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HSP50016 Datasheet PDF : 31 Pages
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HSP50016
Scaling Multipliers
The output of each HDF is passed to a Scaling Multiplier.
The Scaling Multipliers are used to compensate for the HDF
gain, which is between 1 (inclusive) and 0.5 (non-inclusive),
or (0.5, 1.0). The gain through the HDF is dependent on the
decimation factor: when the decimation is an even power of
two, the HDF gain is equal to 1; otherwise, the gain must be
compensated for in the Scaling Multiplier. The HDF gain is
given by the equation:
HDF Gain = R5/2CEILING(5 log2(R))
(EQ. 8)
where R is the HDF decimation factor. The compensating
Scale Factor, which is input to both Scaling Multipliers, is
given by the equation:
Scale Factor = 2CEILING(5 log2(R)) ⁄ R5
(EQ. 9)
where R is the HDF decimation factor.
NOTE: The Scale Factor falls in the interval [1, 2). The
output of the scaling multiplier is symmetrically rounded to
17 bits.
The binary formats of the inputs and outputs of the scaling
multiplier are as follows:
Input from HDF:
Scale factor:
Output:
a0(-20). a1(2-1) a2(2-2)... a17(2-17)
a0(20). a1(2-1) a2(2-2)... a15(2-15)
a0(-20). a1(2-1) a2(2-2)... a16(2-16)
FIR Filter
The Scaling Multiplier output is passed to the FIR Filter,
which performs aliasing attenuation, passband roll off
compensation and transition band shaping. The FIR Filter
Section is functionally two identical 121 tap lowpass FIR
filters, one each for the I and Q channel. The two filters are
each implemented as sum of products, each with a single
multiplier, with the coefficients stored in ROM. The filters'
passbands are precompensated to be the inverse of the
response of the HDF. The frequency responses of the HDF,
FIR, and Composite HDF/FIR filters are shown in Figure 8.
The composite passband of the HDF and FIR filter
frequency response is shown in Figure 9. The FIR
coefficients are scaled so that the maximum gain of the
composite filter is less than or equal to 0dB. The composite
passband ripple is less than 0.04dB.
HDF
0
-50
-100
-150
fS
(R = 16)
4
fS
3fS
fS
2
4
0
-50
-100
-150
FIR
fS
fS
3fS
fS
4
2
4
COMPOSITE
0
-50
-100
-150
fS
fS
3fS
fS
4
2
4
SAMPLE TIMES
FIGURE 8A. DDC HDF, FIR, AND COMPOSITE FILTER
RESPONSE (FOR R = 16)
0
HDF
-20
-40
-60
-80
-100
COMPOSITE
FIR
HDF/FIR
-120
2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32
fS fS
fS
fS
3fS
fS
128R 64R 32R
16R
32R
8R
SAMPLE TIMES
FIGURE 8B. DDC FILTER RESPONSES (FOR R = 16)
3-207
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