Motorola GTX/LCS 2000 Service Manual page 38

GAIN = 16 dB
NF = 1.8 dB
PRE-SELECTOR
RF
3 ELEMENTS
AMP
CERAMIC FILTER
LOSS = 1.5 dB
REJ = -43 dB @ IMAGE
K9.1
SW 5 V
5V_REG
9V3
ADAPT
LO_INJ
PRE-SELECTOR
3 ELEMENTS
CERAMIC FILTER
C. LOSS = 7 dB
LOSS = 1.5 dB
NF = 7 dB
REJ = -43 dB @ IMAGE
IIP3 = 11 dBm
Figure 7-2 Receiver Functional Block Diagram
After the amplifier, the signal enters a second bandpass filter (FL5204), which is
identical to FL5203. The bandpass filter is fixed tuned from 851 to 870 MHz.
After the second bandpass filter, mixer U5211 down-converts the signal to the IF
frequency.
The mixer is a passive double-balance mixer that gets a local oscillator signal in
the range of 805.9 to 824.9 MHz from the synthesizer and works on 4-dBm power.
The IF frequency is 45.1 MHz. A diplexer matches the mixer IF port to 50 Ohm
out of the IF frequency band.
The diplexer consists of R5401, C5404, L5402.
The IF signal is fed to first crystal filter Y5201 through matching network L5401,
C5200.
Crystal filter Y5201 has two poles. The crystal filter is followed by matching
elements, that match the filter output impedance to the input impedance of IF
amplifier Q5201.
Following the IF amplifier is a matching network that matches the IF amplifier
output impedance to the input impedance of a second 45.1-MHz crystal filter
(Y5202). The second crystal filter is followed by matching elements that match
the output of the second crystal filter to the input of the receiver back end circuits.
After the matching circuit, the signal passes a pair of hot-carrier limiting diodes
(D5201) placed in front of the back end circuit.
The receiver back end consists of BBR IC (U5201) that contains the following
functions:
• Preamplifier & mixer to convert the signal to a second IF frequency of
455 kHz.
• Second LO circuit
GAIN = 21 dB
NF = 3.3 dB
IIP3 = 3 dBm
IF BPF
IF BPF
IF
CRYSTAL
CRYSTAL
AMP
2-POLE
2-POLE
LOSS = 2 dB LOSS = 2 dB
REJ = 15 dB @ 25 KHz REJ = 15 dB @ 25 KHz
IIP3 = 10 dBm IIP3 = 10 dBm
Theory of Operation
BPF 455 KHz
BPF 455 KHz
N = 4
N = 4
BW = 10 KHz
BW = 9 KHz
DET_AUDIO
BBR
RSSI
2ND LO
44.654 MHZ
7-3