Mike Maghakian ) on 8/4/03 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. the 6KHz is the
BEST you can get and you rarely get that. the usual is 8Khz. the
ceramic filter I sell is RATED at 6KHZ at 6db but in actuality it is
around 8KHZ, this is not unusual.

I suggest that you read the spec sheets. Most ceramic filters are specified as
having a minimum bandwidth at -6dB and a maximum bandwidth at -60dB. In the
case of higher quality crystal and mechanical filters, you will often find a
typical values given as well.

Typically the ratio of max. bandwidth at -60dB to min bandwidth at -6db is
2.5:1 to 3:1. For example in the case of the LF-D6 which you sell, the ratio
is very close to 2.5:1. The shape factor of a better quality ceramic filter is
typically 1.5:1. This allows a lot of tolerance in the -6dB bandwidth. Again
using the LF-D6 as an example, and assuming a 1.5:1 shape factor, the -6dB
bandwidth could be anywhere from 6.0+ to 10.0- kHz.

This is the way that it has been with ceramic filters for a long time. I will
spare you the trouble of looking up the Murata ceramic filter specs from 20+
years ago. They are no different from the ones that they published before they
stopped making metal cased filters within the last two years. This is very
nice from a manufacturing point of view - it is much easier to meet
specifications. The Murata CFJ series of filters were made to tighter
tolerances, particularly the 455K12, K13 and K14 SSB filters. It is just that
they were difficult for the to obtain in one-off quantities. It was also
possible to obtain actual bandwidths within any specification you wanted from
the Japanese manufacturers - you just had to pay, and buy enough of them.

BTW why do you think that the reviews in Passport to World Band Radio (PWBR)
have included actual bandwidth measurements vs. nominal specs. for many years?
It is precisely because of the broad specifications for low cost ceramic
filters. See, for example the review of the Lowe HF-225 in the 1990 Passport
p. 171.

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

This statement is simply ridiculous. The only deception is in your mind. The
devices which you obtain from Murata, NTKK or others conform to their published
specifications. They never improved their specifications because they did not
have to.

...... and how pathetic are most of the ceramic filters in receivers
costing up to $1000.

If a low cost filter satisfies your market, why put a better one in? As a
manufacturer, you could purchase filters to a tighter spec, and pay for it, or
go to crystal or mechanical filters with a large increase in cost, and a
significant increase in size.

The Sat800 is the best low cost receiver in terms of
selectivity that I have ever seen and it cost only about $400, truly
amazing.

Passport 2003 gives the measured bandwidths as 2.6, 5.8 and 7.1 kHz. Possibly
you would like to do some research and tell us what the nominal figures are -
my guess would be 2.4, 4 and 6kHz. this is not great for AM broadcast band
listening unless you tune to one sideband only.

73 John KC0GGH


PS. I suggest that you correct you web site. The LF-D6 is specified as having
a minimum bandwidth at -6dB of 6 kHz, and a maximum bandwidth at -70dB of
15kHz