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Motorola Electronic Components Datasheet

MC92307 Datasheet

2K - Samples FFT-Processor

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MC92307 pdf
MOTOROLA
SEMICONDUCTOR TECHNICAL DATA
Current information @www.mot.com.sps/ADC
MC92307
Advance Information
2K - Samples FFT-Processor
The MC92307 is a pipelined Fast Fourier Transformation (FFT) processor with a
blocklength of 2048 complex samples. It is especially designed for use in digital ter-
restrial Set-Top boxes according to the DVB-T standard for 2K transmission. One
block of 2048 complex samples can be processed in 224 µs
Feature Summary
• Processing of one block of 2048 complex samples (i.e. one 2K-OFDM symbol) in
224 µs.
• Designed for direct interfacing with Motorola’s OFDM demodulator MC92308.
• Input wordlength 8 bit, output accuracy selectable between 10 and 12 bit.
• Overflow on certain OFDM subcarriers due to co-channel interferes is handled in-
ternally.
• Readout of output samples can be shifted by half the blocklength to start at the
middle of the output block.
• Either multiplexed or non-multiplexed input and output format (the multiplexed
mode allows direct interfacing with Motorola’s OFDM Demodulator MC92308).
• Internal Bit reversal can be disabled to allow the construction of FFT modules for
blocklengths > 2K by using several devices in parallel.
• Low clock rate inputs and outputs.
• 0.5µm CMOS process at 3.3 V.
• 5 V tolerant inputs and outputs.
RESB
CLK
SYMSYNC
DOUT[11:0]
DOUTR[11:0]
NOMUX
OFFSET
RES[1:0]
FFTSTART
DIN[7:0]
DINR[7:0]
Ordering Information
Device
MC92307CI
Package
160QFP
NOMUX
DIN 8
DINR 8
FFTSTART
Input
Buffer
RESB
CLK
FFTSTART
16
Control
FFT (11 stages)
incl. Rounding
Twiddle Factor ROM
24 Output
Reorder
Buffer
OFFSET
RES[1:0]
REVRSB
Figure 1. Block diagram of the FFT processor MC92307
This document contains information on a new product.
Specifications and information herein are subject to change without notice.
© MOTOROLA, INC. 1997
12 DOUT
12 DOUTR
SYMSYNC
8/12/97


Motorola Electronic Components Datasheet

MC92307 Datasheet

2K - Samples FFT-Processor

No Preview Available !

MC92307 pdf
External Interfaces
Fast Fourier Transform in the DVB-T System
Within the DVB consortium several standards for digital
television broadcast were submitted for the different
transmission media, leading to DVB-S (satellite), DVB-
C (cable) and DVB-T (terrestrial). The details of the
DVB-T system are described in reference [1]. To cope
with the special effects of digital terrestrial transmission
the OFDM modulation scheme was chosen: In principle
the whole available bandwidth is divided into a large
number N (e.g. 2048) of separate narrowband subchan-
nels (the OFDM subcarriers). Data associated with
each subcarrier are transmitted independently from and
in parallel with the other subcarriers, leading to a very
low datarate on each subcarrier compared to the overall
transmission capacity.
The superposition of all the subcarriers in the transmit-
ter is achieved very efficiently by an IFFT. Consequently
the corresponding functional block in the receiver is a
FFT. This IFFT -> FFT pair is one of the central ele-
ments in all OFDM systems, it represents the link be-
tween the so-called frequency domain and the time
domain used for the RF link between the transmitter and
the receiver. Therefore one of the important parameters
describing an OFDM system is the blocklength of the
IFFT/FFT, such a block is called an ‘OFDM symbol’.
In the DVB-T specification two modes are described,
the first with 2048 (2K) complex samples per OFDM
symbol, the second with 8192 (8K) samples per block.
The MC92307 device described here is designed espe-
cially for the use in systems working according in the
2K-mode described in reference [1].
External Interfaces
Control Interface
CLK
The overall OFDM transmission clock of app.
36.57 MHz must be provided at this pin. Obviously in
case of a DVB-T frontend composed of a MC92308 to-
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.
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.
OFFSET
After the completion of the FFT processing the frequen-
cy domain data are output by the reorder buffer, starting
together with a pulse of the SYMSYNC pin. The starting
position of the output can be selected with the OFFSET
pin. With OFFSET set to L the first frequency domain
sample of a block that is delivered at the output of the
FFT device belongs the baseband frequency corre-
sponding to the DC component (i.e. the center frequen-
cy in the received time domain signal). The numbers of
the frequency bins increases continuously until bin
#2047 at the end of the block.
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
component in the received time domain signal). Again
the bin numbers increase up to bin #2047. Then a wrap-
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-
face (DOUT and DOUTR) can be used in two modes:
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
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


Part Number MC92307
Description 2K - Samples FFT-Processor
Manufacturer Motorola
Total Page 10 Pages
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