Filter
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CIRCUIT DESCRIPTION
6-2
March 1999
Part No. 001-2009-600
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
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
BPF
L223
L224
BANDPASS
FILTER
L215
L216
52.95 MHz 4-POLE
17.5 MHz TCXO
DATA
CLK
EN
LD
PD
OUT
F
IN
IN
HS CS RX
HS LK RX
U205
BUFFER
Q210
Q211
Q203 Q204
OCXO IN
TX OCXO
QUADRATURE DETECTOR/
843-848 MHz
SYNTHESIZER
SYNTHESIZER
REF
HIGH STABILITY
REF
MAIN
2ND MIXER
6.1.6 SECOND MIXER/DETECTOR
As shown in Figure 6-2, U202 contains second
oscillator, second mixer, limiter, detector and RSSI
circuitry. 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 (±0.1 PPM) output of Y201 is
fed through C231 to tripler Q205. The tripler passes
the third harmonic at 52.5 MHz to the OSC B input of
U202.
Biasing of Q205 is provided by R228, R227 and
R229. RF choke L214 blocks the flow of RF through
R229. An AC voltage divider formed by C236/C235
matches Q205 to the highpass filter. The third har-
monic of the TCXO frequency is then used to drive
the OSC B input at 52.5 MHz. L215, C237 and L216
form a high pass filter to attenuate frequencies below
52.95 MHz. C222 and C238 match the output of the
filter to U202.
Figure 6-2 U202 BLOCK DIAGRAM
MIXER
LIMITER
OSC
AMP
DEMODULATOR
AMP
AMP
FILTER
SQUELCH TRIGGER
WITH HYSTERESIS
AF
QUAD COIL
DECOUPLING 1
DECOUPLING 2
GROUND
SQUELCH IN
FILTER OUT
FILTER IN
RSSI
AUDIO9
10
11
12
13
MUTE14
15
16 MIXER IN
IF IN
Vcc
MIXER OUT
OSC E
7
6
5
4
3
2
OSC B 1
8