I completely agree with all of that except for the very last sentence.
Clearly you've never used a 756Pro (first version) in a major CW
contest. I can remember very loud stations 10 or 15 KHz away completely
desensing my receiver to the point that the station I was trying to copy
simply disappeared. When I switched to the K3 the difference was
astounding.
73,
Dave AB7E
On 5/11/2014 2:03 PM, Al Lorona wrote:
What determines the bandwidth you hear at the loudspeaker? It's not your
roofing filter, despite a continuing notion that it is.
Dave Hachadorian's point in a post a few weeks ago was that you don't need a 1.8 kHz filter to get a 1.8 kHz bandwidth. You're free to set whatever bandwidth you want with any filter.
Before rigs had DSP we got used to the idea that your crystal filter sets your
bandwidth. That's not true any more. It sets your *maximum* bandwidth. You then
have the freedom to narrow and position a bandwidth arbitrarily using the DSP
controls [SHIFT and WIDTH or HI and LO].
Here's a true-false quiz:
1. I'm a contester, so I need a 1.8 kHz roofing filter in the K3.
2. I should purchase the 400 Hz filter if I like to operate CW with bandwidths of 300 - 400 Hz.
3. For SSB, the 2.7 and 2.8 kHz filters are 'too wide'.
4. I have the 2.7 kHz filter installed, so for best results I should set my WIDTH control for a passband of 2.7 kHz.
5. I can use my 2.7 kHz filter in CW mode with my LO=0.30 and HI=0.50 (that is, BW=0.20).
The answers are:
1. False. You do not need a 1.8 kHz filter just to set the BW=1.80. A 2.7 kHz filter can serve well during a contest with a much narrower DSP bandwidth. Refer to Dave's original post.
2. False. You can set the CW bandwidth to 400 using any filter whose bandwidth
is equal to or greater than 400.
3. False. This was Dave's point. You're free to have a 2.7 or 2.8 installed,
yet set the WIDTH to 1.8, 1.5 or anything else you wish.
4. False. You don't need to restrict yourself to only that bandwidth. You can
set it to a narrower value if you wish.
5. True. And you'll probably suffer no ill effects under most conditions.
Furthermore, the "but extremely strong signals will pump my hardware AGC" arguments are probably a bit overrated. Most folks, even before a strong station gets close enough to do that, will give up and leave the frequency because of the QRM, especially in the presence of transmitted phase noise or key clicks as has come up in more recent posts.
So then why have narrow roofing filters to choose from? To maximize the
close-in dynamic range, which is important if you have large antennas in
high-RF environments.
The vast majority of hams does not absolutely need really narrow roofing filters. It's wonderful that the K3 allows this, but it's certainly not mandatory, especially for casual operating.
Finally, note that if you received good training as a Novice with a poor, unselective receiver, you'll be able to copy right through any AGC pumping! It's the operator, more than the filters.
Al W6LX
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