Tandy 2000 Service Manual page 84

Tandy@
Model
2000
Service
I,IanuaI
The
output voltage of the pLL can be written as
shown
in the
following equation:
(
f
ir,
-f
o
)
V0:
K
vco
where
K
vco
f.
1n
f
o
=
conversion
gain of the
VCO
=
frequency
of the input
signal
:
free running
frequency
of
VCO
The
process
of
recovering
FSK
signals involves
the
conversion
of
the
PLL
output
into logic
compatible signars.
For high data
rates,
a
considerable
amount
of carrier-wilr
be present at the output of the pLL due to the
wideband
nature of the loop firter.
To avoid the use of complicated
filters,
a comparator with hysteresis or schmitt trigger
is
required.
Wit.h
the
conversion
gain of
the
VCO
fixed,
the
output voltage as given by
the
above
equation varies
according
to
the
frequency
deviation
of f*-
from
f^.
Since
this differs
from
system
to
system,
^tt is
neEessary
that the hysteresis of the
Schmitt
trigger
be capable of
being
changed
so that it
can be optimized
for a particular
system. This is
accomplished
in
t.he 564 by
varying
the
voltage at pin
15 which
results in a
change
in
the
hysteresis of the Schmitt trigger.
For
FSK
signalsr
dn important factor to
be considered
is
the
drift in the free-running
frequency
of the
VCO
itself.
If
this
changes due
to temperature (according
to the
equation
above), it wilr lead to a
change
in the dc levels of the
pLL
output
and consequently
to errors in the digital
output
signal. This is especially true for
narrow
band
signals
where
the deviation in f._ itself
may
be
less than
the
change
in f^
due
to
temp3Pature.
rhis effect
can
be
eliminated if the dc or
average
value of the signal is
retrieved
and used
as a reference to the comparator.
In
this
manner,
variations in the dc levels of the pLL output
do
not affect
FSK
output.
Due
to its inherent high
frequency performancer
€rn
emitter-coupled oscillator is
used
in the vco. variation
of
the
phase
detector output voltage
changes
the
frequency of
the oscillator.
The frequency
of the oscillator
has
a
negative
temperature coefficient
due
to the positive
temperature
coefficient of the monolithic resistor.
To
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