DID TEURS

tone comes in, it puts out a
low level. In F5K (10 meters
and below), 2295 Hz is used
as the mark time; 2125 Hz is
used as the space. This
gives a downward shift of
170 Hz . In AF5K (6 meters
and above), the tones are
reversed. 51 selects F5K or
AF 5K and will be labeled
during al ignment.
Fig. 3 is the modulator.
The input at J1 is from the
serial output of the UART.
For a high input, we get a
2 295~Hz toneat J2. A low input
gives us 2125 Hz out.
The output of the XR210 is
made into a sine wave by
the low-pass filter so that
our transmit signal will not
be too wide. The filter cutoff
is about 3 kHz. 51 a l­
lows you to ide ntify in CWo
52 is wired in at the mike
jack the sa me way as a PTT
switc h. 53 selec ts F5K or
AF SK .
Conslruction
The interface is built on
a n Apple hobby/prototype
board, Apple part number
A2BOOO1X. This board is
made to plug into the Ifa
slots at the back of the Apple.
51 is brought out in
back of the Apple so that
you can get at it. No layout
is included with this projec t
because nothing is critical.
You sho uld have no trouble
if good general construct
ion practice is used. Capacitors
C1 , C2, and C3
mu st be of good quality,
like mvlart or polystyrene
. It is a good idea to
construct the IC6 circuit on
a n e xperime ntal basis first
and trim C1, C2, and C3 to
get the clock frequency
within 1 % . A frequency
counte r is the only good
way to do this. At 60 wpm,
the frequency at IC6, pin 3,
should be 728 Hz plus or
minus 7 Hz. At 100 wpm, it
will be 1187 Hz plus o r m i­
nus 10 Hz. This tr imming
ca n be done with small value
mica o r ceramic capacitors.
After yo u have built
the ci rcuit on the Apple
board, c heck the clocks
'" "
· I2 V
_12v
'0_'
"
, ," l'
Fig. 2. Demodulator schematic diagram.
3) should be of good quality.
51 , 52, and 53 are
brought out to the front
panel. The c hokes a re the
common 88-mH variety or
equivalent. In all circuits,
resisto rs are quarter- or
half-Watt 5%. All capacitors
not mentioned may be
of a ny type.
Alignment
First do the modula to r.
W it h 51 in normal , hook the
scope to 12 a nd make su re
you ha ve a n a udio sine
wave. Adjust R3 fo r a bout 1
volt or enough to run you r
. ~
I.
'"
• , LO HS$ FlLT(A
_. _. m 00 • ., , ."
'" • -
" f,,,
"
frequency counter. Now
hook the frequency counter
to 12. Put 53 to F5K and
sho rt J1 to ground. Adjust
R2 for 2125 Hz. If you can't
get 2125 Hz, adjust both R1
and R2 for 2125 Hz . Put 53
to AF5K and adjust R1 for
2295 Hz. These co ntro ls are
interacting, so go back and
fo rth several times until
you have the correct tones.
The tones a re very stable
and will stay within 1 %
d rift easily. Now verify the
conditions listed in Table 1.
The output at J2 should
be a nice-loo king sine wave.
L TO oo,e '''PUT
"!) m
TU"SWIHU
Fig. 3. Modulator. Output in FSK: mark time = 2295 Hz; space = 2125 Hz. Output in A FSK
(tones a re re versed): mark time = 2125 Hz; space = 2295 Hz.
• ".
J \ .....
U 210
. ~ v
~ r-'
r
....0Cl >lH
• ~
," ".
.05' " H f " ""L"Y
• /
00
l '
''2' •
•• "
' ''217
" " •
- ", "
7,
"
"
PHO ~f
. uOIO
OUTPur
UOIOH S S
01 IlCY ~ ',LHA
'ROoo
r iG I " 'H
URI." OU T
...
$PU >I ( A 2000H,
rc
2 400'"
..'
';V
.(
_ TP .
.,
[
y~ ., I ""0
,.. no
.~
..
- " .!:
H ,.. Fs" · LO U J" 112 SETS LO '''f O ' 212S H.
n ,.. ~ s ~
"'
AT J'
'"
"'
SETS '" ' ''[0 • n9SH.
.,.
U2'0 S.. OW ..
lct 140_ 00 ' •
H TO SCOPt:
' ~OICUO~
'"
",. m ,u
.. ..
'"
r ±
'" '" ~ , J..
'" -If.' ...
l:"
,
00 ~
• • '" '" '" • I" '"
'",
UO S . S • •
.. •
.
.,.
."C -J , .
'C2 ,.
'" ".
" ,
." .,
• ... ,
L OO"$ 5.. 0 ..... '"
• ,
'" L OO 3' 9 s..o.....
cr
'" HO l 00
.04'
'" I •
'" ;,
•
I ~" 4 •
• , ~'O •
again. They may need a littIe
more trimming.
My 55B phone-jack output
is 8 Ohms, so a speaker
at 11 works fine (see Fig. 2).
If yours has a higher impedance,
you ma y need a transformer
to ma tc h to the
speaker. The power supply
fo r Figs. 2 and 3 is separate
from the Apple. If you wish,
you could use power from
the Apple for these circuits
also. C1 shou ld be a good
quali ty capacitor. 51 is
brought out to the front
panel.
C1 in the modulator (Fig.
lOU. r ,ooE '$ '" UVEL
.:.:d 68 73 Magazine . April, 1981