Preliminary
HT9015
·
·
Limiting IF Amplifier, RSSI (Received Signal
Strength Indicator) and RSSI Comparator
Quadrature detector
Once the signal leaves the IF section, it must
be demodulated so that the baseband signal
can be separated from the IF signal. This is
accomplished by the quadrature detector. A
quadrature detector (Figure 7) uses a 90°
phase shifter (Ci), an L/C tuned circuit, and a
phase comparator to demodulate FM signals.
The basic function of the IF amp is to boost
the IF signal and to help handle impulse
noise. The IF limiter applies very high gain to
the IF frequency such that the top and bottom
of the waveform are clipped. This helps in re-
ducing AM and noise intercepted upon recep-
tion. The limiting IF amplifier consists of four
differential amplifiers (Figure 6).
L
C
-
T
a
n
k
I
F
-
O
U
T
D
E
C
Q
u
a
d
r
a
t
u
r
e
A
M
P
1
A
M
P
2
A
M
P
3
A
M
P
4
D
e
t
e
c
t
o
r
O
F
-
I
N
I
n
t
e
r
n
a
l
2
n
d
I
F
S
(
R
S
S
I
)
Figure 6 Limiting IF Amplifier, RSSI and RSSI comparator
V
D
D
D
i
f
f
e
r
e
n
t
i
a
l
t
o
S
i
n
g
l
e
-
E
n
d
e
d
C
o
n
v
e
r
t
e
r
L
C
-
T
a
n
k
D
e
m
o
d
u
l
a
t
i
o
n
L
C
R
I
L
¯
1
I
L
¯
2
S
i
g
n
a
l
L
P
F
f
0
=
4
7
k
H
z
B
u
f
f
e
r
V
O
¯
C
i
I
1
¯
I
2 ¯
I
F
_
A
M
P
i
V
I
=
1
2
¯
Figure 7 Quadrature detector
16
April 10, 2000
HOLTEK [ HOLTEK SEMICONDUCTOR INC ]
