Z87200 Datasheet, PDF (19/54 Pages) Zilog, Inc. – Spread-Spectrum Transceiver

Zilog
Z87200
Spread-Spectrum Transceiver
To detect this maximum correlation in each symbol period, maximum of 65,533, is stored in addresses 2E_H and 30_H.
the signal power value is compared against a 10-bit user- A counter is used to count the number of missed detects in
programmable threshold value. A symbol clock pulse is
generated each time the power value exceeds the thresh-
each burst, and the system can be configured to automat-
ically abort a burst and return to acquisition mode if this
4
old value to indicate a symbol detect. Since the Acquisi- number exceeds the Missed Detects per Burst Threshold
tion/Preamble symbol and subsequent data symbols can value stored in address 2F_H. Under normal operating con-
have different PN codes with different peak correlation val- ditions, the Z87200 will automatically return to acquisition
ues (which depend on the PN code length and code prop- mode when the number of symbols processed in the burst
erties), the Z87200 is equipped with two separate thresh-
old registers to store the Acquisition/Preamble Threshold
value (stored in addresses 29_H and 2A_H) and the Data
Symbol Threshold value (stored in addresses 2B_H and
2C_H). The device will automatically use the appropriate
value depending on whether it is in acquisition mode or
is equal to the value of the data stored in address 2E_H and
30_H. To permit the processing of longer bursts or continu-
ous data, this function can be disabled by setting bit 6 of
address 30_H high.
Differential Demodulator
not.
Both DPSK demodulation and carrier discrimination are
Since spread-spectrum receivers are frequently designed
to operate under extremely adverse signal-to-noise ratio
conditions, the Z87200 is equipped with a “flywheel circuit”
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 inappro-
priate times in each symbol period and to insert a symbol
clock pulse at the appropriate time if the symbol detection
supported in the Z87200 receiver by the calculation of
“Dot” and “Cross” products using the despread I and Q
channel information generated by the PN Matched Filter
for the current and previous symbols. A block diagram of
the DPSK Demodulator’s I and Q channel processing is
shown in Let I_k and Q_k represent the I and Q channel out-
puts, respectively, for the k^th symbol. The Dot and Cross
products can then be defined as:
is missed. The flywheel circuit operates by its a priori
knowledge of when the next detect pulse is expected. A
Dot(k) = I_k I_k-1 + Q_k Q_k-1; and,
priori, the expected pulse will occur one symbol period af-
ter the last correctly detected one, and a window of ±1
Cross(k) = Q_k I_k-1 - I_k Q_k-1.
baseband sample time is therefore used to gate the detect Examination of these products in the complex plane re-
pulse. Any detects generated outside this time window are veals that the Dot and Cross products are the real and
ignored, while a symbol detect pulse will be inserted into imaginary results, respectively, of complex multiplication
the symbol clock stream if the power level does not exceed of the current and previous symbols. The Dot product
the threshold within the window, corresponding to a alone thus allows determination of the phase shift between
missed detect. An inserted symbol detect signal will be successive BPSK symbols, while the Dot and Cross prod-
generated precisely one symbol after the last valid detect, ucts together allow determination of the integer number of
the nominal symbol length being determined by the value π/2 phase shifts between successive QPSK symbols. Dif-
of Rx Chips Per Data Symbol stored in address 2D_H.
The cross-correlation characteristics of a noisy received
signal with the noise-free local PN code used in the
Z87200’s PN Matched Filter may result in “smearing” of
the peak power value over adjacent chip periods. Such
ferential encoding of the source data implies that an abso-
lute phase reference is not required, and thus knowledge
of the phase shift between successive symbols derived
from the Dot and Cross products unambiguously permits
correct demodulation.
smearing can result in two or three consecutive power val- Implementation of this approach is simplified if the polari-
ues (typically, the on-time and one-sample early and late ties (the signs) alone of the Dot and Cross products pro-
values) exceeding the threshold. A maximum power selec- vide the information required to make the correct symbol
tor circuit is incorporated in the Z87200 to choose the high- decision. For BPSK and π/4 QPSK signals, no modifica-
est of any three consecutive power levels each time this tions are needed: in BPSK, the sign of the Dot product fully
occurs, thereby enhancing the probability that the optimum captures the signal constellation, while, in π/4 QPSK, the
symbol timing will be chosen in such cases. If desired, this signal constellation intrinsically includes the phase rotation
function can be disabled by setting bit 3 of address 30_H needed to align the decision boundaries with the four pos-
high.
sible combinations of the Dot and Cross product polarities.
The Z87200 also includes a circuit to keep track of missed
detects; that is, those cases where no peak power level ex-
ceeds the set threshold. An excessively high rate of
missed detects is an indication of poor signal quality and
can be used to abort the reception of a burst of data. The
number of symbols expected in each receive burst, up to a
For QPSK signals, a fixed phase rotation of π/4 (45°) is in-
troduced in the DPSK Demodulator to the previous symbol
to simplify the decision algorithm. Rotation of the previous
symbol is controlled by the settings of bits 0 and 1 of ad-
dress 33_H, allowing the previous symbol to be rotated by
0° or ±45°. As noted, for BPSK or π/4 QPSK signals, a ro-
tation of 0° should be programmed, but, for QPSK signals,
DS96WRL0400
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