300 or 500 hz filter?

Started by K7JLJ, May 18, 2015, 10:18:13 pm

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K7JLJ

Birthday this week and hoping a W4RT dual filter board for the FT-817  is a gift, but can't decide on which CW filter I should get.

I keep reading the 500 is the one but 300 works better on PSK.

I'd be using it 90% on CW 10% PSK

What's your preference and what is the filter actually doing? 

Appreciate the help!
-Jim

cockpitbob

May 19, 2015, 08:21:50 am #1 Last Edit: May 19, 2015, 08:35:14 am by cockpitbob
I have the 500Hz filter in my 857.  Unfortunately I've never used a 300Hz filter so I can't make any comparisons.  What I read was that if you are a serious contester you want the 300Hz.  For my casual use the 500Hz seemed a bit better.  If the other station and I aren't exactly zero beat with each other I can still hear him through a 500Hz filter. 

It does a good job of blocking QRM.  If things are really bad I also use the rig's DCP filter to listen through a really narrow slot.  If there's a high level of QRN the filter seems to make the background noise louder compared to the signal.  You know, that deep rumbling you get with a band pass filter.  I don't know if that would be any different with a 300Hz filter.  Probably not since since this kind of thing your ears work in dB and there's not that much difference in dB between a 300Hz and 500Hz pass band.


As far as what it's doing, its narrowing the bandwidth of the signal earlier in the receiver's electronics.  Your rig's DSP filter will filter the "base band audio".  Whatever noise and neighboring signals have already been amplified and demodulated through the whole receiver electronics before the DSP filter.  The 500Hz "roofing filter" will filter the signal earlier in the receiver's electronics:  in the IF stage (intermediate frequency).  This means less noise gets amplified and demodulated in the down stream electronics that convert it from the IF frequency to audio.  Also, strong noise can create problems for the demodulating electronics.

K7JLJ

Oh, I was thinking it was a tone difference its just a more narrow window that is pre-filtering before amplification then?

Sounds like the 500 would be what I want then.  What is the "non filtered" window width? Is that the 2.3khz I'm reading about (not the SSB filter rating) but read somewhere that without mechanicals the 817 had a 2.3 or 2.4 filter..

So a 500 would make selectivity ~ 4X greater?


- Jim
-Jim

cockpitbob

Yes, a narrower window.  It also has steeper edges.  That is, as signals go outside the 500Hz bandwidth they get attenuated faster than with the radio's filters.

I'm not sure of the no filter bandwidth.  I just know it's significantly wider than 500Hz and doesn't have very steep edges.

4X greater?  Probably.  I know if I'm listening to a station operating "up 1", without 500Hz the filter I can usually hear people calling him (they sound like 1,700Hz beeps).  With the filter I can't hear the stations calling him up 1.

gil

Hello, I'd get the 500Hz filter. With 300 you might miss stations nearby...

Gil


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K7JLJ

Thanks guys, will get the 500. Not sure if I'm going to bother with the dual board and the SSB filter but if money is there and I'm sending it in for the DSP mod I figured I'd get it done, even if I only get the CW filter for now.


- Jim
-Jim

K7JLJ

Well I got the 500hz and installed it tonight.  Does this video show the extend of what I can expect?

http://youtu.be/bz-RZNWy50k


- Jim
-Jim

gil

Hello,

Yes, but in such a case the benefits won't be apparent.. You will really enjoy your filter when two or more stations are close in frequency and you have a hard time to differentiate them. With the filter you can isolate one.. A station 1kHz away from you might not hear you and have a strong signal. Without the filter you'll hear them full blast, just a lower tone. Hard to deal with if you're trying to listen to a weak signal. With the filter you won't hear it...

If you really want to "see" it, install FlDigi and plug in your external speaker jack/cable to the computer microphone jack, minimum volume. You'll see the waterfall showing the width of your received bandwidth. Now turn the filter on... Voila!

Gil.

K7JLJ

Cool ideal Gil, will try that.


- Jim
-Jim