Z87200 Datasheet, PDF (15/54 Pages) Zilog, Inc. – Spread-Spectrum Transceiver

Zilog
Z87200
Spread-Spectrum Transceiver
Transmitter PN Code Generation
Table 3. DQPSK Differential Encoder Sequence
When the Z87200 is used for burst signal operation, each
Signal
burst is preceded by an Acquisition/Preamble symbol to
facilitate acquisition. This Acquisition/Preamble symbol is
I, Q BIts
0
0
Quadrant
First
Quadrant Diagram
2nd
1st
4
automatically generated by the Z87200’s transmitter be-
fore information data symbols are accepted for transmis-
1
0
Second
3rd
4th
sion. Two separate and independent PN codes may be
1
1
Third
employed: one for spreading the Acquisition/Preamble
0
1
Fourth
symbol, and one for the subsequent information data sym-
bols. As a result, a much higher processing gain may be
used for signal acquisition than for signal tracking in order
to improve burst acquisition performance.
BPSK/QPSK Modulator
The Z87200 incorporates an on-chip BPSK/QPSK modu-
lator which modulates the encoded and spread transmit
The Transmitter Acquisition/Preamble and Transmitter
Data Symbol PN code lengths are completely independent
of each other and can be up to 64 chips long. Transmit PN
codes are programmed in the Z87200 as binary code val-
ues. The number of Transmitter Chips per Acquisition/Pre-
amble Symbol is set by the value stored in bits 5-0 of ad-
dress 43_H, and the Transmitter Acquisition/Preamble
Symbol Code coefficient values are stored in addresses
44_H to 4B_H. The number of Transmitter Chips per Data
Symbol is set by the data stored in address 42_H, and the
Transmitter Data Symbol Code coefficient values are
stored in addresses 4C_H to 53_H.
signal with the sine and cosine outputs of the Z87200’s
NCO to generate a digitized I.F. output signal, TXIFOUT_7-
0^{. Since the NCO operates at a rate defined by RXIFCLK,}
the BPSK/QPSK modulator output is also generated at this
sampling rate, and, consequently, TXIFCLK must be held
common with RXIFCLK to operate the Z87200’s
BPSK/QPSK Modulator. The digital modulator output sig-
nal can then be fed into an external 8-bit DAC (operating
at RXIFCLK) to generate an analog I.F. transmit signal,
where the chosen I.F. is the Z87200’s programmed NCO
frequency or one of its aliases with respect to the output
sampling rate, RXIFCLK. Please note that operation of the
BPSK/QPSK modulator is only specified to 20 MHz; that is,
A rising edge of the MTXEN input or of bit 1 of address 37_H
causes the Z87200 to begin the transmit sequence by
transmitting a single symbol using the Acquisition/Pream-
ble PN code. The completion of transmission of the Acqui-
if RXIFCLK/TXIFCLK is greater than 20 MHz in the system
design, it is recommended that the baseband transmit out-
puts of the Z87200 be used with an external BPSK/QPSK
modulator.
sition/Preamble symbol is indicated with TXACQPLS,
while the ongoing transmission of data symbols is signaled
with TXTRKPLS. Data bits to be transmitted after the Ac-
quisition/Preamble symbol are requested with TXBITPLS,
where a single pulse requests data in BPSK mode and two
pulses request data in QPSK mode. The user data sym-
bols are then PN modulated using the Transmitter Data
Symbol PN code.
When the Z87200 is set to transmit in BPSK mode (by set-
ting bit 0 of address 40_H high), identical signals are applied
to both the I and Q channels of the modulator so that the
modulated output signal occupies only the first and third
quadrants of the signal space defined in Note that the
modulator itself cannot generate π/4 QPSK signals, but the
Z87200 can receive such signals and can be used with an
external modulator for their transmission.
The PN spreading codes are XORed with the data bits (in
BPSK mode) or bit pairs (in QPSK mode) to transmit one
complete code sequence for every Acquisition/Preamble
and data symbol at all times. The resulting spread I and Q
channel signals are brought out as the TXIOUT and TX-
QOUT signals for use by an external modulator and are
also fed into the Z87200’s internal on-chip modulator. In
BPSK mode, only TXIOUT is used by the Z87200’s modu-
lator. If an external QPSK modulator is used, the carrier
should be modulated as shown in Table 3 to be compatible
with the Z87200 receiver.
DS96WRL0400
4-15