Functions Of Phase Detector, Charge Pump And Lpf; Phase Comparator And Low-Pass Filter - Technics ST-9038 Manual

i
PART
II
PLL
SYNTHES]ZER
t'
1-7
Phase
comparator and
low-pass
filter
V1
Programmable
freq
uency
divider output
12.5
KHz +
L'f
Reference
compar¡son
frequency
12.5 KHz
+VDD
Vcc
c
B
o5
a4
To VCO
R1
IS
GND
Muting
signal
(To
lF)
(To AF)
1-7-1
Functions
of
phase detector,
charge
pump and LPF
ln the
above-mentioned circuit, the charge pump by the
phase
detector and C-MOS
is
involved
in
lC
¡rPD861C,
while LPF
is
of
discrete.
When
the reference comparison frequency
is
fed into
one
of the input
sides
of the phasedetector by
the sequential logíc circuit,
and
the output
of the
programmable frequency
divider
is
fed into
the
other, the
phase
or
frequency difference between the two frequencies
is
detected, and
its
output
¡s
the
logic value
of "O" or
"1"
in
accordance
with
the phase
"lead" and "lag" or
the frequency
differences. The results are shown in
the following.
(1)
When
the
programmable frequency divider output
Vr
is lagging
in
phase compared
wlth
the
reference
comparison frequency
Rr,
"1"
is theoutput at
Dr,
and
"0"
at Ur.
(2)
ln
the
reverse case,
"0"
is
the
output at Dr
,
and
"1"
4t Ur
.
(3)
When
both
are
¡dentical in phase,
the output at Dr
and
Ur
is
"1"
(4)
The
outputs
of Dr
and
Ur
will
never be
"0"
4t
any
t¡me.
Since
the
phase
detector is able
to
detect the phase lead and lag but unable
to
convert the current
into
DC power, the charge pump of the C-MOS
is used
to
achieve
this.
lt
is
connected to the
active
low-pass
filter
consisting
of
a
darlington amplif ier and
a CB
network which supplies the
DC
output
to
the VCO.
Suppose
the
frequency
of
the VCO is shifted
by +Af for
some reason,
then
Vr
leads
in
phase
compared
to
Rr,
causing
Dr to
become
"0".
Then,
Or,
channel P, turns ON
to
change
C'thus
decreasing
the output
voltage
of
the active LPF and
lowering
the
VCO
frequency. Next,
VCO,
shifted
by
-Af,
causes
Ur to
become
"0",
therefore
Or of
channel N
turns ON
causing
the
voltage
-t0*
t
I
o6
a7
*
G
chann
Phase
detector'
N
chan nel
Ç