Sony ICF-6800W - VCO1 Frequency Range Adjustment Necessity

CF-6800W
Utilizing these characteristics of the mixer,
a filter is fitted after the mixer 2, so that fre-
quencies above 3.5 MHz will be filtered out
(refer
to
Fig.
8).
This
means
that
when
recei-
ving
a
10
MHz
signal,
only
the
1.055
MHz
frequency will be passed through. This signal
is fed to the phase comparator and compared
with the frequency of the VFO.
This then is how the reception of 10MHz
signal is carried out utilizing the filter. A filter
is incorporated between the mixer 1 and the
mixer 2 too, for the same purpose.
The frequency of VCO] is controlled by first
comparing the signals by the phase comparator,
obtaining the difference in the form of a
voltage difference, and feeding this differential
to the variable capacitance diode of VCO1. As
shown in Fig. 7, whether it is a 1 MHz, 10 MHz,
or 20MHz signal that is being received, the
frequency that is fed to the phase comparator
is always 1.055MHz. This would make it
seem that VCO1 will constantly be oscillating
at the same frequency, however, the oscillator
circuit of VCO] is switched by means of a
switch for 1 MHz level signals, 10 MHz level
signals, and 20 MHz level signals, and so it never
oscillates at the same frequency.
5) Why is it necessary to adjust the frequency range
of VCO1?
As stated earlier, VCO1 is regulated by means of
the voltage produced at the phase comparator.
The voltage here is approximately 0.8 V to 5 V.
The oscillating frequencies of VCO1 at the maxi-
mum and minimum of this voltage range becomes
the frequency range of VCO1.
Fig. 9 shows the relationship between the voltage
that is applied to the variable capacitance diode
and capacity. Fig. 10 shows the relationship
of the oscillating frequency of VCO1 when the
voltage changes.
MAX.
3
=
N
oO
< DEPENDS UPON
3 \n VARICAPS
VOLTAGE —™ MAX.
Fig. 9
MAX.
it
oO
= =
5
y
rs
wy
VOLTAGE -~ MAX.
Fig. 10
The frequency when the voltage fed to the variable
capacitance diode is at a minimum and the fre-
quency when it is at a maximum are determined
in the following manner.
As
shown
in
the
block
diagram
in
Fig.
11,
the
frequencies that are fed to the phase comparator
must satisfy the frequency range of the VFO.
The question now is how far below 0.975 MHz,
and how far above 2.075 MHz are acceptable.
The conclusion is that it must be within the
following ranges:
SHOULD BE BETWEEN
0.975 MHz AND
2.075 MHz
PHASE
DETECTOR
0.975 MHz — 2.075 MHz