On Sat, 22 Mar 2008 23:53:51 -0400, "Tio Pedro"
wrote:


"Michael Black" wrote in message news:fs4hu4
That term applies to a filter used after the first mixer to limit
the passband but which doesn't provide ultimate selectivity.

My point was that I wonder if you can get a few KHz of selectivity
up there. Otherwise, we'd surely see it in commercial equipment
and even home made. All the design books decades ago would say the
best design is to put the ultimate selectivity right after the first
mixer of a receiver, yet as receiver design changed to put the first
IF in the 45 to 70MHz range, I've never seen an instance of a narrow
filter right after the first mixer.

I don't know whether it can be done, but the fact that it isn't done
suggests there are limitations.

The filters in a cellphone would be wide enough for FM, and given that
all the cellphones I've seen convert to a lower frequency where there
is another filter, they probably are relatively loose filters up at
45MHz or so.

MIchael VE2BVW

Michael,

I am not up on this, but I believe narrow BW roofing filters are now
being marketed for many rigs. InRad has some, and I am pretty sure
TenTec offers them.

Pete


Pete,
The Ten Tec Transciever filters are at 9 MHz except in the "general
coverage up convert" receivers. in which case they are not narrow at
all.

Note that INRAD offers roofing filters 4-5 KHz wide at VHF frequencies
from 40 to 73 MHz ( see page 13 of their catalog ) with i/o impedances
of 50 ohms : http://www.inrad.net/inrad.php?mode=catalog

Ed, N5EI