[TenTec] Filters, amps and IMD, etc

[Bill Tippett]

K1FK wrote:
 >Crystal filters can and [DO] contribute towards IMD!  I is not just the 
additional amplifier in the
Orion that should be questioned in regards to IMD when selecting the 
250/500 Hz filters!

         Absolutely correct Dave.  To this day
I do not believe Ten-Tec understands exactly
why the 500 Hz behaves the way it does.  My
web page says "apparently caused" by the extra
amplifier stage, but in fact nobody knows for
certain whether it is the amplifier, the filter
itself, something else or a combined effect.
As Jack Burchfield told me himself, this whole
issue of IMD is "black magic".

         There's a funny story associated
with this.  Shortly after I sent out the 18
original #352 kits in April 2004, one guy
sent his to Rob Sherwood for testing.  The
results were TERRIBLE and I felt badly for
having apparently caused 18 people a lot
of grief for nothing.  I then sent Rob my
filter (using SMD caps) and Barry N1EU sent
his (using disc caps).  The reason I wanted
both types of caps tested was to see if the
type of capacitor could possibly be causing
the poor IMD results.  Rob's measured IMD
results from both filters were consistent
and excellent (WHEW!), so we concluded the
first filter he tested had been improperly
constructed and the type of capacitor used
had no effect.

         I've also had some experience checking
several used TT filters.  I've been very
surprised how much they vary for a simple
thing like insertion loss.  I believe there
are more bad used filters circulating around
than we realize.  If you buy a used filter,
be sure to at least check its insertion loss.  IMD is
another issue entirely and it is very possible that
some filters may have poor IMD.  There is really
no way to tell except to preferably test it in a lab
or observe it under extreme contest conditions.

                                 73,  Bill  W4ZV