Sony ICF-6800W - Circuit Description - PLL Synthesizer

1-2, CIRCUIT DESCRIPTION
This receiver has a high degree of stability which
is achieved by replacing the first local oscillator of
the shortwave double-super heterodyne circuitry
with a PLL synthesizer.
The following is a brief circuit description.
1) Fig. 5 is a block diagram showing the principle of
the PLL circuit employed in this set. Let us
consider this block diagram.
Say a 1 MHz signal comes into the receiver from
the antenna. In order to convert this signal to
the first intermediate frequency [19.055 MHz
(ist intermediate frequency of the receiver)],
the frequency of VCO1 has to be 20.055 MHz.
At this time, the frequency of VCO2 is set at
29 MHz.
When these two signals are fed to the mixer 1,
an 8.945 MHz signal [29 — 20.055 = 8.945 MHz]
is produced at the output of. the mixer 1. This
signal is now fed to the mixer 2. Meanwhile,
a 10MHz signal from a fixed oscillator is also
fed to the mixer 2.
When the two signals are mixed by the mixer 2,
a 1.055 MHz signal (10 — 8.945 = 1.055 MHz) is
produced as the output of the mixer 2, and is
then fed to the phase comparator. Meanwhile,
a separate signal from an oscillator is also fed
to
the
phase
comparator
as
shown
in
Fig.
5
[Here-
inafter, this oscillator will be referred to as the
VFO. ]
SIGNAL FROM
ANTENNA 1 MHz
1ST MIX
20.055 MHz
(21.055 MHz)
29 MHz
(30 MHz}
19.055 MHz
8.945 MHz
ICF-6800W
This phase comparator compares the phase dif-
ference of the output signal of the mixer 2 and
VFO. If there is a phase difference between these
two signals, this difference is produced at the .
output as a DC voltage and applied to the varicap
(variable capacitance diode) of VCO1 to regulate
the oscillating frequency of VCO1. At this time,
as long as the frequency of VFO is 1.055 MHz,
VCO1 will produce a stable oscillating frequency
of 20.055 MHz.
This is the basic operation of the PLL circuit.
2) How are 2 MHz, 3 MHz signals, received?
Let us consider how a 2 MHz signal is received.
The frequencies of each of the circuits will now
be as follows.
2 MHz (3 MHz) (4 MHz)
VCO1 21.055MHZ 22.055MHz 23.055MHz
vVCO2. +30MHz 31 MHz 32 MHz
VFO 1.055 MHz 1.055 MHz 1.055MHz
By having each of the oscillators produce the
frequencies listed above, 2 MHz signals can be re-
ceived, but if one were to attempt to cover | MHz
to 30MHz in this manner, VCO2 and VCO1
would have to be similar type oscillators, which
would be extremely difficult from the standpoint
of production.
10 MHz 1
(FIXED) He on