I am going to put this here so it doesn't get lost.
XPERIMENTER'S WORKSHOP
TWEAK, TUNE, AND MODIFY!
Bill Cheek
email:
bcheek@comtronics.net
Build a Four-Level FSK Data Decoder Interface
I n this issue we present the promised Four-
Level FSK Data Decoder Interface
(4LFSKDDI) that, with a freeware pro-
gram, can decode a few of those elusive and
mysterious signals out there on the airwaves.
The 4LFSKDDI can be built by most hobby-
ists, and the software is a no-brainer. Readers
are cautioned that decoding certain F our-Level
FSK signals could be illegal, depending on
where you reside. Get informed before pro-
ceeding.
See my column last month for the necessary
dual polarity power supply. Since then, I
learned that power requirements aren’t so criti-
cal, but ±15V is the max. Greater than ±1 IV
should be regulated, but lower can be unregu-
lated so long as it is reasonably stable. ±5 V is
an absolute minimum.
A pair of 9V DC adapters or 9V batteries
can be perfect for the 4LFSKDDI. The impor-
tant thing is to measure your intended power
supplies before connecting them to the circuit.
See my May-97, Jul-98, Aug-98, and Feb-99
MT columns for important (but not essential)
background information, particularly on the
simpler 2-Level FSK data decoder interfaces
and processes.
You will need an 800-940 MHz scanner
(cellular not required) with the NFM discrimi-
nator/baseband audio mod installed and known
to be functional. See my Jul-98 column to do
this modification to most any scanner. My
Web site at:
http://ourworld.compuserve.
com/homepages/bcheek/scandata.txt always
offers the latest on this baseband audio mod.
See Table 1 for the Parts List.
■ Getting Started
Cut a piece ofperfboard 18 complete holes
long by 15 complete holes wide. This size
supports the circuit perfectly and fits either of
the suggested enclosures. If you choose the
#270-283 project box, don’t use the
circuit board that comes with it. Save
that board for another project.
Follow Figures 1 and 2 for the
broad details of constructing the cir-
cuit. Begin by removing Pin 1 3 of the
IC socket that will be used for U2.
You can even cut Pin 13 away from
the LM339 chip, too. It’s not needed,
but the empty space for it on the wir-
ing side of the board comes in handy.
Install the IC sockets first, then
solder bare 22-24 ga wire “traces”
among the appropriate IC pins, for
example: U1 pins 5, 10, 13 & 14; U2
pins5,7,&9; U2 pins 10-ll;andUl
Pin 5 to U2 pin 7. This secures the
sockets to the board to keep them
from falling out.
Install and solder jumpers JU1
and JU2. One end of JU1 has to be left
open until later in the construction.
JU3 and JU4 can be added later.
Now begin in earnest by installing
and soldering R2, C 1 , R1 , C2, C6, R5
and D2, in that order. First, bend the
leads of resistors and diodes at right
angles, as close to their bodies as
possible. Use a flat-blade jeweler’s
screwdriver to bend component leads
on the wiring side of the board.
Use protruding component leads
as “traces” where possible (most of
the time.) After a component is in-
stalled flush with the board, sharply
bend its protruding leads on the wir-
ing side in the general direction of the
“trace” it should follow. Solder at
least one end in place to hold the
component before installing another
one.
FIG-1: 4-LEVEL FSK DATA DECODER INTERFACE SCHEMATIC
4-LEVEL FSK DATA DECODER INTERFACE CONSTRUCTION GUIDE
COMPONENT SIDE SOLDER & WIRING SIDE
ABCDEFGH I JKLMNOPQ R
LEGEND
O = unused perfboard holes
O = component lead holes
0 = holes for wires to external connections
= wiring on OTHER side of board
= wiring on THIS side of board
1. Remove pin 13 from U2 socket
and/or from chip before installing
2. VR1 can be replaced by a 4.7k
fixed resistor at holes C6 & C8
3. VR2 can be replaced by a 4. 7k
fixed resistor at holes C9&C11
QPONMLKJ I HGFEDCBA
LEGEND
O = unused perfboard holes
O = component lead holes
O = holes for wires to external connections
= wiring on OTHER side of board
• typically jumpers (JU1 - JU4)
wiring on THIS side of board
1. Remove pin 13 from U2 socket
and/or from chip before installing
2. VR1 can be replaced by a 4.7k
fixed resistor at holes C6 & C8
3. VR2 can be replaced by a 4. 7k
fixed resistor at holes C9&C11
82
MONITORING TIMES May 1999
Starting with R2, Cl, Rl, C2, C6, R5 and
D2 as instructed above, install components,
one at a time, working in and completing one
area at a time. Work in a clockwise direction
around the board. Where possible, tightly bend
the leads on the solder side and route them as
traces, instead of clipping them.
Follow the wiring and parts-placement pat-
terns shown in Figure 2. Shorter traces will
“rigidize” after soldering. Don’t make any-
thing permanent until after the board is tested
and proved up. Observe polarity of diodes and
electrolytic capacitors. Ensure IC’s are cor-
rectly installed, per Pin 1 references.
Preset the adjustments of the two trim pots
(VR1 & VR2) to midway between the ends.
NOTE: the trim pots aren’t absolutely essen-
tial. They can be replaced with 4.7k fixed
resistors. (See Notes 2 and 3 in Fig-2.) VR1
sets the gain of Ula while VR2 is a Low Pass
Filter adjustment for Uld.
I Connect the 4LFSKDDI
Wire the outputs of the 4LFSKDDI to a
female DB9 jack. Wire an RCA jack to feed
the input to the 4LFSKDDI. Connect an ordi-
nary shielded audio patch cable from the RCA
jack to the NFM baseband audio output on
your scanner. If needed, add a plug or adapter
on this end of the cable to mate with the
scanner’s baseband audio jack.
Connect a shielded straight-thru serial cable
with a male DB9 plug to the 4LFSKDDI’s
female DB-9 jack. The other end of this serial
cable should have a female DB9 (or DB25)
plug (or adapter) to mate to the desired
COMport on the PC.
The ± power supply can be connected in
any number of ways, from the old fashioned
hard-wired method to perhaps a stereo jack
and plug with the shell grounded; -V on the
ring and +V on the tip. A pair of monaural
U1
i
LM324 Quad Op-amp
276-1711
U2
i
LM339 Quad Comparator
capacitor; 2.2-uF/35v, tant
276-1712
Cl .3.4
3
RSU1 1295888
C2
1
capacitor; 0.01 -uF
272-1065
C5.6
2
capacitor; 0.1 -uF
276-109
D1-2
2
diode: 1N91 421 N41 48
276-1620
Rl
1
resistor; 1 -M
271-1356
R2
1
resistor; 2.2k
271-1325
R4-5
2
resistor ; 1 5K
271-1337
R6-7
2
resistor; 1 00
271-1311
R8.11
2
resistor; 22K
271-1339
R3.10
2
resistor; 47K
271-1342
Rl 2-1 4
3
resistor; 2.7K
RSU1 1344942
J1
1
RCA Jack
274-346
J2
1
DBS Jack (female)
276-1538
1
Perf board: 1 8 x 1 5 holes
276-1394-6
VR1-2
2
T rimmer pot; 1 0K, mini
(See R3 & R3a below)
271-282.
Optional and Pei iphei al Items
*P3,33
p
resistor, 4.7k *
271-1330
A1
i
Enclosure 270-1802. 270-283A 1
W1
3 ft
Shielded audio cable
42-2370
H2
6 ft
Shielded serial cable
26-117
(6 ' DB9 ferale/0B9 rale cable)
2
IC socket; 14 pin DIP
276-1999
* Can be
usee in place of VR1 & VP2
jacks with common grounds will work; +V on
one center and -V on the other. Just don’t
reverse the +/- power polarities; the IC’s will
blow. Figure 3 illustrates the necessary con-
nections.
■ Wrap-up and Loose Ends
The circuit has been extensively field-tested
and proved, so be patient. Use a bright light
and a strong magnifier to examine your work
as you go! Common errors include missing
traces and jumpers, reverse polarity of capaci-
tors and diodes; cold solder joints; and solder-
blobs or short-circuits in tight places. Despite
the high-density design, cramped quarters are
few, largely in the vicinity of U2 pins 3-5, U 1
pin 12, Cl , R2, and some of the area between
U1 and U2.
Most “traces” are rigid once soldered at
each end, but a few might be “wiggly” unless
anchored in some manner. Pay special atten-
tion to traces E14-I2, F12-Q11, E13-P1, and
H9-Q14. Anchoring can be with super-glue,
hot glue, or even loops of wire passed through
holes on either side of a trace, for instance at
holes B12 and A13 to anchor the comer of
trace E13-P1.
For detailed information on hobby circuit
building, including pcb-making, see my four-
part series, “Cool Ways to Design Circuits,”
Apr-96 through Jul-96. MT offers low-cost
reprints if you don’t have those issues.
■ Test the 4LFSKDDI
Download POCFLEX.ZIP from the fol-
lowing site:
http://www.geocities.com/CapeCanaveral/
Launchpad/4039/PINFO.HTM
Docs and insider information at this site
make this program an excellent test platform
for the 4LFSKDDI, if legal in your area. Unzip
the POCFLEX.ZIP archive into a new direc-
tory, say: \4LFSK
Review the contents, especially the *.htm
files, which are the docs for this program. For
a quick-start, follow the next five steps ex-
actly:
1. Set the scanner to searching the 928.0-
932.0 MHz band segment.
A. Connect the baseband audio output of
the scanner to the RCA jack on the
4LFSKDDI
B. Don’t power up the 4LFSKDDI at this
time.
2. Edit the POCFLEX.INI file in the
\4LFSKDDI directory as follows:
TWOLEVELINT = 0
SPORT = 2 (See NOTE 1 below)
RCVPOLARITY = 0
SHOWNUMERIC = 1
SHOWMISC = 1
TIMESTAMP = 1
KILLLF = 0
pm echo = 0
lpt jiort = 0
screenmode = 2
NOTE 1: Make all settings exactly as shown
above except for the SPORT = 2 line. Change
the “2” to match the COMport (1-4) used by
your 4LFSKDDI.
3 . Edit the file called FILTER.INI as follows:
textscan = 1
scannumeric = 1
scanaddr = 1
filtfile = 1
WINSIZE = 50
beepfreq = 1 1000
beeplen = 3
NOTE 2: Make all settings exactly as shown
above.
NOTE 3 : The POCFLEX.INI and FILTER.INI
files are the configuration settings for the
POCFLEX program. If you don’t understand
config files or don’t know how to edit this type
of file, you’ll need to seek outside help. Make
sure each of these two files contains the set-
tings exactly as shown or discussed above.
You can change them later to suit.
4. Power up the 4LFSKDDI.
5. Run pocflex.exe from a DOS command
prompt, or from Win95/98 in a DOS win-
dow.
Make sure the scanner has stopped on an
appropriate data signal (the 928-932 MHz
band is loaded with appropriate data signals).
If all is well, data should appear on the screen
in a few seconds. If not, don’t panic. Is the
scanner on a data signal, and is the signal
carrying data? Sometimes, these signals are
“silent” between data bursts with clear tones.
You can tell when data is present; check to be
sure.
If all is well with the signal, but no data
appears, then check the settings of the *.INI
configuration files. Check all electrical con-
nections and check for errors in the wiring of
the connections, especially the DB9 wiring
and the ± power polarity. Obviously, check the
4LFSKDDI board, too.
Support for this and all my columns is
freely available by e-mail. If you’re not com-
puterized, please include an SASE with postal
requests.
E-mail:
bcheek@cts.com
WWW:
http://ourworld.compuserve.com/
homepages/bcheek
FAX: (619) 578-9247 anytime
Postal: PO Box 262478; San Diego, CA
92196-2478
May 1 999
MONITORING TIMES
83