Scandata faq q10
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NOTICE OF CERTAIN DATA DECODER LIMITATIONS (and a fix!)
By Bill Cheek
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.
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