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Author Topic: The Double Sawbuck QRM Anihilator!!  (Read 680 times)
ZS6HK
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Posts: 5




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« on: October 03, 2007, 10:33:23 AM »

Quite a long time ago (20 years?) an article appeared in a mag with this as the title. It described a two IC device that when fed with noisy CW it delivered noise free Code. I recall that one of the IC's was a 567 and you could adjust the input tone, the output tone and the audio bandwidth. I would love to build another one, I used it with it's input plug only just into the headphone socket of the rig so that it enhanced the audio yet left the original sound playing in the rig's speaker.Thanks.

73 de ZS6HK, John in Benoni, South Africa
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AA4PB
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« Reply #1 on: October 03, 2007, 04:42:14 PM »

I don't remember that particular article but from your description it sounds like a PLL to decode the received CW tones and key another oscillator for you to listen to. It'll do a perfect job of eliminating band noise but is the CW signal is weak and noisy it may not duplicate the characters properly.

Think in terms of a PLL CW decoder followed by a code practice oscillator. You can probably find info on the two pieces and tie them together.
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KE3WD
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« Reply #2 on: October 03, 2007, 09:38:02 PM »



The LM 567 was a tone decoder chip designed to provide a saturated transistor switch to ground when an input signal is present within the passband.  

Originally designed for inexpensive use as Touch Tone decoder, The circuit consists of an I and Q detector driven by a voltage controlled oscillator which determines the center frequency of the decoder.  

Ah, google coughs up a datasheet:

http://web.mit.edu/6.115/www/datasheets/LM567.pdf

We'll leave the argument as to whether or not the thing is a true PLL for another time, it could be wired up to act like one and so for all intents and purposes I guess it is one.  

Here is a Spanish page that has the PCB on it, mentions Popular Electronics magazine, may be the circuit in question here or close to it, not much else and no schematic I could find there:

http://py2mg.qsl.br/Filtro%20CW%20PLL/FILTRO%20CW%20PLLtxt.pdf

Mainpage:  http://www.py2mg.qsl.br/

It appears to have been set up to act more like a bandpass filter than anything else.  

I once incorporated the 567 as a 50KHz bandpass filter for a specified ultrasonics design, there was a design kit for this chip that may still be available, you would get two chips, various components and the design and application notes, wouldn't be too hard to set one up as a ~600-800Hz bandpass filter for CW.  The thing wouldn't ring, that's likely what the boon would be.  


KE3WD
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ZS6HK
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« Reply #3 on: October 03, 2007, 11:16:15 PM »

Thanks, guys.  Well, I found out where I saw the article!! It is in the October 1979 edition of 73 Magazine and was written by James S. Beeton the Third. I don't suppose anyone has that mag!!?Huh  
Incidentally, I must say how much I am enjoying going through all the writings in this topic. I have been virtually off the air for at least ten years after a direct lightning strike on the tower wiped out everything. Now I am getting it all together again and CW is still the biggest attraction for me. Keep it up, and thanks again.

73

John in Benoni
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