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Author Topic: SCAF or IF Filter?  (Read 14526 times)
VA7CPC
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Posts: 2375




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« Reply #15 on: August 06, 2010, 09:47:07 PM »


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...


Read the IC-718 manual _very carefully_ -- I bet it will generate a CW "spot tone" at the correct audio frequency for "zero-beat".  Just adjust your receiving frequency until the incoming signal's pitch matches the "spot tone".

As an alternative, turn off the IC-718 "BK-IN".  When you touch your paddle, the rig will generate a correctly-pitched AF sound (in loudspeaker or headphones), but _will not transmit_.   So you can match pitches -- and not forget to change the BK-IN setting back to "normal".

                  Charles
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KB4MB
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Posts: 295




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« Reply #16 on: August 08, 2010, 05:06:04 AM »

You have to change the bk in, but that is such a pain (hit set, dial to the setting, then hit set again, then do it all over again so you can send)...  Plus, then your paddles are sending a series of dits or dahs, so getting a nice phasing noise isn't the easiest either Smiley

Luckily I send cq a bit as I am still learning and getting better, so it is up to them to zero me  Tongue
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KE3WD
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« Reply #17 on: August 08, 2010, 07:25:12 AM »

That is just an excuse to avoid the learning curve and get on with becoming familiar with how to zero beat the rig. 

If you have ever tuned a guitar, you can learn to match the sidetone with the received signal's note. 

And that is well worth the small bit of time now for a lifetime's use. 

Have fun with it, that's the most important part.


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VA7CPC
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« Reply #18 on: August 08, 2010, 10:07:23 AM »

You have to change the bk in, but that is such a pain (hit set, dial to the setting, then hit set again, then do it all over again so you can send)...  Plus, then your paddles are sending a series of dits or dahs, so getting a nice phasing noise isn't the easiest either Smiley

Luckily I send cq a bit as I am still learning and getting better, so it is up to them to zero me  Tongue

I just checked the manual -- you're right, of course.  It's easy to change BK-IN on the IC-706 (which I own), and I assumed the IC-718 was similar.  But it doesn't have any "menu soft-keys" like the IC-706.   And I don't think the Ic-718 has a 'CW spot" tone.   Ouch!

I've used a harmonica to generate a "spot tone".  Once you know what hole corresponds to the correct pitch, it's as easy as pressing a "CW spot" key.  I think there's freeware called "eTune" which will give you a sound-to-Hz conversion, using the computer's microphone.

I like putting the problem on the other guy (by calling CQ).  If the wall is too high to climb, walk around it.

                      Charles
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KB4MB
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« Reply #19 on: August 08, 2010, 10:29:18 AM »

Quote
That is just an excuse to avoid the learning curve and get on with becoming familiar with how to zero beat the rig. 

The easiest way to zero beat the rig is to put it in sideband, tune to no note, then flip to cw.  However, if I was tuning around in cw mode, it gets a little trickier Smiley

I keep it at a low note, so it is a bit easier to get it right than if it was higher.  However, any tool will help me develop my ear until I will just know my offset well enough.  I have just always had bad pitch, which is why I have always had trouble playing by ear...  I know when I am wrong, but I don't know when I'm right, if you know what I mean.
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AA4PB
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« Reply #20 on: August 08, 2010, 11:32:20 AM »

There are some circuits around for a home made zero beat indicator using a PLL chip and an LED. You tune the signal until the LED flashes in step with the CW tone and you are right on zero beat.
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N2EY
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Posts: 3877




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« Reply #21 on: August 08, 2010, 11:54:50 AM »

The easiest way to zero beat a CW rig is to use the CW filter when tuning in stations. If the rig is set up correctly, you'll be on frequency when the incoming signal is in the center of the passband.

That's one big reason we went to transceivers in the first place: Eliminate the zero-beat chore.

As for filters, the closer the filter is to the antenna, the better. This was known over 50 years ago; see "What's Wrong With Our Present Receivers?" QST, January 1957.

If you're set on using SSB bandwidth IF filters for CW, at least do this:

Turn the AGC off (if you can, some rigs don't let you).

Turn the RF gain way down.

Turn the AF gain way up.

Use the RF gain for volume control. Use as much AF gain and as little RF gain as possible.

Yes, the S-meter and AGC won't work, but they're nonessentials that can make CW harder to copy.

73 de Jim, N2EY
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KB4MB
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« Reply #22 on: August 26, 2010, 05:20:41 AM »

I finished up the SCAF filter and keyer project.  Keyer just sent a long line of dits, so I screwed up somewhere with that Sad

The SCAF worked just fine and is pretty neat.  However, when I key, I can hear some RF buzz in the headphones... not a huge deal, but I'd prefer it to not be there... it is a metal box, so I am not sure what more I can do, but I guess I could ground the box to the station ground and see what happens.

I am still going to get an IF filter, but for now at least, I have something to help when I am listening!
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KE3WD
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« Reply #23 on: August 26, 2010, 08:00:58 AM »

...However, when I key, I can hear some RF buzz in the headphones... not a huge deal, but I'd prefer it to not be there... it is a metal box, so I am not sure what more I can do, but I guess I could ground the box to the station ground and see what happens.

More likely that is caused inside the box. 

Audio circuit needs what we call a "single point ground".  -- Often called "Star Grounding".  All internal connections that connect to ground should connect at a single point in the circuit. 

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KB4MB
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« Reply #24 on: August 26, 2010, 08:15:54 AM »

Quote
Audio circuit needs what we call a "single point ground".  -- Often called "Star Grounding".  All internal connections that connect to ground should connect at a single point in the circuit. 
All audio ground is connected to the metal enclosure, so they are grounded at the same point there, right?  The ground of the circuit board audio input is attached to the sleeve of the audio input of the jack (which again is tied to the enclosure)... The DC power ground is attached to the metal enclosure as well as the circuit board.  I had no choice with this as they don't sell stand-off coaxial power plugs at RadioShack anymore.

Is this the problem?
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KE3WD
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« Reply #25 on: August 26, 2010, 03:18:14 PM »

I'm not there to put "hands on" so I can only suggest something that may or may not be the problem but sounds like it may be. 

The whole idea behind the single pont ground in audio circuits is to minimize the resistance that each different grounding point may encounter. 

Very small differences in resistance at those points can and often will make the zero point of an audio circuit wander around a bit, often resulting in the audio having some hum, buzz, or other continuous noise ride on it. 

If you don't have a separate ground wire from each jack's ground lug going back to the circuit board, that can do it.  The metal of the case is a great shield but often not a great ground sink for such circuitry.  This problem is an easy enough solve, a few bits of insulated, stranded wire can cure the problem. 

If the offending noise is not htere in receive mode, it very well could be some other problem such as RF.  I think you said you tried ferrite, maybe some bypass capacitance would be the better choice here.  Experiment with a few different pF value disk caps across input and/or output to see if they can snub your noise. 
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KB4MB
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Posts: 295




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« Reply #26 on: August 26, 2010, 05:45:04 PM »

I disconnected the power lead, and to no avail.  When I disconnect the power, the noise isn't there.  So it is the op-amp being powered up that is getting the interference when transmitting. 

The ground situation now is the box is the ground, and the only connection to the circuit board ground is from one jack to the board.

When i put the ferrite on the cable after the jack, no difference...

Any other ideas where I should look?  I appreciate it!

--Kris, KB4MB
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KE3WD
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« Reply #27 on: August 27, 2010, 09:35:27 PM »

Try tacking a 0.1 cap across the IC's power terminal to ground, as close to the IC as possible.  See if Decoupling might clean it up. 
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KB4MB
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Posts: 295




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« Reply #28 on: August 30, 2010, 06:54:45 AM »

Try tacking a 0.1 cap across the IC's power terminal to ground, as close to the IC as possible.  See if Decoupling might clean it up. 

Looking at the schematic, there is already one there Sad Actually, two!
http://newenglandqrp.org/files/NEScaf-schematic.jpg

I might add one at the power input of the box, just for giggles to see...  There were some stations in the passband last night, and it was great to be able to tune them out, as well as the static crashes... lots of fun.
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KB4MB
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Posts: 295




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« Reply #29 on: August 30, 2010, 04:20:47 PM »

Well, the cap didn't work.  I was buttoning up everything (removed any extra wires I originally had), and my arm bumped the volume... guess what - no noise.  I only pick up the distorted audio when the volume is low.  So, no big deal, either operate a little louder, deal with the noise, or add some resistors at the output (I'd rather not redo those sockets!)

Now I just have to find the fault in the keyer in the same box, and I am good to go.  Thanks for your help!  The SCAF filter so far has been great, and I still haven't heard any overloading yet from other stations (but I'm sure that will happen the more I am on!)...  just a few months until Christmas and hopefully Santa will get me the IF Filter  Grin
« Last Edit: August 31, 2010, 05:22:30 AM by Kris Brzescinski » Logged
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