Posted by
Guy, K2AV on
Nov 14, 2004; 5:02pm
URL: http://elecraft.85.s1.nabble.com/Talking-filter-Bandwidths-tp371577p371582.html
Most of the issues you address are seen in the sweep. The problem that
all the content and implications of the sweep can't be put in a 42 or
45 character item name in a catalog. Where there is a picture in a
catalog, the sweep curve would be more meaningful than a picture of
the filter itself. But I'm not holding my breath.
But IMHO the number one function of a filter is to pass the desired
signal and reject all off frequency, nothing in second place. That
would make the 6db point the better characterization of the filter,
more representative of the beginning of the stop band, IF one MUST be
stuck with a single number.
And that, precisely, was what was so disappointing about the Collins
"500 Hz" mechanical filter, that it was a 610/1630 Hz filter 6/60 when
I got around to measuring it (INRAD says 595/1560 on their web page).
A very broad 2.6 shape.
For CW contest operation, a station up or down the typical 500 Hz, is
only reduced by 36 db, as opposed to ~65 db using INRAD's 500 Hz (6
dB) filter. -36 dB not much help copying an S0 or S1 signal 500 Hz
away from a 20 over 9 signal.
I personally would like to see something like BW 595/2.6 in listings,
particularly next to an alternative listed as 510/1.74.
One presumes (more than hopefully?) engineering and manufacturing to
current standards, that is pass band loss and impedance to specs, and
reasonable ripple, and pass band shape for frequency response and
delay appropriate to use. In any event, the few dozen characters of a
filter description could never address the engineering quality issues
you so rightly list. If they did, how would it read?
Cat #1234, 455 center 500 Hz bw, poor quality, cheep
Cat #3456, 455 center 500 Hz bw, better quality, best buy.
Cat #5678, 455 center 500 Hz bw, steep skirts, transparent passband,
Rolls Royce, only the rich need apply.
Maybe this is one of those food labeling law issues, that all ads have
to include certain numbers and you can get slammed for telling lies.
As to overdriving, for many rigs that is a roofing filter issue due to
their choice of IF's, AND one of the many reasons why the Elecraft K2
design works so well.
73
> Totally agree with you about sweeping filters to obtain their
> responses. It
> can be intersting when the stopband is down 90 - 100db, because at
> no point
> must the filter be overdriven. These are hombrew filters. I wish
> that filter
> manufacturers would specify their filters signal level limits,
> because too
> often receiver designers forget that crystal filters especially can
> be
> overdriven and introduce distortion. But perhaps if the
> manufacturers did,
> some might not sell any filters!
>
> 73 Geoff GM4ESD
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