Z87200
Zilog Spread-Spectrum Transceiver
4-3
Z87200 I.F. Interface
The Z87200 receiver circuitry employs an NCO and com-
plex multiplier referenced to RXIFCLK to perform frequen-
cy downconversion, where the input I.F. sampling rate and
the clock rate of RXIFCLK must be identical. In “complex
input” or Quadrature Sampling Mode, external dual ana-
log-to-digital converters (ADCs) sample quadrature I.F.
signals so that the Z87200 can perform true full single
sideband downconversion directly from I.F. to baseband.
At PN chip rates less than one-eighth the value of RXIF-
CLK, downconversion may also be effected using a single
ADC in “real input” or Direct I.F. Sampling Mode.
The input I.F. frequency is not limited by the capabilities of
the Z87200. The highest frequency to which the NCO can
be programmed is 50% of the I.F. sampling rate (the fre-
quency of RXIFCLK); moreover, the signal bandwidth,
NCO frequency, and I.F. sampling rate are all interrelated,
as discussed in Higher I.F. frequencies, however, can be
supported by using one of the aliases of the NCO frequen-
cy generated by the sampling process. For example, a
spread signal presented to the Z87200’s receiver ADCs at
an I.F. frequency of f
I.F.
, where f
RXIFCLK
< f
I.F.
< 2 x f
RXIF-
CLK
, can generally, as allowed by the signal’s bandwidth,
be supported by programming the Z87200’s NCO to a fre-
quency of (f
I.F.
- f
RXIFCLK
), as discussed in Appendix A of
this product specification. The maximum I.F. frequency is
then limited by the track-and-hold capabilities of the
ADC(s) selected. Signals at I.F. frequencies up to about
100 MHz can be processed by currently available 8-bit
ADCs, but the implementation cost as well as the perfor-
mance can typically be improved by using an I.F. frequen-
cy of 30 MHz or lower. Downconversion to baseband is
then accomplished digitally by the Z87200, with a pro-
grammable loop filter provided to establish a frequency
tracking loop.
Burst and Continuous Data Modes
The Z87200 is designed to operate in either burst or con-
tinuous mode: in burst mode, built-in symbol counters al-
low bursts of up to 65,533 symbols to be automatically
transmitted or received; in continuous mode, the data is
simply treated as a burst of infinite length. The Z87200’s
use of a digital PN Matched Filter for code detection and
despreading permits signal and symbol timing acquisition
in just one symbol. The fast acquisition properties of this
design are exploited by preceding each data burst with a
single Acquisition/Preamble symbol, allowing different PN
codes (at the same PN chip rate) to independently spread
the Acquisition/Preamble and data symbols. In this way, a
long PN code with high processing gain can be used for
the Acquisition/Preamble symbol to maximize the proba-
bility of burst detection, and a shorter PN code can be used
thereafter to permit a higher data rate.
To improve performance in the presence of high noise and
interference levels, the Z87200 receiver’s symbol timing
recovery circuit incorporates a “flywheel circuit” to maxi-
mize the probability of correct symbol timing. This circuit
will insert a symbol clock pulse if the correlation peak ob-
tained by the PN Matched Filter fails to exceed the pro-
grammed detect threshold at the expected time during a
given symbol. During each burst, a missed detect counter
tallies each such event to monitor performance and allow
a burst to be aborted in the presence of abnormally high in-
terference. A timing gate circuit further minimizes the prob-
ability of false correlation peak detection and consequent
false symbol clock generation due to noise or interference.
To minimize power consumption, individual sections of the
device can be turned off when not in use. For example, the
receiver circuitry can be turned off during transmission
and, conversely, the transmitter circuitry can be turned off
during reception when the Z87200 is operating in a half-
duplex/time division duplex (TDD) system. If the NCO is
not being used as the BPSK/QPSK modulator (that is, if an
external modulator is being used), the NCO can also be
turned off during transmission to conserve still more pow-
er.
Conclusion
The fast acquisition characteristics of the Z87200 make it
ideal for use in applications where bursts are transmitted
relatively infrequently. In such cases, the device can be
controlled so that it is in full “sleep” mode with all receiver,
transmitter, and NCO functions turned off over the majority
of the burst cycle, thereby significantly reducing the aggre-
gate power consumption. Since the multiply operations of
the PN Matched Filter consume a major part of the overall
power required during receiver operation, two independent
power-saving techniques are also built into the PN
Matched Filter to reduce consumption during operation by
a significant factor for both short and long PN spreading
codes.
The above features make the Z87200 an extremely
versatile and useful device for spread-spectrum data
communications. Operating at its highest rates, the
Z87200 is suitable for use in wireless Local Area Network
implementations, while its programmability allows it to be
used in a variety of data acquisition, telemetry, and
transaction system applications.
