12-10-2012, 11:56 AM
Digital FM exciter – compact and versatile
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Input interface
Analog (left / right) and digital audio signals
(AES/EBU) are applied separately
(FIG 1). Additionally, you can supply an
MPX signal which has been generated
in an external stereo coder.
The input interface provides three analog
inputs for RDS, SCA and DARC signals.
The R&S ® SU800 digitizes these
signals and adds them to the internal
MPX signal. It decodes the AES signal
and converts it into a constant sampling
rate. This allows you to process
input data at different sampling rates up
to 192 kHz and to eliminate jitter effects
simultaneously.
To suppress interference, all incoming
analog data input signals are first routed
via analog lowpass filters, their level
adjusted, and then separately digitized
and fed to the digital signal processing
system. Digitization is handled by highquality
A/D converters – with 24-bit resolution
for analog audio signals and with
16-bit resolution for MPX and additional
data. You can define a specific level for
nominal deviation for each input.
Digital signal processing
The fully digital signal processing function,
which is handled by a powerful
field programmable gate array (FPGA),
ensures a consistently high level of signal
quality, free from random sources of
error such as temperature fluctuations or
quality variations of the component. Due
to the consistent use of digital technology,
the entire module needs virtually no
adjustment.
The digitized audio signals are routed via
a lowpass, and a bandpass suppresses
interfering signal components outside
the data signal spectrum. The internal
stereo coder processes the filtered audio
signals to yield a standard-conforming
MPX signal. For stereo signals, you can
set the level of the pilot tone. The digital
MPX signal is used for frequency modulation
of a high-precision direct digital
synthesizer (DDS).
Control
A powerful microcontroller configures
and monitors the R&S ® SU800. It initializes
the hardware from a compact
flash memory that contains the complete
software and firmware as well as
all settings.
The R&S ® SU800 concept is based on the
R&S ® Nx8000 transmitter platform and is
also used in the TV Exciter R&S ® Sx800
[2]. Important control features are thus
uniform, which simplifies maintenance
and operation significantly. During operation,
for example, the exciter is configured
and monitored by the Control Unit
R&S NetCCU ® 800 via the Ethernet interface
on the rear panel.
RF section
The RF section converts the digital signal
into an analog signal by means of a
fast 16-bit D/A converter. The signal is
then filtered – to meet the requirements
for spurious and spectrum masks – and
converted to the output frequency. The
selected switching concept allows frequency
switching without adjustments.
The synthesizer provides all necessary
clock and mixer frequencies. If you need
exceptionally good frequency stability,
the exciter allows you to synchronize the
internal clock to an external reference
(10 MHz or 1 pps).