Z87200 Datasheet, PDF (42/54 Pages) Zilog, Inc. – Spread-Spectrum Transceiver

Z87200
Spread-Spectrum Transceiver
Zilog
THEORY OF OPERATION
The Z87200 receiver’s downconverter circuitry allows use
of two distinct modes, where the mode chosen will depend
upon the application. In applications where the received
PN chip rate is less than approximately 1/8 of the I.F. sam-
ple clock (RXIFCLK) rate, the Z87200 can be used with a
single A/D converter (ADC) and operate in Direct I.F. Sam-
pling Mode. For higher chip rate applications, it is neces-
sary to use the Z87200 in the full Quadrature Sampling
Mode; that is, using a quadrature signal source, two ADCs,
and the on-chip NCO in its quadrature mode.
Using the Z87200 with a Single ADC in Direct I.F.
Sample Mode
Direct I.F. Sampling Mode allows one rather than two
ADCs to be used, as will be explained below. If appropriate
for the application, use of Direct I.F. Sampling Mode can
reduce the system cost since quadrature downconversion
with its associated 90° signal separation and the second
ADC used in Quadrature Sampling Mode are not required.
The trade-off, however, is in the lower maximum PN chip
rate that can be supported by the Z87200 in Direct I.F.
Sampling Mode as compared to the maximum rate that
can be supported by Quadrature Sampling Mode.
In Direct I.F. Sampling Mode, the sampled signal is pre-
sented as input to the receiver’s I channel input (RXIIN)
and the Q channel input (RXQIN) is held to zero (where
“zero” is defined by the ADC input format ). As a result,
only two of the four multipliers in the Downconverter’s
complex multiplier are used and the device does not make
a true single-sideband downconversion from I.F. to base-
band. In Quadrature Sampling Mode, by contrast, quadra-
ture inputs to two ADCs provide I and Q inputs to the
Z87200 and the full complex multiplier is used. An illustra-
tion of the operation of Direct I.F. Sampling Mode is shown
in the frequency domain in Figure 11, where the spectra
have been drawn asymmetrically so that spectral inver-
sions can be readily identified.
INPUT SPECTRUM
1
BANDWIDTH: B
–f_{1 0} f₁
SPECTRUM OF
2
SAMPLING PROCESS
–f_SA
0
f_SA
SPECTRUM
3
AFTER A/D
–f_SA
–f_{1 0} f₁
f_SA
QUADRATURE
4
NCO SPECTRUM
–f_SA
–f₁ 0
f_SA
5
SPECTRUM AFTER MIXER
2 f₁
f_SA –2 f₁
–f_SA
0
f_SA
6
SPECTRUM AFTER IDEAL DIGITAL LOW PASS FILTER
–f_SA
0
f_SA
Figure 11. Spectra of Signals in Direct I.F. Sampling Mode
FREQ.
FREQ.
FREQ.
FREQ.
FREQ.
FREQ.
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DS96WRL0400