Sony ICF-6800W - Page 11

1C3 (phase comparator)
As shown in Fig. 21, IC3 comprises a digital
phase comparator consisting of a combination of
gates and an active low-pass filter amplifier.
The digital phase comparator compares the
phases of the signals that come into the terminals
@ and (8) r
Phase comparison is carried out at the leading
edge of the pulse. (Refer to Fig. 19)
1
7
q n in
in
1
1
1
t
|
t
1
1
@
input
~
;
ae
|
f
|
1
‘
1
1
a :
‘ 1
t
al \
yt 1
il |
i t
tl l
@
output
ag
a
oes
ae
in-phase phase lag phase advance
Fig. 19
As shown in Fig. 19, when the phase of input
is lagging, the output is low (0) level. When the
phase of input is advanced, it will be high (1)
level.
The signals that appear at the terminal ©) after
going through the phase comparator are fed to the
active filter. Fig. 20 shows an approximation of
this active filter.
phase Ro CF
comparator
CAPACITANCE —=
ICF-6800W
Basically, the active filter is composed of Rl
and CF. By applying negative feedback from the
return
of
the
CF,
the
linear
zone
is
expanded
at
the
6 dB/oct curve formed by RI and CF.
As for its overall functioning, if for example
the frequency of VCO1 is higher than the intended
one:
@ A positive pulse signal will be sent out from
the output of the comparator.
@ The output is filtered by the active filter, while
the polarity of this amplifier is also reversed,
and so a DC signal that tends to become close
to the ground potential is produced from the
filter output.
@ The reverse bias fed to D27 drops, and the
equivalent capacity increases.
® Since D11 is connected as an element of the
oscillator’s L-C of VCO1, the oscillating fre-
quency is lowered and locks on the desired
frequency.
Relationship between voltage difference between
both ends of variable capacitance diode and capacity
3 x
DEPENDS UPON
oF = a VARICAPS
= MAX.
VOLTAGE ACROSS VARICAP
Fig. 22
RIS! 47k