[HBR] Re: filters

[Hopperdhh at aol.com]

In a message dated 5/17/2006 7:05:57 A.M. Eastern Daylight Time,  
waltah at earthlink.net writes:


And  certainly a bargain at $5 each.   However the bandwidth isn't  
stated.   

If you get one of these for an HBR project  (and the bandwidth 
proves suited to your planned use) then one could build  it as a 
single conversion receiver.   Just put the filter after  the 1st mixer 
and use two 1650 kcs IF stages.  Ditch the second  conversion and 
the 100/85 kcs IFTs and all  that.





If anyone knows the BW of these filters, please let us know.
 
Here's some more food for thought: I made a successful AM  filter for my 
Collins 75A4 (IF=455 KC) using 2 tiny ceramic filters out of  a er.. solid state 
CB radio.  The radio was a Delco with a matched pair of  filters.  I used 
matching networks into and out of the ceramic filters  each made from one coil of 
an IF can with tapped capacitors for getting a match  to the 2K impedance of 
the ceramic filters.  The insertion loss of this  combination is much lower than 
the SSB mechanical filter, and performance is  great on AM.  This still would 
require double conversion for bands above  perhaps 80M, but it is an easy way 
to get decent AM IF selectivity.  Old CB  radio circuit boards are a dime a 
dozen at hamfests.
 
My vote for the best receiver of all time, by the way,  is NOT the  75A4.  It 
would have to go to the Drake 2-B.  I remember the first one  I ever heard.  
It was at a local ham club meeting when I was in high  school.  It was out of 
the cabinet, and one of the old timers there said,  "Boy, that's about a 
gutless wonder, isn't it?"  Just recently on the air I  heard a comment, "Every ham 
should own a Drake 2-B."  Bob Drake was a  filter guru, and studying the 
circuit of any of the old Drake  receivers (1-A, 2-A, 2-B or 2-C) is an education 
in filter design --  not just the 50 KC filter, but throughout the whole 
receiver.  I once  dissected the 50 KC filter from my 2-B, and made a detailed 
schematic of it.  (Details are not shown in the receiver schematic.)  With  
today's circuit modeling programs it would be interesting to plug  in the values, 
and see its bandpass shape in each selectivity position.  A  50 KC filter along 
the lines of the 2-B filter could pretty easily be built with  the high-Q 
cores available today.  That's the direction I'm leaning toward  for my HBR 
receiver project.
 
Dan K9WEK