Theory of Operation
6-3
DRS2000HS/DRS6000HS
User and Technical Manual
Each BDC operates by mixing a Local Oscillator (LO) signal
together with the incoming RF signal and producing a UHF
output signal in the range of 110 to 860 MHz. The incoming
signal is filtered and then amplified by a Low Noise Amplifier
(LNA) and is sent along to the mixer. See
Figure 6-2
for a
functional block diagram of the block downconverter circuitry.
Figure 6-2: Block Diagram of Downconverter
The LO generates either a lower or a higher frequency
(depending upon the BDC’s operating band) using a Phase Lock
Loop (PLL) synthesizer. The 2 GHz BDCs use a low-side local
oscillator (its frequency is on the low side of the RF band) while
the 7 GHz BDCs use a high-side local oscillator. The
synthesizer’s frequency is fixed and programmed via the on-
board micro-controller. This LO frequency is then amplified and
filtered to reduce spurious signals before being applied to the
mixer.
In the mixer, the incoming RF signal is mixed with the LO signal
resulting in a signal equal to the difference between the two
signals. This signal is then amplified and low-pass filtered to
remove the signal representing the sum of the two signals.
RF Input
Band
Filter
Low-noise
amplifier
Mixer
Amplifier
Low pass
filter
Local
oscillator
Amplifier
Low pass
filter
Regulated DC power (+8.5 or +18.5 VDC) for the BDCs is
supplied via the BDC connectors on the same UHF cable
that carries output from the BDCs.
6.1.2
COFDM Diversity Demodulator
The COFDM diversity demodulator contains the Coded
Orthogonal Frequency Division Multiplexing (COFDM)
demodulation and diversity reception circuitry.
The IF signal is processed by the main demodulator
integrated circuits (ICs). The COFDM demodulator
automatically detects the key parameters of the incoming
signal via the TPS information and configures it accordingly.
These parameters include Modulation Type, Forward Error
Correction (FEC), Guard Interval (GI), and Spectrum
Inversion (SI). Only the bandwidth of the signal (6, 7, or 8
MHz) and the receive channel and frequency need to be
specified by the user.
The Diversity demodulator takes advantage of the spatial
antennas diversity provided by its six demodulators coupled
with six independent antennas, to provide reliable reception
even in the worst RF situations experienced at transmission
sites, Electronic News Gathering vehicles (ENG), or mobile
applications.
The receiver includes 6 sets of RF tuners and DVB-T
demodulators that are used in a diversity way. It supports all
DVB-T modes and can operate in VHF and UHF bands for
channel spacings of 6 MHz, 7 MHz, or 8 MHz. The MPEG
transport stream is presented as a serial ASI interface.
The demodulator has been especially designed for mobile,
portable and fixed applications by using special features for
difficult environments that improve significantly their
reception:
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