Z87200 Datasheet, PDF(31/54 Page) Zilog, Inc. – Spread-Spectrum Transceiver

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Z87200 Datasheet, PDF (31/54 Pages) Zilog, Inc. – Spread-Spectrum Transceiver

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Zilog

Z87200

Spread-Spectrum Transceiver

that viewport control affects both I and Q channels of the The choice of the threshold values will be determined by

PN Matched Filter output.

several factors. Arithmetically, the digital baseband sam-

Table 11. Matched Filter Viewport Control

Bits 1-0

ISUM, QSUM

Bits 7-0

4 ples of the received signal are multiplied by the PN

Matched Filter tap coefficients each baseband sample

clock cycle and the results are summed to provide a corre-

lation value. The I and Q PN Matched Filter correlated out-

put values are then used to estimate the signal power ac-

Bits 8-1

cording to the following approximation:

Bits 9-2

Max{Abs(I),Abs(Q)}+1/2 Min{Abs(I), Abs(Q)}.

Saturation protection is implemented for those cases when

the PN Matched Filter output signal level overflows the

scaled range selected for the Power Estimator and DPSK

Demodulator. When the scaled value range is exceeded,

the saturation protection limits the output word to the max-

imum or minimum value of the range according to whether

the positive or negative boundary was exceeded.

The magnitude of the estimated power thus depends on

several variables, including the setting of the Integrate and

Dump Filter viewport, the PN code length and autocorrela-

tion properties, and the magnitudes of the incoming

RXIIN7-0 and RXQIN7-0 signals. The actual threshold val-

ues that should be programmed will therefore vary from

application to application.

Acquisition and Tracking Processor

Registers

The Acquisition and Tracking Processor Registers allow

the user to configure how the PN Matched Filter outputs for

the Acquisition/Preamble symbol and the data symbols

that follow thereafter are treated in the Symbol Tracking

Processor. Since operation of the Z87200 receiver pre-

sumes symbol-synchronous PN modulation, processing of

the PN Matched Filter outputs can be used for symbol syn-

chronization prior to DPSK demodulation. The Acquisi-

tion/Preamble symbol and the data symbols may have dif-

ferent PN spreading codes, however, and so the PN

Matched Filter outputs may exhibit different signal levels

due to the different code lengths and auto-correlation

properties. The control registers in this block allow such

differences to be treated, as well as permitting specifica-

tion of the number of receive data symbols per burst and

other parameters associated with burst data communica-

tions.

Addresses 29H and 2AH:

Acquisition/Preamble Threshold

Addresses 29H and 2AH contain the unsigned Acquisi-

tion/Preamble Threshold value, as shown in This value is

used for comparison with the estimated signal power from

the PN Matched Filter to determine whether a successful

correlation has been detected in acquisition mode. The

Acquisition/Preamble Threshold value must be set by the

user to satisfactorily detect the correlation peak in noise

obtained when the received PN-spread Acquisition/Pre-

amble is correlated against a local version of the Acquisi-

tion/Preamble PN code by the PN Matched Filter. Once

the power estimation value exceeds the threshold value, a

successful correlation is assumed to have been detected.

Note that the Symbol Tracking Processor does not insert

missed detect pulses when the device is in acquisition

mode.

Table 12. Acquisition/Preamble Threshold Storage

The I and Q channel outputs of the PN Matched Filter are

processed to estimate the correlation signal power at each

baseband sampling instant. This estimated signal power is

compared with the contents of the Acquisition/Preamble

and Data Symbol Threshold registers, as appropriate, to

determine whether âsuccessfulâ correlation has been de-

tected. Successful detection in acquisition mode immedi-

ately switches the receiver to despread and track the ex-

pected subsequent data symbols, while successful

detection thereafter yields symbol synchronization. The

threshold register values must be set by the user to satis-

factorily detect the correlation peak in noise obtained when

the received PN-spread signal is correlated against a local

version of the PN code by the PN Matched Filter. Once the

power estimation value exceeds the threshold register val-

ue, a successful correlation is assumed to have been de-

tected. Further operations in the Symbol Tracking Proces-

sor then handle the possibility of multiple detects per

symbol, missed detects, etc.

ADDR 2AH

Bits 1-0

Acq. Thresh. Bits 9-8

ADDR 29H

Bits 7-0

Acq. Thresh. Bits 7-0

Addresses 2BH and 2CH:

Data Symbol Threshold

Addresses 2BH and 2CH contain the Data Symbol Thresh-

old value, as shown in This value is used for comparison

with the estimated signal power from the PN Matched Fil-

ter to determine whether a successful correlation has been

detected for each data symbol. The Data Symbol Thresh-

old value must be set by the user to satisfactorily detect the

correlation peak in noise obtained when the received PN-

spread data symbol is correlated against a local version of

the data symbol PN code by the PN Matched Filter. Once

the power estimation value exceeds the threshold value, a

successful correlation is assumed to have been detected.

If bit 2 of address 30H is set low, then the Symbol Acquisi-

DS96WRL0400

4-31

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