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Author Topic: CW 250 or 500 HZ filter for PSK31?  (Read 925 times)
KB3MDT
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Posts: 194




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« on: March 17, 2006, 03:54:37 PM »

Hi,
I have an Icom IC-718 100 watt transceiver. I like the PSK31 mode. Would I see any advantage to adding the 250 Hz or 500 Hz CWv Crystal filter to the radio?

The idea is that the filter would keep the bandwidth narrow so the AGC wouldn't get faked out by strong adjacent signals. The PSK31 signal is only 31 Hz wide.

Do the CW filters work on the IC 718 when in USB or LSB mode? I read somewhere you fake out the receiver by telling it you are installing a narrow SSB filter instead of the CW Filter. Then the filter can be turned on or off when in USB or LSB mode.

Will I notice notice any improvement? Or should I spend my $150 on something else.

Thoughts?

Ken
KB3MDT
73
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KB4QAA
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Posts: 2340




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« Reply #1 on: March 17, 2006, 03:59:12 PM »

Sure a narrow filter is going to help reception by rejecting signals outside the pass band and increasing your signal to noise ratio.

73

Bill
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K5DVW
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Posts: 2193




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« Reply #2 on: March 17, 2006, 04:34:52 PM »

Actually, not always will it help. The bandwidth filtering for PSK31 is done in DSP by your computer. Unless some strong adjacent signal is interfering, you'd see little improvement even compared to a 2.3KHz filter. A narrower IF filter on PSK wont make a weak signal suddenly full copy.
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N6AJR
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Posts: 9915




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« Reply #3 on: March 17, 2006, 04:51:50 PM »

since the average BW is 31.5 cycles, even a 250 is going to be "wide banded" in psk 31 use. I may help a bit on nearby signals, but turning down the input gain will probably work better.  

Spend you $150 on a nice antenna and get it up high, with some good coax and enjoy the better siognals.. like a 20/40 80 inverted V fan dipole on a 40 foot push up mast with a non metal stub on top.

using the wires as top guys and run double dacron rope to guy the lower part of the mast and lean it up on the side of the roof.

get a good signal for psk 31 and other modes.. put the wires up, 80 north south, 40 east west, and 20 caddy corner, with rope for the ends to a sutible anchor.. ( I use  Duck bill anchors, cheep and effective.)

about $150 should do it nicely.
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AA4PB
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Posts: 12801




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« Reply #4 on: March 17, 2006, 05:24:54 PM »

The 250Hz filter will help in those cases where you have a strong signal inside the 3KHz SSB filter but outside the 250Hz filter. While the DSP filters in the PSK31 software are much narrower than 250Hz, they are outside of the AGC and cannot keep an interferring signal from pushing up the AGC and reducing the receiver sensitivity.
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K0RFD
Member

Posts: 1368




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« Reply #5 on: March 17, 2006, 06:18:03 PM »

K5DVW wrote:

>The bandwidth filtering for PSK31 is done in DSP by
>your computer.

Sure it is.  But that's not going to help copying weak signals next to strong ones.  By the time the signal gets to your computer, it's all the way through AGC loop of your rig and into the audio.  In the 718, the DSP filtering is done in the audio stage, so it's no better.  That's not going to help AGC pumping when a strong station is in the passband.  A narrow filter in the IF stages WILL help that.

To the original poster, the answer is an unqualified "it depends."

It depends on how you operate.  In my Kenwood, I have both 400 and 270 Hz filters.  I don't do much PSK contesting, because even a brief contest exchange is abysmally slow in PSK.  But in that environment, the narrower filter definitely helps.  For casual DXing, even the wider CW filter helps when there are strong signals elsewhere in the passband. It lets you see a little more of the waterfall, but the tradeoff is that if there is a strong station in the passband, your AGC will pump down and obliterate weak stations. Either one is better than nothing, but the narrower filter really helps when the passband is crowded.
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N2JSO
Member

Posts: 21




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« Reply #6 on: March 20, 2006, 10:16:53 AM »

I don't own an IC-718, but when I was shopping for a rig, the '718 made my "short list" because you CAN use the 500 or 250 Hz filter in USB/LSB mode.  You need to turn the Expanded Filter mode on, see page 24 and 25 of the manual.  The manual downloadable from Icom if you don't have a paper copy ( I downloaded a bunch of rig manuals when I was shopping - it made comparisons easier).

A 250Hz filter is perfect for PSK31, RTTY or other narrow modes when the band is crowded or a strong signal is nearby but outside of the 250Hz passband.  There are wider digital modes, but they are not as common as PSK31 or RTTY.   It's also useful for CW , of course.  When tuning around looking for a signal, use the wide filter, then switch to narrow if needed.
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