Sony ICF-6800W - Page 7

(1) Down to minus 0.975 MHz below 0.975 MHz
(2) Below 3.5 MHz and above 2.075 MHz
Unless within the ranges of (1) and (2) above,
the PLL circuitry will not function normally.
(1) is determined in the following manner.
The frequency of 0.975MHz is an absolutely
indispensable frequency because of the relation-
ship with the VFO. Unless the frequency of
VCO1 does not come down to 0.975 MHz even
when the voltage to the variable capacitance diode
is lowered, there will be no change in frequency
when the MW/SW TUNING DIAL is turned, and
so it must be below 0.975 MHz.
Concerning frecuency below 0.975 MHz, in
reality,
there
could
not
be
a
frequency
of
minus
0.975
MHz,
but
here,
we
shall
refer
to
anything
below zero as a minus frequency.
~-0.975 MHz 2 | 0.975 MHz
MHz
Fig. 12.
The frequencies that are compared in the phase
comparator
are
neither
plus
nor
minus
but
are
compared in terms of absolute values, and so the
frequency of minus 1 MHz will be handled as
1 MHz by the phase comparator.
Let
us
consider
what
happens
when
the
frequency
goes
down
below
minus
0.975MHz
when
the
voltage
of
the
variable
capacitance
diode
is
at
its
minimum.
Let us say for the sake of that it
goes
down
to
minus
1MHz
when
the
voltage
of the variable capacitance diode is at its mini-
mum.
Now if the signal that is being fed to the phase
comparator is minus 0.98MHz (the VFO fre-
quency
is
0.975
MHz),
this
frequency
will
be
handled
as
0.98
MHz
,
and
so
it
will
be
judged
as
being
higher
than
the
VFO
frequency,
and
a voltage in the direction of lowering the capaci-
tance of the variable capacitance diode will be fed
to the variable capacitance diode from the phase
comparator.
Meanwhile,
2
minus
0.98
MHz
signal
being
fed
to
the
phase.
comparator
means
that
the
signal
that
is
being
fed
to
the
mixer
2
may
also
be
considered
to
be
minus
0.98MHz.
The
signal
from
VFO1
that
is
fed
to
the
mixer
1,
if
VCO2
is
producing
a 28
MHz
signal,
will
be
27.020
MHz.
This is because VCO] — VCO2= output of the
mixer
2;
VCO1
will
be
oscillating
at
a
frequency
that is 0.98MHz lower than the frequency of
argument
| ICF~-6800W
VCO2. (The word minus in minus 0.98 MHz is
applied when the frequency of VCO1 is lower than
that of VCO2.) ,
VCO1 is oscillating at 27.020 MHz. However, the
voltage that is being fed to the variable capacitance
diode tends to lower the oscillating frequency, and
so the frequency comes down even further, going
as far down as the minimum voltage being fed
to the variable capacitance diode.
In this instance, the frequency will come down to
minus 1 MHz because of the voltage being fed
to the variable capacitance diode, and so the
oscillation will continue to the frequency that
has gone down to minus 1 MHz, and then stop
there. In this instance, it will come
down to 27 MHz and then stop there.
In this manner, when it drops below minus
0.975 MHz, the PLL will no longer function
normally.
The upper range of 2.075MHz to 3.5 MHz is
determined in the following manner. Concerning
any frequency below 2.075 MHz, the situation is
the same as stated earlier in that it must not go
down because of the situation with the VFO.
The reason that it must not go above 3.5 MHz
is that prior to being fed to the phase compsrator,
the signal goes through the low-pass filter.
This low-pass filter will only pass frequencies up
to around 3.5 MHz, and concerning frequencies
above 3.5 MHz, the signals will not be fed to the
phase comparator.
This means that the phase comparator will deter-
mine that the frequency is low, and therefore
feed a voltage to the variable capacitance diode
that will tend to raise the frequency. Accordingly,
VCO1 will now oscillate at an even highe fre-
quency to the point where the voltage beiig fed
to the variable capacitance diode will attain its
maximum level, and there the oscillatin; fre-
quency will settle.
From the foregoing, the frequency that is led to
the phase comparator will be determimd as
follows:
(1) Must be below 0.975 MHz down to minus
0.975 MHz when the voltage being fed to the
variable capacitance diode is at its minimum.
(2) Must be above 2.075MHz up to 3.)MHz
when the voltage being fed to the viable
capacitance diode is at its maximum.
Refer to Page 45 for the information on adjssting
the frequency range of VCO1. ,