Sony ICF-6800W - VCO2 Operation and Frequency Control

ICF-6800W |
3) VCO2 of ICF-6800W
Accordingly, the circuitry of the ICF-6800W is
put together as shown in Fig. 6, in determining
the oscillating frequencies of VCO2.
1/10 PULSE
DIVIDER GENERATOR
(X’TAL}
Fig. 6
(The oscillating frequencies of VCO2 are generated
in increments of 1 MHz between 28 MHz and
37MHz. Further explanations on these oscilla-
tions will be provided later on.)
a. VCO2 determines stability of PLL
With a PLL circuitry as shown in Fig. 5, it is
the stability of the three oscillators, VCO2,
the 10MHz fixed oscillator, and the VFO,
that determine the overall stability of the PLL.
By using a crystal oscillator for the 10 MHz
fixed oscillator, ample stability can be ob-
tained. As for, the VFO, since its oscillator
frequency is not so very high, stable oscillation
can be obtained even with an L-C oscillator.
Therefore, VCO2 determines the ultimate
stability of the PLL.
Fig. 6 shows a block diagram of the circuitry
of VCO2. By forming another PLL to stabilize
VCO2 oscillate as stable as VCO1.
b. Functions of VCO2 (refer to Fig. 6)
10 MHz is divided to 1/10 by means of a divid-
ing circuit, to generate 1 MHz pulse signals.
Next these pulse signals are made into integral-
fold pulse signals of 1 MHz by means of a pulse
generator.
A pulse generator is a circuit that generates
integral pulses of the input signals when this
happens, pulses of (n x 1 MHz) (n are integral
numbers of 1, 2, 3, and so on) are generated.
These pulse signals are now fed to the phase
detector. Meanwhile, the signal from VCO2
is also fed to the phase detector, and the phases
of these two signals are compared, with any
difference being produced as a DC voltage.
This voltage is fed to the variable capacitance
diode of VCO2, to enable it to generate stable
oscillations per every 1MHz (28, 29 and
so on up to 37 MHz).
vco2
PHASE
DETECTOR
4) Frequencies of each section of receiver
Next, each of the frequencies of the different
sections of the receiver have to be changed in ac-
cordance with the frequency of the signal being
received. However, from the standpoint of the
makeup of the circuitry, it is difficult to change
VCO2 in the same manner as VCO1. Accordingly,
these changes are accomplished in the following
manner.
a. Functions of controls
When the 10MHz-step selector control for
SW BAND SELECTOR is turned, the oscilla-
ting frequency of VCO1 is also switched in
steps of 10 MHz.
When the 1 MHz-step selector control for SW
BAND SELECTOR is turned:
1. The oscillating frequency of VCO1 is also
switched in steps of 1 MHz (0O—9 MHz);
2. The oscillating frequency of VCO2 is also
switched in steps of 1 MHz (28—37 MHz);
When the MW/SW TUNING DIAL is turned:
The VFO frequency changes.
In. this manner, the frequencies of each of the
sections change as the three controls referred
to above are moved.
Refer to Page 10 for the relationship between
the oscillating frequencies of each of the sec-
tions of the receiver,
b. Oscillating frequecny ranges of each section
and frequencies during reception
VCO1: produces oscillations from 18.975 MHz
to 49.075 MHz dividing this spectrum
into three bands.
VCO2: produces oscillations from 28 MHz to
37 MHz in steps of 1MHz (28, 29,
30, 31, 32, 33, 34, 35, 36 and 37 MHz),
VFO: produces oscillations continuously from
0.975 MHz to 2.075 MHz.
These three oscillators enable reception fromm
1MHz to 30MHz. This is accomplished in
the following manner.
Band switching is carried out per 1 MHz by
changing the oscillating frequency of VCO2,
while for frequencies in between, overall
reception is provided by changing the oscilla-
ting frequency of the VFO.
For example, when a 7.5 MHz signal is: to be
tuned in, the frequencies of each of the sectioras
becomes as follows:
VCO1: 19.055 MHz + 7.5 MHz = 26.555 MHz
VCO2: 28 MHz +7 MHz = 35 MHz
VFO: = 1.055 MHz + 0.5 MHz = 1.555 MHz