Specifications

Other
Radios
Transmit audio is
injected
through a 5
to lOmF, non-polarized capacitor through
a 5KW resistor
into the
modulator. This is typically a varactor diode
in a
transmit
oscillator stage.
The schematic symbol
for a varactor
diode
is
-1ft,
which makes them
pretty
simple to find by
tracing fbr-
ward
in the radio's schematic from the
microphone.
Crystal-Control
led FM Transceivers
Norn:
True FM
multimode
radios
using
varactor modulation
in
a crystal oscillator stage
seem to work best-
they
tend
to have
a
slightly wider IF
fllter
(i.e.,
the CFW455E) and
pass
data
with less distortion.
(Fortu-
nately,
most
synthesized
radios
can be mnde
to work with extremely simple
mods.) However if
your
radio
is crystal controlled, it can be made to
work
if it uses true
FM: it's simply a matter of connecting
directly
to the FM modulator-which will always be in a crystal oscillator
stage.
If
your
crystal-controlled rig is true FM
(varactor
modulator in an oscillator
stage), inject transmit
audio
from the PK-96 into the cathode of the varactor diode. However, many crystal-controlled
rigs are
phase
modulated and need a varactor modulator added to the transmit crystal oscillator.
Use an abrupt
junction
type varactor diode, such as the
MV2105 liom Motorola, and adjust
the capaci-
tance in the crystal circuit to compensate lbr the additional
capacitance ol'the
varactor. If there ale
lixed-
value capacitors, remove or
"pad"
them.
Il'not, change the trimmer to one of
a lower minimum
value.
You'll need
to bias the varactor.
I've used the transmit oscillator V", and a
20KW trimpot to
ground,
feed-
ing
bias through two lOKW to
47KW fixed resistors, feeding the TNC's transmit audio
through a 5mF
to
1 OmF capacitor to the center
junction
of these, and
feeding
the
free
end
to the cathode of the
varactor. The
bias
needs
to be adjusted
for
best
received
eye
pattern
on a service monitor or
receiver. See the
fbllowing
diagram:
Transmit
Oscillator
Vcc
100K
to TCXO
XTAL
4-20
pF
to TCXO
g!'ound
TNC TX
,
Audio
J
EIIE
1000pFfi
=
MV2105