Transmitter Section - NEC PASOLINK Training Course

ROI-S04605

2.1 Transmitter Section

2. FUNCTIONAL OPERATION
This chapter describes the functional operation of the ODU which
comprises of transmitter section, receiver section, and alarm and control
section. A functional block diagram is shown in Fig. 2-1.
This section describes Engineering Orderwire (EOW) signal demodulation
and IF-to-RF conversion.
The 850 MHz IF signal which comprises an EOW signal, monitor (MON)
signal, alarm and control signals and DC component from the IDU is
separated through the MPX circuit. The EOW signal is processed at the
CPU and fed to the RX LEV MON jack. The DC component is applied to
the PS module to produce regulated DC voltages which are used in the
ODU. The 850 MHz IF signal enters the mixer (MIX) through the AGC
AMP which compensate for any input IF signal level variations.
The MIX mixes the incoming 850 MHz IF signal with a [ ] GHz band local
signal generated by the synthesized RF local oscillator to produce the
specified transmit RF signal.
The transmit RF signal enters a BPF which eliminates undesired
components generated in the IF-to-RF conversion. The filtered RF signal
goes to the RF amplifier where it is amplified to the required level by an
automatic level control (ALC) circuit. The amplified RF signal is then fed
to the antenna through the Duplexer.
The automatic transmitter power control (ATPC) function automatically
varies the TX output power according to path conditions. Fading exerts
heavy influences on propagation, causing the receive signal level at the
opposite station to vary. The ATPC function operates by controlling the
transmitter output power of the local station according to the variation of
the received signal level at the opposite station. The received signal level
variation at the opposite station is informed to the local station using the
ATPC bits in the overhead.
A constant transmit output power is maintained using the ALC function
which is provided in the RF CKT module. The ALC circuit detect the
transmit output power using a diode to obtain a DC voltage proportional to
the transmit power. The gain of the RF amplifier is controlled inversely
with this detected DC voltage to maintain the transmit output power within
the specified limits.
FUNCTIONAL OPERATION
2-1