11
FN3365.10
October 6, 2008
If internal multiplexing is selected (INT/EXT = 1), the data
stream input through AIN0-15 is fed to both the upper and lower
processing legs as shown in Figure 11A. The output of each
processing leg is then multiplexed together to produce the
interpolated sample stream at twice the input sample rate. In
this mode the device is clocked at the interpolated data rate to
support the multiplexing of each processing leg’s output into a
single data stream. The upper and lower processing legs each
run at the input data rate of CLK/2 as indicated by the “ †”
marking the various registers and processing elements in
Figure 11A. In this mode, data samples are clocked into the part
on every other rising edge of CLK. The SYNC signal is used to
specify which set of CLK cycles are used to register data at the
part’s input. Specifically, every other rising edge of CLK starting
one CLK after the assertion of SYNC will be used to clock data
into the part. With internal multiplexing the minimum pipeline
delay through the upper processing leg is 15 CLK’s and the
pipeline delay through the lower processing leg is 48 CLK’s,
(2[19+3]+4).
If external multiplexing is selected (INT/EXT
= 0), the upper
and lower processing legs are output through AOUT0-15
and BOUT0-15 for multiplexing into a single data stream off
chip.This allows the processing legs to run at the maximum
clock rate which coincides with an interpolated output data
rate of 104 MSPS. NOTE: The samples output on
BOUT0-15 precede those on AOUT0-15 in sample order.
This requires a multiplexing scenario in which BOUT0-15 is
selected before AOUT0-15. With external multiplexing, the
minimum pipeline delay through the upper processing leg is
9 CLK’s and the pipeline delay through the lower processing
leg is 26 CLK’s as shown in Figure 11B. In this mode SYNC
has no effect on part operation.
Down Convert and Decimate Mode (MODE1-0 = 10)
In Down Convert and Decimate Mode a real input signal is
spectrally shifted -f
S
/4 which centers the upper sideband at
DC. This operation produces real and imaginary
components which are each filtered and decimated by
identical 67-tap halfband filters. For added flexibility, a
positive f
S
/4 spectral shift may be selected which centers
the lower sideband at DC. The direction of the spectral shift
is selected via USB/LSB as described in the Quadrature
Down Convert section. A spectral representation of the
down convert and decimate operation is shown in Figure
12 (USB/LSB = 1). NOTE: Each of the complex terms
output by the Filter Processor are scaled by two to
compensate for the attenuation of one half introduced
by the down conversion process.
The Down Convert and Decimate mode is most easily
understood by first considering the transversal
implementation using a 7 tap filter as shown in Figure 13.
By examining the combination of down conversion, filtering
and decimation, it is seen that the real outputs are only
dependent on the sum-of-products for the even indexed
samples and filter coefficients, and the imaginary outputs are
only a function of the sum-of-products for the odd indexed
samples and filter coefficients. This computational
partitioning allows the quadrature filters required after down
conversion to be realized using the same poly-phase
processing elements used in the previous two modes.
A functional block diagram of the polyphase implementation
is shown in Figure 14. In this implementation, the input data
stream is broken into even and odd sample streams and
processed independently by the even and odd tap filters. By
decomposing the sample stream into even and odd
samples, the zero mix terms produced by the down convert
LO drop out of the data streams, and the output of each of
the filters represent the decimated data streams for both the
real and imaginary outputs.
INPUT SIGNAL SPECTRUM
DOWN CONVERTED SIGNAL
FILTERED SIGNAL
FILTER PASSBAND
DECIMATED OUTPUT SIGNAL SPECTRUM
0f
S
/2 f
S
-f
S
/2
0f
S
/2 f
S
-f
S
/2
0f
S
/2 f
S
-f
S
/2
0f
’
S
2f
’S
-f
’
S
f
S
= INPUT SAMPLE RATE
f
’
S
= DECIMATED SAMPLE RATE, f
S
/2
FIGURE 12. DOWN CONVERT AND DECIMATE OPERATION
HSP43216
