MC92307 Datasheet, PDF (2/10 Pages) Motorola, Inc – 2K - Samples FFT-Processor

External Interfaces
Fast Fourier Transform in the DVB-T System OFFSET
Within the DVB consortium several standards for digital After the completion of the FFT processing the frequen-
television broadcast were submitted for the different
cy domain data are output by the reorder buffer, starting
transmission media, leading to DVB-S (satellite), DVB- together with a pulse of the SYMSYNC pin. The starting
C (cable) and DVB-T (terrestrial). The details of the
position of the output can be selected with the OFFSET
DVB-T system are described in reference [1]. To cope pin. With OFFSET set to L the first frequency domain
with the special effects of digital terrestrial transmission sample of a block that is delivered at the output of the
the OFDM modulation scheme was chosen: In principle FFT device belongs the baseband frequency corre-
the whole available bandwidth is divided into a large
sponding to the DC component (i.e. the center frequen-
number N (e.g. 2048) of separate narrowband subchan- cy in the received time domain signal). The numbers of
nels (the OFDM subcarriers). Data associated with
the frequency bins increases continuously until bin
each subcarrier are transmitted independently from and #2047 at the end of the block.
in parallel with the other subcarriers, leading to a very
low datarate on each subcarrier compared to the overall
transmission capacity.
If OFFSET is set to H the first frequency domain sample
belongs to the baseband frequency bin #1024, corre-
sponding to an offset of one half of the block (i.e. the DC
The superposition of all the subcarriers in the transmit- component in the received time domain signal). Again
ter is achieved very efficiently by an IFFT. Consequently the bin numbers increase up to bin #2047. Then a wrap-
the corresponding functional block in the receiver is a
n ^{FFT. This IFFT -> FFT pair is one of the central ele-}
ments in all OFDM systems, it represents the link be-
tio ^{tween the so-called frequency domain and the time}
domain used for the RF link between the transmitter and
a _{the receiver. Therefore one of the important parameters}
describing an OFDM system is the blocklength of the
rm IFFT/FFT, such a block is called an ‘OFDM symbol’.
fo ^{In the DVB-T specification two modes are described,}
the first with 2048 (2K) complex samples per OFDM
In _{symbol, the second with 8192 (8K) samples per block.}
The MC92307 device described here is designed espe-
y _{cially for the use in systems working according in the}
r _{2K-mode described in reference [1].}
ina _{External Interfaces}
lim _{Control Interface}
e _CLK
r _{The overall OFDM transmission clock of app.}
P _{36.57 MHz must be provided at this pin. Obviously in}
around to bin #0 occurs.
Table 1. Function of OFFSET
Sample #
after
SYMSYNC
s₀
s₁
...
s₁₀₂₂
sd₁₀₂₃
s₁₀₂₄
...
s₂₀₄₆
s₂₀₄₇
(Baseband)
Bin #
(OFFSET = L)
DOUT₀
DOUT₁
...
DOUT₁₀₂₂
DOUT₁₀₂₃
DOUT₁₀₂₄
...
DOUT₂₀₄₆
DOUT₂₀₄₇
(Baseband)
Bin #
(OFFSET = H)
DOUT₁₀₂₄
DOUT₁₀₂₅
...
DOUT₂₀₄₆
DOUT₂₀₄₇
DOUT₀
...
DOUT₁₀₂₂
DOUT₁₀₂₃
Note that the bin numbers given in the table above cor-
respond to the baseband positions.
NOMUX
The input interface (DIN and DINR) and the output inter-
case of a DVB-T frontend composed of a MC92308 to- ^{face (DOUT and DOUTR) can be used in two modes:}
gether with the MC92307 both devices have to be sup-
plied with the same clock signal. Normally the
36.57 MHz are generated by a VCXO in the tuner which
in turn is controlled by the OFDM timing synchronization
circuitry.
Normally the complex samples are provided in the
MULTIPLEXED mode, i.e. at first the real part of one
sample is transferred using the DIN or DOUT lines fol-
lowed by the imaginary part of the same sample using
the same physical pins. The pins labelled DINR and
RESB
Synchronous Reset: Normally H, a L together with a ris-
ing edge at CLK puts all outputs in tri-state and aborts
any processing, leaving the FFT device in the idle
mode.
DOUTR pins are not used, DINR should be tied to a
fixed value and DOUTR should be left unconnected.
The multiplexed mode is selected by setting the NO-
MUX line at L level, note that it is the only mode the FFT
device MC92307 can be used together with the OFDM
device MC92308.
MOTOROLA
2
MC92307_DS - 8/12/97