Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!watmath!clyde!burl!ulysses!mhuxj!houxm!ihnp4!zehntel!hplabs!sri-unix!andy@aids-unix From: andy%aids-unix@sri-unix.UUCP Newsgroups: net.ham-radio Subject: Re: Why is 250 Hz filter CHEAPER than 500 Hz? (ICOM R71A) Message-ID: <12783@sri-arpa.UUCP> Date: Sun, 7-Oct-84 17:46:15 EDT Article-I.D.: sri-arpa.12783 Posted: Sun Oct 7 17:46:15 1984 Date-Received: Thu, 11-Oct-84 05:47:06 EDT Lines: 33 From: Andy CromartyThe cost of a filter is usually less dependent on its bandwidth than on (a) its shape factor (ratio of the 6dB and 60dB bandwidths) and (b) the IF stage (more specifically, the frequency) for which it was designed, at least for a crystal filter. This is because for any given frequency the crystals cost about the same whether they're 250 or 500 Hz apart; the thing that costs more is *more* crystals (aka more "poles"), which does not alter the 6dB bandwidth but improves the shape factor. Improved shape factor corresponds to an increased ability to select against strong stations on a nearby frequency. In addition, filter cost varies with frequency, since it is generally harder to build an IF stage with good gain and selectivity at a higher frequency -- that's the reason most superhet receivers use a low-frequency IF stage somewhere along the way, most often at 455kHz. (They use the high-frequency IF stage(s) for image rejection.) This would predict that the more expensive component has a better shape factor and/or is for a higher IF frequency. If that isn't the case, then I'd suspect either (a) different filter technology (there are other sorts of filters, such as ceramic or mechanical filters for low IFs) or (b) an artifact of the production process, such as improved production efficiency through automation (the sort of thing that Yaesu claims is responsible for the remarkably high feature/cost ratio for the FT-757GX transceiver) or larger volume production of one filter than the other. [Corrections and additions are invited.] 73, Andy N6JLJ