2-6-99

NOTICE OF CERTAIN DATA DECODER LIMITATIONS
              (and a fix!)
==========================================

My SCANDATA.FAQ and related periodic newsgroup 
posts on data decoding attempt to define a 
2 Level FSK Data Decoder Interface that will 
work on a majority of all scanners.

And it does. But there are probably a few scanners
with which it won't work as designed.  I just
discovered one, the Sony ICF-SC1PC. And I learned
why the 2LFSKDDI didn't work, and what to do about
it.  First, the cause and why a few other scanners 
may be affected.

When a known good 2LFSKDDI wouldn't work on my
Sony ICF-SC1PC, I immediately measured the RMS
value of the baseband audio signal, and much to
my dismay it was 0.150 volts (AC). Aha!

  Oddly enough, that same scanner worked fine
  with a 4 Level FSK Data Decoder Interface!
  Of course, it has GAIN to overcome weak
  signal inputs, so that was clue enough.

The baseband audio output of most scanners I
have ever measured runs from 0.350v to 0.800v,
so that's the range for which I optimized the
design of the 2LFSKDDI that I regularly post
on the newsgroups and in my SCANDATA.FAQ file.

http://ourworld.compuserve.com/homepages/bcheek/scandata.txt

Extensive tests proved that the Sony ICF-SC1PC
was not defective in any manner; it just has a
low level NFM discriminator signal.  If one
scanner has a markedly low signal, then others
might, too.  Here's how to find out about yours:

Connect a voltmeter (set to the AC-volts function
at a range of about 1-volts) to the baseband audio
output jack on your scanner.  Tune the scanner to
a known trunked data control signal (855-868 MHz).
Record the measured voltage.  If 0.350 VAC or
better, then the 2LFSKDDI as presented in my FAQ
and newsgroup posts will work just fine.  But what
if the signal is less than 0.350v?

The 2LFSKDDI weak-signal fix is simple.  Change 
the 3.3k resistor (at U1 Pin 3 to ground) to 2.2k 
if the signal is between 0.250-0.350v.  

If the signal is weaker than 0.250v, then change 
that 3.3k resistor to 1k. Easy as that.  1-2-3.

OPTION:  You could build your 2LFSKDDI with a 10k
         trimmer potentiometer in place of that
         resistor, so as to make the one decoder
         interface serve a variety of scanners.
         A 1/4" hole drilled in the DB25 shell just
         over the trip pot adjustment will let you
         tweak it for optimum performance at any time.

        U1 Pin 3 >----o----/\/\/\-----o----> Ground
                              ^       |
                              |       |
                              o-------/

       Just wire the trim pot as a variable resistor.

NOTE:  Some "data slicers" and "hamcom" interfaces
       might call for a 10-k resistor in place of
       my 3.3k. This only makes the 2LFSKDDI more
       prone to weak-signal failure.

NOTE:  Other "data slicers" and "hamcom" interface
       designs go for the simplicity by eliminating
       that one resistor by grounding Pin 3 of U1. 
       These designs might also eliminate another 
       resistor, too. Granted, these designs will 
       decode practically any level of signal......
       .......including.....NOISE!  The purpose of
       the 3.3k resistor is to set a threshhold above
       the noise for improved accuracy.  So it is
       essential, in my opinion.  Just that depending
       on the output level of your baseband audio, 
       the optimum value of that resistor might be 
       anywhere from 1k to 10k. For most scanners,
       it remains 3.3k.

CAVEAT:  Before going off half-cocked, make sure
there is nothing amiss with your Baseband Audio
modification.  Errors and incorrect installations
can cause weak signals, too.  The correct mod
has the (+) leg of a 2.2-uF to 10-uF tantalum 
capacitor soldered to the NFM Discriminator audio 
output pin.  The (-) leg of this capacitor goes 
out via a hookup wire or shielded cable to the hot 
lead or center lead of the jack on your scanner 
that  serves as the baseband audio output.  The 
shell or outer lug of the jack should be grounded 
to scanner ground.  The shield of the shielded cable
must also be soldered to the ground lug.

If...and only if....your baseband audio mod was
properly done, and then if the signal is weaker than
0.350 VAC, you can alter the design of the 2LFSKDDI
as indicated above, for good results.

------------------------------------------------        --
Bill Cheek - President/CEO/Publisher            \      /
COMMtronics Engineering/World Scanner Report     \    /
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E-mail:  bcheek@cts.com                           /  \
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