Digital Signal Processing; Analog Signal Processing - ABB ETL600 Instruction Manual

ABB Switzerland Ltd
3.2.1.

Digital signal processing

The FDM-signals of all AF interface boards 1 (O4LE) are fed to RF
converter module 4 (P4LT) via TDM bus, where
•
the pilot signals (one for each AF channel
•
the AF channel data signals,
•
the NSD600 AF signals and
•
the DPLC signal
are Single-SideBand (subsequently = SSB) modulated, combined into
a single low frequency (LF) signal and converted to the desired RF
band.
Traditionally, the conversion to the RF band has been achieved with
several stages of frequency conversion and filtering implemented in
analog technology. The reverse procedure was employed for
recovering the LF signal. It required critical analog filters for achieving
the desired filter characteristics. However, in ETL600, the AF signal is
first converted into a digital signal (a serial sequence of codewords
consisting of Zeros an Ones) by an analog to digital converter. This
digital signal is processed by Digital Signal Processor (DSP) circuits
located on the RF converter module type P4LT. Using suitable
software algorithms, this digital data is manipulated by the DSPs to get
an another digital data stream which when reconverted into an analog
signal gives the same effect as if the analog signal was SSB
modulated into RF. The reverse procedure is employed for
demodulation. One advantage of this technique is that by just
changing a few software parameters, a lot of settings such as carrier
frequencies, pilot frequencies, orientation of the spectrum etc. can be
changed. Another important advantage is that a lot of hardware for
modulation, demodulation, filtering etc. is eliminated, thereby reducing
the failure rate of the equipment and the necessary tuning efforts
drastically. Also, due to the digital nature of the involved hardware,
variations due to changes of climatic conditions (temperature,
humidity) as observed with analog modulation techniques are virtually
nonexistent.
3.2.2.

Analog signal processing

For ETL640/80, the RF signal modulated and converted into analog
form by the P4LT module is fed to the power subrack 14, where the
signal is amplified by the power amplifier 8. A hybrid 11 is used to
separate the transmitted RF signal from the received signal. The
transmit filter 9 allows for parallel connection of several equipment to
the same coupling device. The transmit filter can be placed either in
the transmit signal path (Fig. 3-10) or in the two wire signal path (Fig.
3-11). Refer to section 3.3.11 regarding the difference between these
two arrangements.
The output power may be doubled to 80 W by stocking a second
power subrack 14 on top of the first, equipped with another 40 W
1
If no AF channels are configured, a pilot is provided in the DPLC band.
3-16
1KHW001489-EN
November 2005
ETL600
1
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Structure and Function