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CIRCUIT DESCRIPTION
BANDPASS FILTER
A201
RF IN
L102/L103/L104
SYNTHESIZER
RF DATA
REF IN
RF CLOCK
U401
SYN CS RX
SYN LK RX
V
R
F IN
Second Mixer/Detector
As shown in Figure 6-2, U203 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 Y401 and tripler
Q204. The 17.5 MHz (±2.5 PPM) output of Y401 is
fed through C275 to tripler Q204. The tripler passes
the third harmonic at 52.5 MHz to the oscillator input
of U203.
OSC B 1
MIXER
OSC E
2
MIXER OUT
3
Vcc
4
IF IN
5
DECOUPLING 1
6
LIMITER
AMP
DECOUPLING 2
7
QUAD COIL
8
Figure 6-2 U201/U203 BLOCK DIAGRAM
August 2000
Part No. 001-2001-200
RF AMP
BANDPASS FILTER
U103A
L108/L109/L110
17.5 MHz TCXO
Y401
BUFFER
MULTIPLIER
Q401
Q204
Q402
CHARGE PUMP
VOLTAGE MULTIPLIER
Q406, Q407
Q403, Q404
Q408, Q409
Q405, CR402
BUFFER
VCO
Q410
A006
Q411
Figure 6-1 12.5 kHz IF RECEIVER BLOCK DIAGRAM
16 MIXER IN
15
GROUND
14
MUTE
13
RSSI
12
SQUELCH IN
FILTER OUT
11
AMP
FILTER
10
FILTER IN
AF
9
AUDIO
AMP
MIXER
52.95 MHz 4-POLE
U101
CRYSTAL FILTER
Z211A/B
IF DETECTOR/AMP
U203
Z213/Z214
VCO
FIRST
SECOND
BUFFER/AMP
INJ AMP
INJ AMP
Q131
Q134
Q133
Q132
Biasing of Q204 is provided by R258, R259 and
R260. RF choke L222 blocks the flow of RF through
R261. An AC voltage divider formed by C280/C281
matches Q204 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. L223, C282 and L224
form a high pass filter to attenuate frequencies below
52.95 MHz. C283 and C284 match the output of the
filter to U203.
The 450 kHz second IF is then fed to ceramic fil-
ter Z213/Z214, then into the IF amplifier. The center
frequency of Z213/Z214 is 450 kHz with a bandwidth
of 9 kHz used to attenuate wideband noise. The lim-
iter amplifies the 450 kHz signal 92 dB which
removes any amplitude fluctuations.
From the limiter the signal is fed to the quadra-
ture detector. An external phase-shift network con-
nected to U203, 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. Z215 is adjusted
to provide maximum undistorted output from the
detector. The audio signal is then fed out on U203,
pin 9.
6-2
52.95 MHz 4-POLE
IF AMP
CRYSTAL FILTER
Q202
Z212A/B
U204A
RSSI
AUDIO
U204B
RX WBAND
BANDPASS
AMP
INJECTION FILTER
L140/L141
U102
INJECTION
TEST VOLTAGE
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