ISL5217 Datasheet, PDF (13/43 Pages) Intersil Corporation – Quad Programmable Up Converter

ISL5217
the symbol rate. This would clear the NCO accumulator
every 3 seconds or at a 1/3 Hz rate. The frequency of the
FID carryout can range from Fclk to Fclk/2^32. The value of
FID is determined from:
FID (31:0) = [(fclk / fco)]
Where fco is the desired frequency of the carryout, which in
the previous example is 1/3 Hz and the fclk is and integer
multiple of the sample frequency, say 65MHz. The resultant
value for the FID would be (65MHz/1/3Hz) or 195e6. The
programmed integer values for the FID are loaded 16 bits at
a time into Control Words 2 and 3.
0x2, bits 15:0 = FID (31:16)
0x3, bits 15:0 = FID (15:0)
Loading 195e6 into the FID would result in 0x2, being
0x0b9f, and 0x3 being 0x76c0.
SAMPLE FREQUENCY
ZERO
SYNCSEL
SYNCIN
WR CW3
START
EDGE
GEN
WR CW21
R
E
>^G
RST
R
E
>^G
IP(1:0)
46
2
EN
REG <
ACC
∑
EnNCO
(CARRIER NCO)
48
RESET
EDGE
GEN
> REG
SAMPCK
(MSB)
SHIFTER
> REG
† ALL REGISTERS ARE
CLOCKED AT CLK
12
4, 3, OR 2
FINE
PHASE
COARSE
PHASE
FIGURE 11. RE-SAMPLING NCO BLOCK DIAGRAM
Fixed Coefficient 11-TAP Interpolating
Half-band
Following the post-FIR gain profile block is a fixed coefficient
11-tap interpolate by 2 Half-Band filter. The default mode is
to bypass the filter with the setting of control word 0x0d, bit 9
enabling the filter. If bypassed, the data to the filter is zeroed
which reduces power consumption. The halfband filter
coefficients are:
3, 0, -25, 0, 150, 256, 150, 0, -25, 0, 3
The output of this filter is rounded to 20-bits. The output is
checked for saturation and limited if necessary. The data
exits the halfband filter as a parallel I<20:0> and Q<20:0>
data stream at the rate of fs*IP*2. Figure 12 shows the
frequency response of the Half-Band filter.
Interpolation Filter
The shaped sample data is input to the interpolating filter at
the interpolation rate. The Interpolator filter resamples the
shaped I and Q data to establish the final output sample rate
of the channel. The output sample rate is always the clock
rate. The Interpolator uses the fine phase values from the
Symbol Rate NCO to compute the fine interpolated samples
at the clock rate. The number of interpolated samples is set
by the following ratio: n_IS = f_CLK / f _S / IP.
The nulls in the interpolation filter frequency response align
with the interpolation images of the shaping filter. The
impulse response of the Interpolation filter is shown in
Figures 13A through 13C for varying interpolation ratios.
0
-20
-40
-60
-80
-100
-120
-140
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
NORMALIZED FREQUENCY (NYQUIST=1)
FIGURE 12. HALF BAND FILTER RESPONSE
0
INTERPOLATION FILTER RESPONSE
-20
-40
-60
-80
-100
-120
512 1024 1536 2048 2560 3072 3584 4096
SAMPLE TIMES
FIGURE 13A. INTERPOLATION FILTER IMPULSE RESPONSE
L = 16; FOUT = 4096
13