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PDF STEL-2000A Data sheet ( Hoja de datos )
| Número de pieza | STEL-2000A | |
| Descripción | Fast Acquisition Burst Processor | |
| Fabricantes | Intel Corporation | |
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STEL-2000A
Data Sheet
STEL-2000A+45
(45 MHz)
STEL-2000A+20 DataShee
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(20 MHz)
Digital, Fast Acquisition
Spread Spectrum
Burst Processor
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GENERAL DESCRIPTION
The STEL-2000A is a programmable single-chip
spread spectrum transceiver. The device performs all
the digital processing required to implement a fast
acquisition direct sequence (i.e., pseudonoise- or PN-
modulated) spread spectrum full- or half-duplex sys-
tem using differentially encoded BPSK, QPSK, or π/4
QPSK. A block diagram of the STEL-2000A is shown
in Figure 1, while the package style and pin configu-
ration are shown in Figure 2. The STEL-2000A is
available in two speed grades; the STEL-2000A+20
(20 MHz maximum clock frequency), and the
STEL-2000A+45 (45.056 MHz maximum clock fre-
quency). The 45 MHz version features a high thermal
conductivity package for superior heat dissipation,
allowing the device to operate continuously at this
speed.
the PN code. Since PN modulation is symbol-syn-
chronous in the STEL-2000A, the data rate is defined
by the PN chip rate and length of the PN code; i.e., by
the number of chips per symbol. When operating
with BPSK modulation, the maximum data rate for a
PN code of length N is 11.264/N Mbps (STEL-
2000A+45 only, 5/N Mbps in the STEL-
2000A+20). When operating with QPSK modulation
(or π/4 QPSK with an external modulator), two bits of
data are transmitted per symbol, and the maximum
data rate for a PN code of length N is 22.528/N Mbps
(STEL-2000A+45 only, 10/N Mbps in the STEL-
2000A+20). Conversely, for a given data rate Rb, the
length N of the PN code employed must be such that
the product of N x Rb is less than 11.264 (for BPSK) or
22.528 (for QPSK) Mcps (STEL-2000A+45 only).
The STEL-2000A integrates the capabilities of a digital
The data rate (Rb) and the PN code length (N), how-
downconverter, PN matched filter, and DPSK
ever, cannot generally be arbitrarily chosen. United
demodulator into a single receiver, where the receiver
States FCC Part 15.247 regulations require a minimum
input is the analog-to-digital converted I.F. signal.
processing gain of 10 dB for unlicensed operation in
STEL-2000A transmit functions include a differential
the Industrial, Scientific, and Medical (ISM) bands,
BPSK/QPSK encoder, PN modulator (spreader), and
implying that the value of N must be at least 10. To
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BPSK/QPSK modulator, where the transmitter
output is a sampled digitally modulated signal ready
implement such a short code, a Barker code of length
11 would typically be used in order to obtain desir-
for external digital-to-analog conversion (oDra, taifSheet4Ua.cbolemauto- and cross-correlation properties. With the
preferred, the spread baseband signal may be output
STEL-2000A, a PN code length of 11 implies that the
to an external modulator). These transceiver
maximum data rate supported by the STEL-2000A in
functions have been designed and integrated for the
compliance with FCC regulations is 2.048 Mbps using
transmission and reception of bursts of spread data.
differential QPSK (STEL-2000A+45 only). The STEL-
In particular, the PN Matched Filter has two distinct
2000A further includes transmit and receive code
PN coefficient registers (rather than a single one) in
overlay generators to insure that signals spread with
order to speed and improve signal acquisition
such a short PN code length possess the spectral
performance. The STEL-2000A is thus optimized to
properties required by FCC regulations.
provide reliable, high-speed wireless data
communications.
The STEL-2000A receiver circuitry employs an NCO
and complex multiplier referenced to RXIFCLK to
The STEL-2000A operates with symbol-synchronous
perform frequency downconversion, where the input
PN modulation in both transmit and receive modes.
I.F. sampling rate and the clock rate of RXIFCLK
Symbol-synchronous PN modulation refers to opera-
must be identical. In Òcomplex inputÓ or Quadrature
tion where the PN code is aligned with the symbol
Sampling Mode, external dual analog-to-digital con-
transitions and repeats once per symbol. By synchro-
verters (ADCs) sample quadrature I.F. signals so that
nizing a full PN code cycle over a symbol duration,
the STEL-2000A can perform true full single sideband
acquisition of the PN code at the receiver simultane-
downconversion directly from I.F. to baseband. At
ously provides symbol synchronization, thereby sig-
PN chip rates less than one-eighth the value of
nificantly improving overall acquisition time.
RXIFCLK, downconversion may also be effected
The receiver clock rate (RXIFCLK frequency) must
be at least four times the receive PN spreading rate
using a single ADC in Òreal inputÓ or Direct I.F.
Sampling Mode, as discussed in Appendix I.
and is limited to a maximum speed of 45.056 MHz
The input I.F. frequency is not limited by the capabili-
(STEL-2000A+45 only, 20 MHz in the STEL-
ties of the STEL-2000A. To avoid destructive aliasing,
2000A+20). As a result, the maximum supported PN
the NCO should not be programmed above 50% of
chip rate is 11.264 Mchips/second (5 Mcps in the
the I.F. sampling rate (the frequency of RXIFCLK);
STEL-2000A+20), where a ÒchipÓ is a single ÒbitÓ of
moreover, the signal bandwidth, NCO frequency, and
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are, theoretically, the optimally despread I and Q
threshold. A maximum power selector circuit is
symbols.
incorporated in the STEL-2000A to choose the highest
To detect this maximum correlation in each symbol
period, the signal power value is compared against a
10-bit user-programmable threshold value. A symbol
clock pulse is generated each time the power value
exceeds the threshold value to indicate a symbol
of any three consecutive power levels each time this
occurs, thereby enhancing the probability that the
optimum symbol timing will be chosen in such cases.
If desired, this function can be disabled by setting bit
3 of address 30H high.
detect. Since the Acquisition/Preamble symbol and
The STEL-2000A also includes a circuit to keep track
subsequent data symbols can have different PN codes
of missed detects; i.e., those cases where no peak
with different peak correlation values (which depend
power level exceeds the set threshold. An excessively
on the PN code length and code properties), the
high rate of missed detects is an indication of poor
STEL-2000A is equipped with two separate threshold
signal quality and can be used to abort the reception
registers to store the Acquisition/Preamble Threshold
of a burst of data. The number of symbols expected
value (stored in addresses 29H and 2AH) and the Data
Symbol Threshold value (stored in addresses 2BH and
2CH). The device will automatically use the appro-
priate value depending on whether it is in acquisition
in each receive burst, up to a maximum of 65,533, is
stored in addresses 2EH and 30H. A counter is used to
count the number of missed detects in each burst, and
the system can be configured to automatically abort a
mode or not.
burst and return to acquisition mode if this number
Since spread spectrum receivers are frequently
designed to operate under extremely adverse signal-
to-noise ratio conditions, the STEL-2000A is equipped
with a Òflywheel circuitÓ to enhance the operation of
the symbol tracking function by introducing memory
to the PN Matched Filter operation. This circuit is
designed to ignore false detects at inappropriate times
in each symbol period and to insert a symbol clock
pulse at the appropriate time if the symbol detection
exceeds the Missed Detects per Burst Threshold value
stored in address 2FH. Under normal operating con-
ditions, the STEL-2000A will automatically return to
acquisition mode when the number of symbols pro-
cessed in the burst is equal to the value of the data
stored in address 2EH and 30H. To permit the pro-
cessing of longer bursts or continuous data, this func-
tion can be disabled by setting bit 6 of address 30H
high.
is missed. The flywheel circuit operates by its aDpraitoariSheet4UD.cioffmerential Demodulator
knowledge of when the next detect pulse is expected.
Both DPSK demodulation and carrier discrimination
The expected pulse will occur one symbol period after
are supported in the STEL-2000A receiver by the cal-
the last correctly detected one, and a window of ±1
culation of ÒdotÓ and ÒcrossÓ products using the
baseband sample time is therefore used to gate the
despread I and Q channel information generated by
detect pulse. Any detects generated outside this time
the PN Matched Filter for the current and previous
window are ignored, while a symbol detect pulse will
symbols. A block diagram of the DPSK Demodula-
be inserted into the symbol clock stream if the power
torÕs I and Q channel processing is shown in Figure 4.
level does not exceed the threshold within the win-
dow, corresponding to a missed detect. An inserted
Let Ik and Qk represent the I and Q channel outputs,
respectively, for the kth symbol. The dot and cross
symbol detect signal will be generated precisely one
products can then be defined as:
symbol after the last valid detect, the nominal symbol
length being determined by the value of Rx Chips Per
Dot(k) = Ik Ik-1 + Qk Qk-1; and,
Data Symbol stored in address 2DH.
Cross(k) = Qk Ik-1 - Ik Qk-1.
The cross-correlation characteristics of a noisy
received signal with the noise-free local PN code used
in the STEL-2000AÕs PN Matched Filter may result in
ÒsmearingÓ of the peak power value over adjacent
chip periods. Such smearing can result in two or
three consecutive power values (typically, the on-time
and one-sample early and late values) exceeding the
Examination of these products in the complex plane
reveals that the dot and cross products are the real
and imaginary results, respectively, of complex mul-
tiplication of the current and previous symbols. The
dot product alone thus allows determination of the
phase shift between successive BPSK symbols, while
the dot and cross products together allow
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