O U T D O O R U N I T A N D A N T E N N A
13
Cable Interface
There are two functions accomplished in the cable interface:
•
The power supply of the radio unit,
•
The provision of signal transportation to the IDU over the coaxial cable.
The supply voltage, which can be in the range from 20 to 60 V with both polarities in relation to
the ground, is supplied from the IDU over the cable.
A 350 MHz frequency signal, modulated with the filtered data signal, is received from the IDU.
The cable interface contains the FM demodulation unit. The data signal from the output of the FM
demodulation unit is forwarded to the modulation input of the voltage-controlled oscillator in the
transmitting part of the synthesizer module. Thus the data signal is transported from the IDU to
the ODU. The cable can be so long until the maximum attenuation of the signal does not exceed
20 dB. The FM demodulators produce a voltage, which is proportional to the logarithm of the 350
MHz carrier power (cable RSSI). This voltage is supplied to the controller module and is used to
check if the allowed length of the coaxial cable is not exceeded.
The transportation of the data signal from the ODU to the IDU is carried out as follows: limited
(constant level) 140 MHz intermediate frequency signal is supplied to the coaxial cable through the
amplifier and the band-pass filter in the cable interface. This signal is frequency-modulated (the
modulation is performed in the transmitter of the opposite station) and the length of the cable has
no effect on the transmitted data signal, unless the 20 dB attenuation limit is kept up. Since the
filtration and restriction of the intermediate frequency signal is performed in the radio unit, the
transportation of the signal to the IDU has no effect on the sensitivity of the receiver.
Additional data signal with a transmission rate of 9 kbps is transmitted over the same coaxial cable
from the IDU to the ODU and vice versa. Both signals carry telemetry data between ODU and
IDU. This is performed using two carriers with pulse-modulated amplitude. The data from the
ODU to the IDU are transmitted on a 6 MHz carrier, but from the IDU to the ODU – on a
4.5 MHz carrier.
Intermediate Frequency Module
The IF module is used for the second conversion and filtering of the signal. The 1542 MHz
intermediate frequency signal is passed to the IF block from the first converter in the receiver.
Synthesized 1402 MHz oscillator jointly with a mixer transforms the first intermediate frequency
into the second intermediate frequency of 140 MHz. The second IF signal is filtered with a surface
acoustic waves (SAW) filter, after the filtering the signal arrives to the logarithmic amplifier that
has a signal limiter at the output. The logarithmic amplifier has an additional function – an exact
measurement of the power of the received signal. The direct voltage on the output of this amplifier
is supplied to the controller module and is used for the measurements.
Synthesizer Module
The synthesizer module has two very similar frequency synthesizers. One synthesizer supplies the
oscillator signal to the first mixer. The second has a modulation input and it supplies the signal to
the transmitter. The synthesizers are based on the voltage-controlled oscillators (VCO). The
frequency of VCOs is stabilized and modified, using a phase-locked loop (PLL).
Receiver
The receiver performs amplification of the received signal by means of a low-noise amplifier
(LNA), which is a monolithic microwave integrated circuit (MMIC), enclosed in a special package.
The signal after that is forwarded to the mirror-channel filter, which is installed with the purpose
to reduce the noise factor of the receiver. From the point of view of the input signal, sufficient
attenuation of the mirror channel is provided by the duplexer filter.
The amplified signal is passed to the subharmonic mixer, where it is converted to the frequency of
1542 MHz. This mixer is also a monolithic microwave integrated circuit.
RX block has a waveguide input.
Summary of Contents for CFM-13-LM
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