CIRCUIT DESCRIPTION
6-2
September 1994
Part No. 001-2008-300
Figure 6-1 RECEIVER BLOCK DIAGRAM
RECEIVE
RF IN
L201/L202/L203
FILTER
BANDPASS
Q201
RF AMP
L204/L205/L206
FILTER
BANDPASS
U201
MIXER
CRYSTAL FILTER
Z201/Z202
Q202
IF AMP
CRYSTAL FILTER
Z203/Z204
AUDIO AMP
U203A
RSSI
U203B
RSSI
Q205
MULTIPLIER
BUFFER
U209
SYN CS RX
SYN LK RX
BUFFER
VCO
BUFFER
Q214/Q215
AMP
AMP
U204A
Y201
RX INJ
TP
Z205
U202
RX WB AUDIO
Q216
Q217
Q210
Q211
Q208
Q209
A201
DETECTOR
PHASE
A006
450 kHz
52.5 MHz
52.95 MHz 4-POLE 52.95 MHz
52.95 MHz
RF DATA
RF CLK
DATA
CLK
EN
LD
PD
OUT
F
IN
IN
RF
QUAD DETECTOR/2ND OSC
BPF
L223
L224
BANDPASS
FILTER
L215
L216
52.95 MHz 4-POLE
17.5 MHz TCXO
753-771 MHz
Biasing of Q205 is provided by R228, R227,
R229 and R230. RF choke L214 blocks the flow of
RF through R229. A bandpass filter is formed by
C236, C235 and L215 is tuned to the third harmonic
of Q205. C237 couples the third harmonic of Q205 to
another bandpass filter formed by L216, C222 and
C238. The third harmonic of the TCXO frequency is
then used to drive the oscillator input at 52.5 MHz.
The 450 kHz second IF is then fed to ceramic fil-
ter Z205, then into the IF amplifier. The 6-pole
ceramic filter has a center frequency of 450 kHz and a
bandwidth of 15 kHz used for added receiver selectiv-
ity. The limiter amplifies the 450 kHz signal, then
limits it to a specific value.
From the limiter the signal is fed to the quadra-
ture detector. An external phase-shift network con-
nected to U202, pin 8, shifts the phase of one of the
detector inputs 90
°
at 450 kHz (the other inputs are
unshifted in phase). When modulation occurs, the fre-
quency of the IF signal changes at an audio rate as
does the phase of the shifted signal. The detector,
which has no output with a 90
°
phase shift, converts
the phase shift into an audio signal. Z213 is adjusted
to provide maximum undistorted output from the
detector. The audio signal is then fed out on U202,
pin 9.
6.1.6 SECOND MIXER/DETECTOR (U202)
As shown in Figure 6-2, U202 contains second
oscillator, second mixer, limiter and detector. The
52.95 MHz IF signal is mixed with a 52.5 MHz signal
produced by TCXO Y201 and tripler Q205. The 17.5
MHz (
±
1 PPM) output of Y201 is fed through C231 to
tripler Q205. The tripler filter passes the third har-
monic at 52.5 MHz to the oscillator input of U202.
Figure 6-2 U202 BLOCK DIAGRAM
MIXER
LIMITER
O
S
C
AMP
D
E
M
O
D
U
L
A
T
O
R
AMP
AMP
FILTER
S
Q
U
E
L
C
H
T
R
IG
G
E
R
W
IT
H
H
Y
S
T
E
R
E
S
IS
AF
QUAD COIL
DECOUPLING 1
DECOUPLING 2
GROUND
SQUELCH IN
FILTER OUT
FILTER IN
RSSI
AUDIO
9
10
11
12
13
MUTE
14
15
16
MIXER IN
IF IN
Vcc
MIXER OUT
OSC E
7
6
5
4
3
2
OSC B
1
8
