AN9614 Datasheet, PDF (2/3 Pages) Intersil Corporation – Using the PRISM® Chip Set for Low Data Rate Applications

Application Note 9614
for PCMCIA applications are not widely available at these
specifications and a custom design may be required.
B. Limitations of HFA3724 LPFs
The HFA3724 includes a set of baseband low pass filters as
the final filtering stage of the complex spread waveform.
These are placed before the In phase (I) and Quadrature (Q)
A/D converters for baseband processing. These filters are
shown on Figure 1, as LPFs (Rx) and LPFs(TX). There are
four cut off frequencies that can be selected for these LPFs.
The cut off can be selected to be 17.6MHz (for a chip rate of
22 MCPS), 8.8MHz (for a 11 MCPS rate), 4.4MHz (for a
5.5 MCPS rate) or 2.2MHz (for a 2.75 MCPS rate). In
addition these cut off frequencies are tunable through an
external resistor by ±20%. The user can select one of the
four discrete cut off frequencies. The lowest cut off is set for
a spread rate of 2.5MHz chip rate and any chip rates lower
than this will require the design of external filtering between
the HFA3724 outputs and the HSP3824 A/D inputs. The
HFA3724 I and Q LPFs are fifth order Butterworth filters and
equivalent external filters need to be designed at the lower
cut off specifications.
C. Selection of Carrier Frequency and Clock
Oscillators
The HSP3824 performs the baseband demodulation
function. The design includes digital signal acquisition and
tracking loops for both the symbol timing clock and the
carrier frequency.
The primary concern when the radio needs to be operated
with a low instantaneous data rate is that it requires a wide
bandwidth to accommodate oscillator frequency tolerances.
As an example at 2400MHz and ±25 PPM, the radio
frequencies at each end of the link can be off by as much as
120kHz from each other. This offset must be well within the
basic data bandwidth of the radio in order for it to be
tolerated without degrading the performance of the link. If it
is not, a frequency sweep would be needed to find the
signals and this is not built into the radio design. Operating
the radio with wide data bandwidth and low data rate is
inefficient and would cause unacceptable loss in
performance.
If the PRISM is used as a spread spectrum system with
11 chips per bit spreading ratio, this then gives it an IF
bandwidth of nominally 22MHz null to null at 1 MSPS. We
filter to 17MHz to allow closer packing of the channels. While
this seems wide compared to the frequency offset,
remember that this is a direct spread system. The first stage
of processing the signal despreads it and collapses it to the
data bandwidth. In PRISM this is done in a time invariant
matched filter correlator. This correlator has an FIR filter
structure where the PN sequence is substituted for the tap
weights. The filter is operated at baseband, so the I and Q
quadrature components are separately correlated with the
same sequence. The outputs of the I and Q correlators are
the vector components of the correlation. These will show a
distinct peak in magnitude (compressed pulse) when
correlation occurs. Correlation performance falls off when
the signal is not stationary (i.e. has offset). The correlator
convolves a stationary signal, (the PN sequence) with the
input signal. The vector correlation is being rotated
throughout the correlation by the offset frequency. This
means that the signal correlates at one angle at the start of a
symbol and at a different angle at the end. If this angular
difference is small, no great loss occurs. The net correlation
goes as the vector sum of all the correlation angles between
the start and the end of the symbol as shown below. Thus
the magnitude falls off to zero if the offset causes a
baseband phase rotation of one cycle per symbol. The
magnitude is obviously maximum at no offset and falls off
about 0.22dB at 45 degrees rotation. This corresponds to
the 120kHz offset (~1/8th of 1 MBPS).
BEGINNING OF
SYMBOL
CORRELATION
VECTOR
AVERAGE SYMBOL
CORRELATION
VECTOR
END OF SYMBOL
45^o
CORRELATION
VECTOR
90^o
180^o
270^o
FIGURE 2. PRISM™ CORRELATION PERFORMANCE vs
FREQUENCY OFFSET
Crystal oscillators of better than ±25 PPM accuracy can be
purchased, but their cost goes up significantly as the
tolerances are tightened. Given this offset, we must be sure
that the receiver can accept the offset. At a data rate of
250 KBPS, the same offset loss occurs with a frequency
offset 1/4th as large. This means that to get the same
performance, we need oscillators specified to ±6 PPM. To go
lower in data rate means tightening up the specification even
further.
Similar consideration needs to be taken for the clocks that
are used to run the baseband processor itself. The symbol
timing clock tracking algorithm operates over 128 symbol
integration intervals. To maintain acceptable BER
performance the symbol timing phase drift must be less than
1/8th chip over the 128 symbol integration interval.
Remember that we are tracking the peak of the compressed
pulse which is 2 chips wide and must keep the straddling
loss low by sampling close to the peak. For a 0.25 MBPS
data rate, the chip rate is 2.75 MCPS. With this rate, the
integration interval is 512ms which translates to an
oscillator within ±89 PPM to keep the drift less than 1/8th
chip (0.045ms). Since the spread rate to data rate ratio is
not changed at the lower data rates, this tolerance is not
effected by lower data rates.
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