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Author Topic: SCAF or IF Filter?  (Read 16172 times)
KB4MB
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« on: August 04, 2010, 08:02:25 AM »

I am on the fence on whether to get an Idiom Press SCAF filter for my CW usage, or an W4RT filter for my Icom 718.  I am not crazy about opening the little guy, and I will come out a bit cheaper building the filter (plus have more fun, and maybe attenuate some SSB if I need to).

I don't notice CW signals capturing the AGC, so other than tidiness, do you think that an outboard filter will be good enough?  Or should I go with the mechanical filter?

Thanks in advance,
Kris, KB4MB
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N4IAG
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« Reply #1 on: August 04, 2010, 08:26:57 AM »

The 500 Hz W4RT filter would be my first choice. I had one in my 718 and it worked very well. The installation was no problem, took 10-15 min. as I recall.
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I want to die peacefully in my sleep like my grandfather, not screaming in terror like his passengers.
VA7CPC
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« Reply #2 on: August 04, 2010, 10:12:44 AM »

Get the 500 Hz filter (or a 300-Hz filter) in the IF.  I have one in my FT-817, and it works very well.

Eventually, you _will_ get a situation where your AGC will be affected by a nearby signal.  And you'll wish for a narrow filter at the IF stage, rather than the AF stage.

If you operate your FT-817 at home, and you want to try a narrow AF filter, try some of the PC-based "DSP filter" software.   It's inexpensive, or free, and works pretty well.

You can set up "CW Get" to send filtered data from the PC's soundcard input to the soundcard output.  Presto -- a 300-Hz (or 140 Hz, or 70 Hz) audio filter!

               Charles
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STAYVERTICAL
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« Reply #3 on: August 04, 2010, 02:54:06 PM »

The choice is clear - get the IF filter.
Outboard filters are useful and do a reasonable job, but if you have ever compared the two you will see the advantage immediately.
For a start your signal to noise ratio improves immediately, and some stations are so strong that you really need to keep them out of your IF.
This is particularly true in CW and Digimodes work, where stations tend to "snuggle" close together.
For my CW work, I use a number of programs off the net, my current favourite is Brunifilt (freeware), which gives you all types of audio filtering, notching, peaking, dynamic volume and frequency control and even allows you to change the pitch of the signal by DSP.
All of these functions are adjustable by onscreen sliders and checkboxes.
This all happens in the standard windows PC and I just listen to the audio output from the earphone port on the PC.
The only caveat is that you may need two soundcards, one for input and one for output.
Since I already use a USB signalink for digimodes, I have the second one, but a cheap audio input dongle may do the job as well.
If you don't want to use a PC at the same time as you operate, this option is not available, but there is nothing to stop you building the scaf filter later on. If you are concerned about installing it, I am sure if you ask, you will be steered to sites where the procedure is detailed, or you may be able to have someone experienced install it for you.
Good luck and 73s.

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AA4PB
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« Reply #4 on: August 04, 2010, 03:12:53 PM »

The issue with the SCAF filter is that other signals inside the IF passband will pump the receiver AGC even though the SCAF filter may keep you from actually hearing the adjacent signals. The IF filter will get rid of the adjacent signals before they hit the AGC, a much better choice in my opinion.
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W0XI
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« Reply #5 on: August 04, 2010, 04:03:53 PM »

On crowded bands, the earlier in the receiver chain a filter is the better. When well designed, the type of filter does not really matter but placement in the receiver chain does. Mechanical filters are old but can be very good. SCAF technology is a couple of decades old now, so I'd go with DSP if you wish to play at the audio level. In some cases, you might consider turning off the internal AGC, adding the audio DSP filter and following that with an audio AGC. Yea; this is complex but might be fun. If you are op oriented rather than circuit/build oriented I'd go straight to the manufacturer's mechanical IF filter. 73s. Phil, W0XI.
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N2EY
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« Reply #6 on: August 05, 2010, 06:41:43 PM »

IF filter first.

73 de Jim, N2EY
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KE3WD
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« Reply #7 on: August 05, 2010, 08:00:12 PM »

SCAF is so seventies.  Or eighties. 

IF filter first. 

If you want more, consider addon audio DSP over SCAF IMO. 


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KB4MB
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« Reply #8 on: August 06, 2010, 04:18:57 AM »

It's pretty funny, because you read the reviews of either the 718 or the SCAF here and many times the respondents say there they prefer the SCAF, never use the narrow filter, never needed to, etc.

I still haven't had my agc pump yet, but since the W4RT filter is so close in price, I will just wait and ask for my birthday in December Smiley  In the mean time, I will use the brain filter (slightly faulty), and if shift if there is a neighboring signal.

Still, the NEScaf? one is only thirty and I can throw that into a box along with the picokeyer I never finished and have a scaf/keyer box...  Maybe I should do that too so I have both options.
« Last Edit: August 06, 2010, 07:51:46 AM by Kris Brzescinski » Logged
W0XI
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« Reply #9 on: August 06, 2010, 07:29:56 AM »

KB4MB,

Hi. Funny? I'd say the comments are individual experience based and they may or may not hit the mark. It's doubtful that most comments are based on a deep comparison of filters, their placement in the receiver(s), and a large logging of listening experiences. However, I think the collection of recalled experiences or opinions is useful when collected as a whole.

It is clear - from research - that best results are generally achieved when the filter is placed in the IF rather than in the audio chain. I'll bet many hams know that. It's also clear that using no filter at all reduces the S/N even without adjacent interfering signals. It's also clear that the skirts and bandwidth of the filter can improve or distort the desired audio signal received. Taken as a whole, the topic is indeed complex. Perhaps the best thing to do is built flexibility into your filtering options.

I've not tried it but I wonder how much distortion is added when the filter is so wide that the harmonics of a high-speed burst (say 40 WPM) of code are included? Hm. 73s. Phil.



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AA4PB
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« Reply #10 on: August 06, 2010, 08:13:52 AM »

I don't think anyone is saying never to use an audio filter. Your question was one or the other, audio or IF filter. Most anyone who knows anything about receiver design knows that you need to put the filter as close to the front end as practical, and certainly ahead of the AGC loop. For most receivers that means the first or second IF.

For me, I'd use the IF filter first, perhaps 300-500Hz for CW. That'll get most of the interferring signals out of the AGC loop. Then I'd follow it with a narrow DSP type audio filter where you can further limit the bandwidth, adjust notches, etc.
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KB4MB
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« Reply #11 on: August 06, 2010, 08:39:11 AM »

I just didn't want to waste the precious little money we have  Roll Eyes

Seeing so many reviews saying they never use the if filter, I just thought I would ask and see what everyone elses experience is..

I am going with the nescaf filter kit, put it in with the keyer I should have finished, and I will have a good little setup for me here to tide me over.  Since that kit is only $31 over $100, that is a big cost savings...  plus, it will help me zero beat, since I can get the bandwith down to 90hz using that kit until my ear gets good enough...

Then in a few months for the birthday or xmas, I will ask for the IF filter, and have both worlds.

I have only had a precious few cw contacts, but I am having a blast going slow on 40m at night.  Getting a little better every day...  Wink

Thanks everyone for their input!  The final solution is both! Grin
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AE4RV
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« Reply #12 on: August 06, 2010, 10:33:32 AM »

I think you made the right decision for now. Someday you will have and really appreciate an IF filter, especially if you take up contesting or QRP sprints. Speaking of, the NAQCC sprint is coming up soon. I finally participated last month and had quite a time.

http://home.windstream.net/yoel/sprint201008.html

73 Geoff

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W0XI
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« Reply #13 on: August 06, 2010, 10:44:56 AM »

Good thinking Kris,

Enjoy the ride! You might look at several CW programs to increase speed and accuracy (all downloadable off the web:

1. JUSTLEARNMORSECODE
2. G4FON,
3. Morse Mail,
4. ebook2CW.

I like the first one since it does characters, most common 100 words, prosigns, Q-signals, and more.

73s,  Phil.
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KE3WD
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« Reply #14 on: August 06, 2010, 08:21:48 PM »

Going for both types of filter is a very good idea. 

It should not only get you out of any sort of trouble where there may be frontend overload or where you just want a nicer sound during a QSO. 

And having both types and being able to switch between one or the other - or sometimes both - may just prove to be worth the price of admission in hands on education alone. 

Good decision. 

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