On Sun, 20 Jul 2014, dave wrote:


Inrad doesn't stock 5-pole filters, but I get it. My ham HF/6m transceiver IF
is 8215 KHz, an aeronautical or military band as I recall. There might be a
big pile of old Bendix or Collins radios somewhere full of such crystals. The
best filters I used for SWL DXing were the Collins Torsional Mechanical BPFs
in my R390A. Unfortunately they had a 90 pound 26 tube infrastructure
required to use them. The Sony 2010 had fairly good ceramic filters when used
with the synchronous detection. (The K3 has synchronous AM reception, done in
DSP). The good old days of strong backs and cheap electricity are long gone.
If I was a rabid SWL I'd get a K3/10 with the General Coverage input filter
set. Otherwise something direct conversion DSP that doesn't need Windows. I
make do with an HF-150, which is the opposite of filtering.


That's the advantage of doing bulk buying. If you have to order filters
in large enough quantity, you can pick the frequency, and thus maybe
arrange it to limit the number of spurs. A lot of commeercial ham
transceivers had odd frequencies, I think I have a selection of SSB
filters that I've gotten really cheap at hamfests. I can't imagine what
else they'd be for on such odd frequencies (AM and FM tended to
standardize, even for two way radio work). But sadly, without any BFO
crystals, those filters can't have much value even if they are SSB
filters, I'd still have to put money into ordering crystals for the BFO.

The few remaining desktop receivers are the ones that may allow for
mechanical filters. They at least have the space, so if they dont' come
with a mechanical filter, they may have that as an option.

I have a TS-830S ham transceiver, and it has a filter in the HF range,
then one down at 455KHz. I seem to recall the default 455KHz was "okay"
but the option was there to put in a better filter.

I seem to recall the R390 had a 500KHz IF, which meant the model in the
line that had a mechanical filter (not all of them did), you couldn't
reuse it somewhere else. The ony mechanical filter I have is at 250KHz,
in an RCA mobile SSB transceiver; it was intended to be trunk mounted, all
those tubes.

But even if you had a 455KHz mechanical filter lying around, many of the
portable receivers of recent times use a 450KHz IF, it having to do with
the math and the synthesizer IC used. I know for a long time I watched
for an SSB CB set cheap, thinking if it was dual conversion the 455KHz
ceramic filter would be a nice one. I finally found one last year, for
five dollars, complete with a mystery switch on the back. But, it's ony
single conversion, a crystal filter in the HF range. Worse, it seems to
be a filter that isn't so narrow, since the same filter is used for SSB
and AM.

Then there was the modification going around about 20 years ago for the
Sony 2010, adding a mechanical filter (or maybe it was two mechanical
filters, one for each sideband?) since while the phasing method detector
did okay getting rid of the opposite sideband, signals in the IF passband
could still pump the AGC.

Mechanical filters are one thing that seems to hold their price. Even
used, they are highly valued. There was that period when Japanese imports
would include mechanical filters, those Kyocera? ones that had foam
inside, and over time the foam would fall apart (just like the wheels on
my Major Matt Mason vehicle). But that same period, the Radio Shack
DX-150 was said to have a mechanical filter, which seems unlikely, too
cheap a receiver. I suspect in some cases, something was lost in
translation, and on those cheaper sets (I remember some Lafayette CB sets
claimed to have "mechanical filters) it was really just ceramic filters,
which were a new thing at the time. One could even argue ceramic or even
crystal filters are mechanical, since they do vibrate, just not the same
way mechanical filters are mechanical.

Michael