be acceptable. The shape is quite nice and could be used upper or lower side no problems at all. the width is 2300hz. Good enough for the kinds of girls I go out with, as they say.


Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.