Erik_Moe@raider (Erik Moe) (02/29/88)
I am looking for some sources for a high-Q notch filter used in cable
systems. I have heard of someone building one from a coffee can and
some copper tubing, but I never got the plans for such a system. If
anyone has some ideas let me know.
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+----------------------------------------------------------------------+dmt@ptsfa.UUCP (Dave Turner) (03/05/88)
In article <0.2228E72A@raider> Erik_Moe@raider (Erik Moe) writes: >I am looking for some sources for a high-Q notch filter used in cable >systems. I have heard of someone building one from a coffee can and >some copper tubing, but I never got the plans for such a system. If >anyone has some ideas let me know. > I've read about coffee can notch filters too and designed one that might work. I haven't found a reference on how to make helical notch filters so I had to guess at how to make a shunt filter. I want to use a capacitor to tune the filter. The design references say to use f = 1.5*fo. The frequency to be removed is halfway between the video carrier and the sound carrier. Add 2.25 MHz to the video carrier or add 3.5 MHz to the lower edge of the TV band. For channel 3 the filter should be tuned to 63.5 MHz. Here is the design: ------------------------- | | | | | C | [[[[|-----------------------|]]]] | CCC | | CCC | | H | | HHHHHHHHH | | HHHHHHHHH | | HHHHHHHHH | | HHHHHHHHH | | HHHHHHHHH | | HHHHHHHHH | | HHHHHHHHH | | HHHHHHHHH | | HHHHHHHHH | | HHHHHHHHH | | HHHHHHHHHHHHHHHH| | | | | | | | | | | | | The above is trying to show two F connectors connected with a small air variable capacitor (C) midway between the connectors. The helix (H) is connected to the capacitor and the side of the can. This is supposed to show a shunt L-C filter. The dimensions the helix are calculated by: Qu = 50D(fo)**.5 squareroot of fo N = 1900 / (fD) b = B - D/2 Zo = 98000 / foD where Qu is the unloaded Q, D is the inside diameter of the can B is the inside height of the can d is the diameter of the helix b is the height of the helix N is the number of turns in the helix f is 1.5 * fo fo is the resonant frequency Zo is the impedance of the helix. Based on the design formulas for helical filters and the dimensions of a 1 lb coffee can (3.875 inches in diameter by 5.1 inches high). Assuming 3/16 inch tubing: Wind 5 turns on a 2.0 inch dia. form. Space the winding over 3.1625 inches. Connect the bottom end of the tubing 1.0 inch from the bottom of the can. Install the F connectors 1.0 inch from the top of the can. Install a copper strip between the F conectors. Install a small air variable capacitor in the center of the copper strip. Connect the top of the helix to the capacitor. Tune to 63.5 MHz. The above will give: Qu = 1544 @ 63.5 MHz N = 5 (5.15 really) d = 2.1875 in. b = 3.1625 in. Zo = 398 ohms. You'll need about 5 feet of 3/16 inch copper tubing. I haven't tried this but it might work. More references are in the ARRL Handbook and Reference Data for Engineers (Sams). If you build it, let me know what happens. -- Dave Turner 415/542-1299 {ihnp4,lll-crg,qantel,pyramid}!ptsfa!dmt