ISL5217_05 Datasheet, PDF (10/43 Pages) Intersil Corporation – Quad Programmable Up Converter

ISL5217
modulator. The maximum phase step that can occur in one
clock is ±180 degrees. Table 1 provides the change in phase
weighting of the input bits.
TABLE 1. PHASE WEIGHTING
dφ(nT)/dt
DEGREES/SAMPLE
1000 0000 0000 0000
-180
0000 0000 0000 0000
0
0111 1111 1111 1111
~+180
Shaping Filter
The shaping filter provides the necessary pulse shaping
required on the input data to implement various QASK and
shaped FM modulation formats. Two identical shaping filters
(one each for the I and Q paths) are provided. The shaping
filter architecture uses a NCO controlled interpolating FIR,
capable of 4, 8, or 16 interpolation phases. The number of
interpolation phases, (IP) is loaded into FIR Control (0xd,
bits 1:0). The span of the impulse response of the polyphase
filter can vary from 4-16 data samples. The desired sample
Data Span, (DS) value minus one is loaded into FIR Control
(0xd, bits 7:4). Thus, the required number of coefficients (or
filter span) becomes:
# Coefficients = (DS)(IP)
(EQ. 2)
The Interpolation Phase also determines the rate to compute
a polyphase output by selecting the appropriate timing from
the Sample Rate NCO to drive the shaping filter at 4x, 8x, or
16x the input sample rate. The Data Span selects the
number of samples to convolve. Each convolution requires
DS reference clocks for each phase of the filter. An output is
calculated (IP) times for each input sample. To allow
sufficient processing time for each output, the reference
clock must be as follows:
CLK ≥ (DS)(IP)(f_S)
(EQ. 3)
Conversely, the input sample rate requires:
f_S ≤ f_CLK ⁄ [(IP)(DS) ]
(EQ. 4)
where f_CLK is the frequency of the reference clock, IP is the
shaping filter interpolate rate; and DS is the number of data
samples in the filter span. For example, if f_CLK = 104MHz,
the filter span is 16 samples, and the interpolation rate is 16,
then the maximum input sample rate, f_S is 104/256 =
406.25kHz. Table 2 shows several examples of calculations
for FIR input sample rates based on master reference clock
rate, number of data samples, and interpolation rate. The
data exits the shaping filters at the interpolated rate.
TABLE 2. EXAMPLE CALCULATIONS
EXAMPLE
1
f_CLK
104MHz
DS IP
MAX f_S
16 16 104/256 = 406.25kHz
2
104MHz 16 8 104/128 = 812.5kHz
3
104MHz 16 4 104/64 = 1.625MHz
4
104MHz 10 4 104/40 = 2.600MHz
5
104MHz 8 4 104/32 = 3.250MHz
6
104MHz 4 4 104/16 = 6.500MHz
The shaping filters have programmable coefficients which
must be loaded via the microprocessor interface. The QPUC
supports loading coefficients for two shaping filters, with FIR
Control (0xd, bit 8) selecting the active filter. The I and Q
shaping filters are identical and may be loaded
simultaneously or separately, allowing for different gains and
responses through the filter if desired.
TABLE 3. FIR CONTROLS
STARTING ADDRESS
STARTING ADDRESS
IP W/FIR CONTROL (8) = ‘0’ W/FIR CONTROL (8) = ‘1’
4
0
8
8
0
8
16
0
128
Because 16 interpolation phases are possible, the
coefficients are structured in sets of 16, one set for each
phase of the shaping filter. The convolution algorithm
sequentially steps through each of these phases, beginning
with phase 0. The coefficients for the shaping filters are
generated by designing the prototype filter at the
interpolated rate. The coefficients are then divided into
interpolation phases by taking every n^th tap of the prototype
filter and storing the coefficient as an element of a coefficient
set. The IP value determines the addressing interval through
the prototype filter to create the coefficient sets for the filter
phases. The first coefficient set begins at address 0. The
next coefficient set begins at address 1 and continues in a
like manner for the remaining coefficient sets. For a 16 tap,
interpolate-by-4 filter, the calculations for filter 1 are:
Polyphase output 0 = (C0*D[n]) + (C4*D[n-1]) + (C8*D[n-2])
+ (C12*D[n-3])
Polyphase output 1 = (C1*D[n]) + (C5*D[n-1]) + (C9*D[n-2])
+ (C13*D[n-3])
Polyphase output 2 = (C2*D[n]) + (C6*D[n-1]) + (C10*D[n-2])
+ (C14*D[n-3])
Polyphase output 3 = (C3*D[n]) + (C7*D[n-1]) + (C11*D[n-2])
+ (C15*D[n-3])
If FIR Control (8) is set the calculations for filter 2 are:
Polyphase output 0 = (D0*D[n]) + (D4*D[n-1]) + (D8*D[n-2])
+ (D12*D[n-3])
10
FN6004.3
July 8, 2005