Posted by
Bill VanAlstyne W5WVO on
Oct 24, 2010; 1:58pm
URL: http://elecraft.85.s1.nabble.com/2-8kHz-roofing-filter-tp5665995p5667708.html
Given the fact that the filter passband shapes aren't absolutely flat, as
Bill W4ZV pointed out, you can't expect PERFECT balance between the
sidebands. But you CAN tweak a lot of this audible difference out by
adjusting the filter offset using the K3 Utility, as has been stated
earlier. Even the 8-pole filters can show some required offset in any
specific K3. And the specified offset for a 5-pole filter can benefit from a
little tweaking, depending on which K3 you put it in. I have a 400 Hz
8-pole, a 1.8 kHz 8-pole, and a 2.7 kHz 5-pole. Of the three, only the 1.8
kHz came out pretty close to where it was supposed to be (only -0.01 kHz
offset applied made the sidebands sound pretty much the same). The 400 Hz CW
filter required an offset of -0.11 kHz to make CW and CW-R sound the same,
and the 2.7 kHz 5-pole filter, which was marked with an offset of -0.90 kHz,
actually showed a better audible center with an offset of -1.09 kHz in my
particular radio.
An earlier poster on this (or a related) thread explained how to do this
without test equipment, using just the K3 Utility, the antenna noise floor,
and your ears. You can search for that post in the archives.
Now, in tweaking the offsets using this by-ear method, it's entirely
possible that you're actually shifting the filter a little past center to
compensate for the filter's unflat shape. I can see an argument that this is
not a good thing to do, as well as an argument that it is a fine thing to
do. :-) I'm of the latter persuasion, at least at the moment,
notwithstanding any future arguments to the contrary.
Bill W5WVO
-----Original Message-----
From: Bill W4ZV
Sent: Saturday, October 23, 2010 20:51
To:
[hidden email]
Subject: Re: [Elecraft] 2.8kHz roofing filter
ab2tc wrote:
>
> In USB there is a definite upward tilt of the passband and the opposite
> for LSB. This is quite audible. Is this normal and is there anything I can
> do about it (other than asking Elecraft to add by mode equalizer)?
>
Probably nothing you can do other than tweaking EQ per mode. It's caused by
the filter having an actual amplitude tilt in its passband. In USB the
upward tilt is on the upper side and in LSB it's on the lower side. I don't
think you would notice the small 8-pole offset (typically 50 Hz) in a 2.8k
filter, but they are more noticeable in CW filters.
73, Bill
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