[TenTec] Roofing filters

[K3BU at aol.com]

In a message dated 12/24/03 10:12:56 PM Eastern Standard Time, 
K3MIY at CSONLINE.NET writes:

>>Please explain the term "roofing filter". What type of filter is it, and how
does it get its name? How does it differ from the standard filters with
which I am familiar?<<

It doesn't, "roofing" name is more associated with how the filter is used. 
Most of "general coverage" rigs use up conversion into the VHF region for the 
first IF and the first, "roofing" filter is typically around 20 kHz wide in 
order to allow FM and other signals to pass. You can't get too good (steep) curves 
at those frequencies, so those filters serve as a front line (wide as a barn 
door) selectivity, but generally too wide for CW and SSB. Then you have second 
mixer and 2nd IF around 8 - 9 MHz with good "standard" filters, followed by 
the 3rd or 4th IF around 455 kHz and 24 kHz or so, where DSPs come in.

Older radios (Drake R4 series) had tunable preselectors for out of band 
strong signal rejection and the first filter was also a "roofing" filter but a real 
selectivity filter. So Orion "new" technology is really back to good old low 
IF (9 MHz) and by using proper (roofing) filters to achieve maximum 
selectivity for the particular mode, you get better rejection of strong signals that end 
up outside of the filter's bandpass curve. Having good mixer on the front of 
it helps with magic of being able to receive weak signals next to the strong 
ones that are pushed outside of the filter's bandpass. The idea is to get the 
maximum selectivity, sharpest filters as close to the receivers front end as 
possible, to prevent strong signals getting into the active devices in the 
receiver path from being overloaded and causing intermods and junk.

Yuri, K3BU.us