roy@phri.UUCP (Roy Smith) (09/04/85)
Ever wonder what was in those "signal splitter" boxes you can buy in Radio Shack so you can run VHF, UHF, and FM through a single antenna feed? I always figured there must be some matching transformers and a few bandpass filters. Well, the other day I opened one up (a couple of not-too heavy whacks with a hammer on the sealed plastic case did the job nicely) and was a bit surprised to find *NO* components except two 150-ohm 1/8th watt resistors going from a little PC board to the FM terminals. The other terminals just fed into traces on the board. This was, BTW, a Gemini model #TV-50; looks like your generic el-cheapo signal splitter with a Gemini label stuck on it. Upon closer inspection, it looks like there really *are* some filters -- the reactive elements being made up entirely of copper traces on the board! There are 3 each of the following type patterns: | | +----------- | -----------+ +---------+ +----------- | | +-----+ | | -----------+ | | *-+ | | +----------- | | +---+ | | | ---------+-+ +-------+ | +------- +-+------- The one on the left is obviously a capacitor, the one on the right an inductor (the "*" is a pad to which a jumper is attached). I suppose I could measure the X-Y dimensions with a ruler (overall cap size is about 1 x 2cm with 13 "plates", inductor is about the same size, with 3 full turns). I could get the copper trace thickness with a micrometer (for the area of the plates). Given that I could probably come up with a fair estimate for the cap value, but I'm not sure how to even begin the inductance calculations. Not having done much RF work, I don't know if this is a standard trick or something new, but it sure impressed me. I haven't made any serious attempt to figure out the values, but they must be small, I wouldn't guess more than a few pF for the capacitor. -- Roy Smith <allegra!phri!roy> System Administrator, Public Health Research Institute 455 First Avenue, New York, NY 10016