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Can't leave well enough alone...

The first thing that I put on my NEScaf board was a diode - in the hole for the +ve power lead.  Too darn easy to get the power leads wrong!

And then I started playing!!

Discovered that when I used the filter for RTTY, it appears to work about the same as my 250 hz 8.8mhz filter - as far as the waterfall is concerned anyway.  Of course the crystal filter will have other benefits - and is a lot pricier....

Then I started to play,  added a 1.something K resistor in parallel with the 2  2k7s - voila - Q of 20 now.  Then I took a line from the notch output of the 2nd filter & ran it to the output (bypassed the audio amp) - now I have a ~ 270 hz filter with a notch in the middle...

I didn't get this done until last night - and of course all of the rtty guys are busy recovering after a tough weekend!!  Since the SCAF chip's gain is unity, the output of the 2nd stage can be feed directly into the sound card for rtty demodulation....

In my case, I just moved the plug from the sound card input to the input of the SCAF and then ran a patch cord from the the SCAF output through a .33 ufd ceraminc (bypassing the LM386 entirely) to the audio out jack.  A patch cord from the audio out jack of the SCAF to the sound card line in....

Didn't even have to adjust any levels!!

What I should really do is add a 2nd pot to control the Q of the 2nd (now notch) filter, alternatively, just play with the fixed resister in Q setting circuitry..

I did play around with the unmodified filter - looking for VLF aircraft beacons.  These beacons have a constant carrier (the beam) about 400hz away from the identifier.  With the umodified SCAF tweaked properly, one could copy the ident without hearing any trace of the beam - even from the local international airport less than 10 miles away.

Sounds like my next digikey order will have to include a couple of SCAF chips..  Now if someone would make a circuit board with just the runs for the circuitry around the SCAF chip...

Villy  VE6SQ

 

 

 

 

 

 

Comments

Thanks for sharing your experience modding the NEScaf for RTTY and chasing the VLF NDBs, Villy. Experimentation is what it's all about. Above all, have fun!

73,Bruce, N1RX