Service Manual

11
C505
LNA
TUNE BPF
TUNE FROM MCU
C504
C523
L508
L506
Q500
D503
L507
D506
1
23
L501
C526
C519
C524
L502
C521
C506
TUNE FROM MCU
D504
D505
D507
TUNE BPF
Figure 4
The Rx signals input from the antenna is filtered to remove out-of-band signals at the electrically
tunable bandpass network (D503, D504, L501, L502, C503, C505 and 507) and then amplified by
low-noise amplifier (LNA) Q500 to obtain a certain level required by reception. The signals pass
through the three-level bandpass network (mainly consisting of D505, D506, D507 and periphery
components) to effectively restrain the out-of-band interference and to derive pure Rx RF signals,
which will be fed to the mixing stage.
The electrically tunable control signals are provide by MCU. The required level can be obtained
through table looking up or formula computing to accurately control varactors to operate within
proper voltage range. It constitutes a bandpass filter with the periphery inductive capacitors to track
the Rx frequency under the change of MCU control voltage and to obtain the preset Rx sensitivity
requirements and out-of-band interference requirements.
Mixing Circuit (MIXER) and IF Bandpass Network (BPF)
Block Diagram for the Mixing Circuit
L53022nH
C533
C534
C532
C531
XF1
Q501
38.85MHZ
1stIF BPF
RF MIXER
AN29160
C528
Figure 5
The mixer mainly processes the local oscillator signals and Rx RF signals output by VCO. The first
IF from the mixer is used by the demodulator chip to discriminate frequency. Here the active mixer