David and All,
Not necessarily! I think Bill is pretty much on point since that is the
posted spec for the 250 hz filter (not just some random test). I am led to
believe these filters do not vary widely in manufacturing. So it would
appear that a 400 hz/250 hz combinations is not a particularly good
combination. The 200 hz may well be a better choice if you want to go that
narrow. I have the 400 hz/200 hz combination, but I find I don't use the
200 hz filter nearly as much as I thought I would. I think that is
primarily due to the fact I just don't seem to need to go that narrow, which
speaks well for the K3 generally. I have used narrow filters a lot in other
radios, but those were audio filters, not roofing filters. The DSP in the
K3 seems to provide just about any additional filtering I need while using
the 400 hz filter. Digital ops may actually find the 200 hz filter more
useful. But I haven't really given my K3 a good baptism under contest fire.
I missed the CQ WPX contest last week, and that would have been a great test
for the narrow filter I think.
I can also tell you from my experience that you need to set the 200 hz
filter (and presumably the 250 hz filter) up with more gain added than they
recommend in the manual. There seems to be a very noticeable reduction in
signal level when the 200 hz filter is engaged. I believe this is on the
"to do" list that Wayne has--to improve this signal level issue.
I think Bill has analyzed the filter combination issue more than just about
anyone. You may want to go back through the archives and read some of his
prior posts to get some good insight about selecting various filter
combinations. He may not be "the last word" on the subject, but I think it
will give you added perspective.
Dave W7AQK
----- Original Message -----
From: "David Woolley (E.L)" <[EMAIL PROTECTED]>
To: <elecraft@mailman.qth.net>
Sent: Thursday, May 29, 2008 3:03 AM
Subject: Re: [Elecraft] K3 Filter selection
Bill W4ZV wrote:
400 and 250: A waste of money IMHO. Since the 400 is actually 435 and
the
250 is actually 370, there is only 65 Hz difference in this combination
and
you'll probably never notice the difference between them (370/435 = only
15%
narrower). Total cost $250.
I think you are generalising from a single sample.
It may well be the case that to guarantee a bandwidth of at least 250 Hz,
without doing expensive select on test procedures for the capacitors, one
will have some samples at 370 Hz.
What you may be getting is 250Hz with a given shape factor, but individual
units may achieve that shape factor by going flat to a higher frequency
and then cutting off much more sharply.
--
David Woolley
"The Elecraft list is a forum for the discussion of topics related to
Elecraft products and more general topics related ham radio"
List Guidelines <http://www.elecraft.com/elecraft_list_guidelines.htm>
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