You may be a little confused about filters in receivers and the ways they
work.  A roofing filter goes after the first mixer and provides some
selectivity before the signal is converted to a second and/or third
intermediate frequency.  I'd have to look at the FT450 manual in detail, but
it has many similarities to the 950 that I use.  The FT950 has a first
intermediate frequency of 69.455 MHz.  In that part of the circuit, a 15
KHz, a 6 KHz or a 3 KHz roofing filter can be switched in.  Because this is
a rather high frequency for crystal filters, a narrow bandwidth cannot be
achieved.  On my 950, I can hear some difference between the 15 and 6 KHz
filters on AM, but none between 6 and 3 KHz.  A roofing filter mostly guards
against interference many KHz away from the frequency you are tuned to.
When installed at a 69 MHz IF, it cannot be made narrower than a few KHz.  I
would be very surprised if you could hear the difference between a 10 and a
3 KHz roofing filter while on SSB or CW.

Rigs like the Elecraft K3, Tentec Orion and Yaesu FTDX-5000  have positions
for roofing filters at a 9 MHz IF, and these can be quite narrow.
Unfortunately, the FT450 CW selectivity cannot be adjusted narrower than 500
Hz, which is one of the reasons I didn't buy it.  You might get a narrower
filter for CW by obtaining an outboard audio filter.  This isn't as
effective as having the bandwidth set in the rig itself, but it might be
helpful in some CW situations.

You can write me off the list if you have more receiver questions or I'm not
understanding what you're trying to do.
73,
Lloyd Rasmussen, W3IUU, Kensington, Maryland
[log in to unmask]
Home:  http://lras.home.sprynet.com
Work:  http://www.loc.gov/nls
 

> -----Original Message-----
> From: For blind ham radio operators [mailto:[log in to unmask]]
> On Behalf Of Michael Ryan
> Sent: Wednesday, September 22, 2010 12:33 PM
> To: [log in to unmask]
> Subject: Narrowing External Filter
> 
> Hi List:=20
> 
> I'd like to add an external filter to narrow up the SSB and CW.=20
> Right now, my FT-450 has only one roofing filter, at 10K and I'd like to =
> add a device that will give me the equivelent of the 3K roofing
> filter.=20=
> 
> Anyone have any ideas on what comes close that I could take a look at?=20=
> 
> 
> TNX & 73:=20
> 
> Michael DE VO1RYN=20