On 15/02/17 08:52, Michael Black wrote:
On Tue, 14 Feb 2017, Ralph Mowery wrote:
In article ,
says...
I recently saw a notice that Rockwell Collins has stopped production
of Mechanical Filters.
Many of the newer radios are starting to use electronic circuits or more
likely software fro the filtering now.
Many newer radios do not have much RF circuity in them, mainly
microprocessors doing most of the work. This trend started about a
dozen years ago and is progressing more and more every year.
Instead of being limiated to just 2 or 3 filters (that often cost around
$ 100 each) you just turn a knob or move the mouse and set up any type
of filtering you want.
That's why in the past, some receivers went down to a 50KHz IF, if you
wanted a selection of filters, it was cheaper using LC circuits down
there than a bunch of crystal or mechanical filters at some higher
frequency.
So long as there is nothing 100KHz away from your desired signal,
and no two signals spaced 50KHz apart that intermodulate in your
RF front end. Imaging is a killer. Double conversion can help, but
nothing beats RF selectivity.
Some of the modern receivers are taking advantage of the availability
of devices with much better linearity and headroom, which reduces
the intermod - and they're using I/Q conversion to defeat imaging -
but they cannot achieve the performance of a good communications
receiver.
Software hacks love SDR because it makes them feel like RF experts,
but they're not.
Clifford Heath