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Theory of Operation
: VHF Receiver
2-21
2.13 VHF Receiver
2.13.1 Receiver Front-End
The RF signal is received by the antenna and applied to a low-pass filter. For VHF, the filter consists
of L3531, L3532, C3532 to C3563. The filtered RF signal is passed through the antenna switch. The
antenna switch circuit consists of two PIN diodes(D3521 and D3551) and a pi network (C3531,
L3551 and C3550).The signal is then applied to a varactor tuned bandpass filter. The VHF bandpass
filter comprises of L3301, L3303, C3301 to C3304 and D3301. The bandpass filter is tuned by
applying a control voltage to the varactor diode (D3301) in the filter.
The bandpass filter is electronically tuned by the DACRx from IC404 which is controlled by the
microprocessor. Depending on the carrier frequency, the DACRx will supply the tuned voltage to the
varactor diodes in the filter. Wideband operation of the filter is achieved by shifting the bandpass
filter across the band.
The output of the bandpass filter is coupled to the RF amplifier transistor Q3302 via C3306. After
being amplified by the RF amplifier, the RF signal is further filtered by a second varactor tuned
bandpass filter, consisting of L3305, L3306, C3311 to C3314 and D3302.
Both the pre and post-RF amplifier varactor tuned filters have similar responses. The 3 dB bandwidth
of the filter is about 12 MHz. This enables the filters to be electronically controlled by using a single
control voltage which is DACRx .
The output of the post-RF amplifier filter is connected to the passive double balanced mixer which
consists of T3301, T3302 and CR3301. Matching of the filter to the mixer is provided by C3317,
C3318 and L3308. After mixing with the first LO signal from the voltage controlled oscillator (VCO)
using high side injection, the RF signal is down-converted to the 44.85 MHz IF signal.
Figure 2-14. VHF Receiver Block Diagram
Crystal
Filter
Mixer
Varactor
Tuned Filter
RF Amp
Varactor
Tuned Filter
Pin Diode
Antenna
Switch
RF Jack
Antenna
Control Voltage
from ASFIC
First LO
from VCO
Second LO
IF Amp
455kHz
Filter
Switch
455kHz
Filter
455kHz
Filter
Switch
Demodulator
RSSI
Limiter
Recovered Audio
BW SEL
U3220
Содержание PR860
Страница 1: ...PR860 Two Way Portable Radio Detailed Service Manual ...
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Страница 5: ...iv Notes ...
Страница 9: ...viii Table of Contents Notes ...
Страница 13: ...Notes xii List of Figures ...
Страница 17: ...1 4 Introduction Radio Model Information Notes ...
Страница 51: ...2 34 Theory of Operation Voltage Control Oscillators VCO Notes ...
Страница 81: ...4 4 Schematic Diagrams Overlays and Parts Lists Flex Layout Notes ...
Страница 193: ...4 116 Schematic Diagrams Overlays and Parts Lists Low Band Section Notes ...
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