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Author Topic: ICOM751A Optional Narrow CW Filter?  (Read 792 times)
AB3CV
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Posts: 9




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« on: November 12, 2005, 01:23:35 PM »

I'd like to get some guidance on an narrow CW filter option for my ICOM751A. Inrad has 400hz and 250hz filters for this. The ICOM already has a 400hz filter standard but they also offer an optional 400hz filter which I presume has a better shape factor.

Any recommendations on whether the Inrad 116 (400hz) would show some improvoment over the existing ICOM or whether I should go for the 250hz Inrad.

tnx
jim ab3cv
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AB3CV
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Posts: 9




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« Reply #1 on: November 12, 2005, 06:38:55 PM »

Talking to myself again...

It seems the 500hz CW filter is in the preceding 9Mhz IF stage. The optional filter slot is an alternative to the standard filter used in common for SSB, CW and RTTY. This standard 455Khz IF filter is 2.3Khz wide.

So my question is would the 400Hz Incomm filter provide a noticeable improvement in cleaning up whatever got past the 500hz filter in the preceding IF stage or would the 250Hz filter be a better choice?

Note that this is the "optional" filter socket and is otherwise unused. The optional filter is activated by a button push. Currently pushing that button does nothing. All the existing filters would remain in place. I'm strictly interested in CW at this point.

73

jim ab3cv

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K7UNZ
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Posts: 691




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« Reply #2 on: November 12, 2005, 09:17:30 PM »

Hi Jim....

In answer to your question, I think you'll find the 400 Hz filter to be much more usefull to you then the 250 Hz item.  As to actual "improvement" you will see, well that will also depend on your radio itself.  

Going to the narrower filter will improve selectivity for sure, but will NOT remove QRN or other atmospheric noise.  In fact, you may be better off investing in a good, external, audio filter if that is your main problem.

The 250 hz filter is excellent if you are a dedicated contester, and the band is wall to wall with super sized signals.  Say in maybe 5 years from now.....hi!  Otherwise, it may be more a hinderance than a help as it will cut the signal level you hear, and could actually cause you to loose the signal in the noise.  There are other drawbacks to the very narrow filter, but I won't go into them.

As a CW guy myself, I usually run on the wide filter in all my rigs, switching to a 500 HZ as a last resort.  I find that an external audio filter (a 20 year old Bencher XZ-2 in my case) does wonders in cutting the noise, and peaking the signal I want to hear.  All my radios run thru it via a switch box, and that's everything from my old Drake twins, Corsair, FT-990, FT-840, and TS-570.  And they ALL have narrow filters installed, which are rarely used.......hi.

Jim/k7unz




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AB3CV
Member

Posts: 9




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« Reply #3 on: November 13, 2005, 08:30:32 AM »

Hi Jim

Thanks for the advice. I'm leaning toward the 400hz filter.

I was curious about your statement of not reducing noise however. Since the CW signal is band limited then narrowing the filter around the signal should reduce the amount of noise power accordingly. I know there will be loss associated with the filter but both the signal and inband noise would be affected to the same degree. Turning up the volume should overcome that.

All this assumes that the noise originates ahead of the 455Khz stage which it should with good design.

What am I missing?

73

jim ab3cv
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K7UNZ
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Posts: 691




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« Reply #4 on: November 13, 2005, 10:17:05 AM »

By noise, I'm referring to atmospheric noise....QRN, etc.

As it is appears all across the band, it will also be within the filter's bandwidth.  You might reduce it a bit 'cos your "sample" is so narrow, but you won't eliminate it.  Sometimes the "ringing" of a narrow filter can actually accentuate it, and make copy of a low signal level even harder.

Just one of those things where you have to push the button and see if the filter helps or hinders copy.

Jim/k7unz
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AB3CV
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Posts: 9




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« Reply #5 on: November 13, 2005, 10:39:46 AM »

Hi Jim

By atmospheric noise I think you're referring to lighning crashes rather than white noise.

Makes sense now to me if that is the case.

I'm still learning...

thanks

jim ab3cv
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K7UNZ
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Posts: 691




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« Reply #6 on: November 14, 2005, 05:33:57 PM »

Yep, that's what I mean...(hi).

By the way, for what it's worth, I still think the 751A is one of the best rigs Icom ever put on the market. Hope you're having fun with yours!

Jim/k7unz
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AB3CV
Member

Posts: 9




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« Reply #7 on: November 14, 2005, 06:31:53 PM »

Hi Jim

I like the 751A a lot. I got it early this year for just SWL use, caught the HAM bug and got licensed in April.

I'm going to look into PC soundcard audio DSP for while before doing anything internal.

I just tested out a little passive closed loop ALC circuit to take the min power down from 7W to reliably below 5W to hold QRP levels. Seems to work fine so far.

73

jim ab3cv
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VE7BGP
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Posts: 80




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« Reply #8 on: November 16, 2005, 11:57:20 AM »

Hello Jim
I have 2 Icom IC-751A's in my shack. I am thinking of going the same route with one of them and installing an Inrad 116 400hz filter in the spare slot. I understand the passband tuning is much tighter and you have better selectivity when you have the 2 filters. I was in the recent Nov CWSS contest and the second filter would really improve things for serious contesting. I don't think it is really needed for casual QSOing on the bands. I like you Jim really like my IC-751A's a lot they are a great rig and very easy to use no inboad Auto Tuner and pages of menus to go thru all the time. I am also looking for comments on Inrad vs Icom FL 52A filters.
73 Gerry
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AB3CV
Member

Posts: 9




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« Reply #9 on: November 16, 2005, 02:55:47 PM »

Since my PC is always sitting next to my ICOM751A I decided to try PC DSP software.

http://fmyers.com/ has a DSP software app at the bottom of the page that seems to work well.

In addition to the bandpass filter it also has a noise filter which seems pretty effective at eliminating lightning crashes.

73

jim
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VE1HE
Member

Posts: 8




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« Reply #10 on: November 26, 2005, 04:08:25 PM »

I have a 751A and I use my 250 Hz filter a lot but only when the QRN is low and the signal is relatively strong.
If the signal is too weak, it makes it even worse.

I like the filter also to listen to a specific calling frequency (nets, etc...) when I'm doing something else so I can concentrate on my work.

It's really nice for that.

I can't comment on the 500 kHz filter because I've never tried it.

GL
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VE7BGP
Member

Posts: 80




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« Reply #11 on: April 20, 2006, 04:33:54 PM »

Hello All Fellow IC-751A users
I took the plunge and picked up a FL-52A filter for my 751A and I installed it. Wow has that radio now have Brick Wall Selectivity. For normal use the stock FL-32A is great but when the going gets tough if the station is netted right on you does the narrow filter ever clean up the QRM and QRN. I think it was a good move am I ever looking forward to Field Day this June I will be one of VE7NA's cw ops for that and our Canada Day contest on July 1st. Just a few comments.
73
Gerry VE7BGP
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