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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 Cromarty 

The 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