I have been corresponding with a filter expert this year and here is
what he has to say:


Hi Mike,

The curves I've plotted on the LF-C6 and LF-C2A would
seem to indicate that the number in NTK filters is the bandwidth so
LF-D6 would indeed be 6 kHz filters. The 6 kHz is a minimum, however.
The LF-C6 I have plotted on my website is actually about 9 kHz wide.
The Murata CFK455I's spec'ed at 4 kHz measure about 6 for the several
I tested. So ceramic filters spec'ed at 6 kHz would probably be 8-9
kHz typical, a little too wide for DXing' although now that I've found
the spec's, perhaps useful to replace the "wide" filter in a rx with
one that's still wide, but better shape factor.

See my earlier post for links to NTK specifications.

The LF-D6 is spec'ed at 6 kHz @ -6 db and 15 kHz @ -70 db.
Interpolating, that's probably 12.6 kHz wide at -60 db, about a 2:1
shape factor. The LF-D series seems to be the the best of the
different series that NTK made.


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N8KDV wrote in message ...
Mike Maghakian wrote:

one myth I believed for about 25 years was that a filter rated at 6KHz
at 6db had a 6 KHz bandwith at 6db. that is not true.

Yes it is true! If one looks at the plot of the response of the filter,
and if it is 6 kHz wide at the -6db point then that's exactly what it is,
6 kHz.... at the -6db point... not 8, not 12, not 4....

the 6KHz is the
BEST you can get and you rarely get that. the usual is 8Khz.

Show me the plot you are looking at to come up with this. How do you come
up with 8 kHz?

the
ceramic filter I sell is RATED at 6KHZ at 6db

How can that be? When clearly it is 8.27 kHz at -6db, (per the plot).
Where are you getting these 'ratings' from? That is, the 6 kHz 'RATED' at
-6db, the plot clearly does not show that.

but in actuality it is
around 8KHZ, this is not unusual.

Not unusual at all, because at the -6db point it IS around 8 kHz.

I have done a lot of research this
year and am surprised at how much deception there is out there.

Where is this 'deception' and 'trickery'?

and
how pathetic are most of the ceramic filters in receivers costing up
to $1000. the Sat800 is the best low cost receiver in terms of
selectivity that I have ever seen and it cost only about $400, truely
amazing.

Nothing amazing at all about it. If a filter is rated at the -6 and -60 db
it's easy to tell what the selectivity and shape factors are.

Some confusion might exist if a different set of figures are used.