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Author Topic: Icom IC746Pro CW Filters  (Read 281 times)
9V1VK
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Posts: 15




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« on: September 27, 2005, 10:57:23 PM »

Hi.  So I've been using my ICOM IC-746Pro for some 20m CW.

The bands get busy so I try to narrow down the filters on the fellow I'm working to block out the QRM.

The problem with going to a 50 or 100hz filter (the minimum possible on the 746pro) is that the audio gets a terrible ringing that is just as distracting as the QRM of a wider filter.

Any advice on how to get nice narrow filters but maintain the audio quality of the guy I'm trying to work?

73
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WA6BFH
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« Reply #1 on: September 27, 2005, 11:04:14 PM »


Thus is the nature of physics!

You could talk to the guys at Inrad, maybe they have a better handle on possible slope factors.

Or, you could make a resonant speaker at a desirable frequency, 700 or 800 Hz etc It may sound crazy, but it really works quite well!
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K5LXP
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« Reply #2 on: September 28, 2005, 06:28:07 AM »

BFH- There are no user-installable filters in the 746Pro, DSP only.

VK- Think about what you're trying to do- 50 or 100Hz?  Consider that a 'very narrow' mechanical CW filter is 250Hz, with 500Hz being very common as a 'narrow' filter.  At 50 or 100Hz I would expect some ringing.  Try widening it up to between 250 to 500Hz and you'll find signals a little easier to copy.  DSP is good, but it's still not perfect.  It's not quite able to do 'single signal' in a very crowded band quite yet.

Mark K5LXP
Albuquerque, NM
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NI0C
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« Reply #3 on: September 28, 2005, 08:40:08 AM »

Have you tried using the "soft" shape factor filter characteristics?  That might alleviate the ringing you are experiencing.

Strictly speaking, "ringing" refers to an undesirable transient response of a filter to the rise and decay of a CW signal that appears to stretch out the signal making it difficult to copy.  The transient response characteristics are a function of the filter design.  

However, what many people refer to as "ringing" is simply an unavoidable result of using extremely narrow filters.  As you narrow the bandwidth of a filter, the random noise in the background becomes more and more like a sinewave at the center frequency of the filter.  With extremely narrow filters, it becomes more and more difficult for the ear to distinguish the difference between a weak signal and the background noise.  That's why widening the bandwidth of CW filter can sometimes make a weak signal easier to copy.

I am a big fan of selectivity, using cascaded 250 Hz crystal filters in my rig, and augmenting those with DSP audio filtering.  My audio DSP is the MFJ484B, capable of a 30 Hz CW bandwidth.  For weak signal work, I set my DSP filter to around 150 Hz BW, certainly no narrower than about 100 Hz.

73,
Chuck  NI0C
 
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