AD6652BC/PCB Analog Devices Inc, AD6652BC/PCB Datasheet - Page 38

• Manufacturer Part Number: AD6652BC/PCB
• Description: BOARD EVAL W/AD6652 & SOFTWARE
• Manufacturer: Analog Devices Inc

Specifications of AD6652BC/PCB

• Module/board Type: Evaluation Board
• For Use With/related Products: AD6652
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

AD6652
RAM COEFFICIENT FILTER
The final signal processing stage is a sum-of-products decimat-
ing filter with programmable coefficients. A simplified block
diagram is shown in Figure 49. The data memories I-RAM an
Q-RAM store the 160 most recent complex samples from th
previous filter stage with 20-bit resolution. The coefficient
memory, CMEM, stores up to 256 coefficients with 20-bit
resolution. On every CLK cycle
are calculated using the same coefficients. The RCF output
consists of 24 bits of I data and 24 bits of Q data.
RCF DECIMATION REGISTER
Use each RCF channel to decimate the data rate. The decima-
tion register is an 8-bit register that can decimate from 1 to 256
The RCF decimation is stored in 0xA0 in the form of M
The input rate to the RCF is f
RCF DECIMATION PHASE
Use the RCF decimation phase to synchronize multiple filters
within a chip. This is useful when using multiple channels
within the AD6652 to implement a polyphase filter, requiring
that the resources of several filters be paralleled. In such an
application, two RCF filters would be processing the same data
from the CIC5. However, each filter is delayed by one-half th
decimation rate, thus creating a 180° phase difference between
the two halves. The AD6652 filter channel uses the value stored
in this register to preload the RCF counter. Therefore, instead
starting from 0, the counter is loaded with this value, thus
creating an offset in the processing that should be equivalent to
the required processing delay. This data is stored in 0xA1 as an
8-bit number.
RCF FILTER LENGTH
The maximum number of taps this filter can calculate, N
given by the following equation. The value N
the channel register within the AD6652 at address 0xA
w
separated by the comma, that appear within the brackets.
here min indicates th
N
taps
≤
Q IN
Figure 49. RAM Coefficient Filter Block Diagram
I IN
min
⎛
⎜
⎜
⎝
160 × 20B
256 × 20B
160 × 20B
f
C-RAM
Q-RAM
CLK
I-RAM
at N
f
SAMP
×
taps
M
SAMP5
5
is the lesser of the two values,
RCF
, one tap for I and one tap for Q
.
,
160
⎞
⎟
⎟
⎠
Σ
Σ
Q OUT
I OUT
taps
− 1 is written to
2.
RCF
taps
−
, is
e
Rev. 0 | Page 38 of 76
e
1.
d
of
.
The RCF coefficients are located in addresses 0x00 to 0x7F and
are interpreted as 20-bit twos complement numbers. When
writing the coefficient RAM, the lower addresses are multiplied
by relatively older data from the CIC5, and the higher coeffi-
cient addresses are multiplied by relatively newer data from
CIC5. The coeffici
coefficient length, N
are symmetric, th
written into the coefficient RAM.
Although the base memory for coefficien
long, the actual length is 256 words long. There are two pag
each of 128 words long. The page is selected by Bit 8 of 0xA4.
Although this data must be written in pages, the internal core
handles filters that exceed the length of 128 taps. Therefore, the
full length of the data RAM can be used as the filter length
(160 taps).
The RCF stores the
160 × 20 is assigned to I data and 160 × 20 is assigned to Q data.
The RCF uses the RAM as a circular buffer, so that it is difficult
to know in which address a particular data element is stored.
When the RCF calculates a filter output, it starts by multiplying
the oldest value in the data RAM by the first coefficient, which
is pointed to by the RCF coefficient offset register (0xA3). This
value is accumulated with the products of newer data words
multiplied by the subsequent locations in the coefficient RAM
until the coef
Table 16. Three-Tap Filter
Coefficient Address
0
1
2 = (N
The RCF coefficient offset register has two purposes. The ma
purpose of this register is for rapid filter changes, by allowin
multiple filters to be loaded into memory and then selected
simply by changing the offset as a pointer. The other use of t
register is to form part of symbol timing adjustment. If the
desired filter length is padded with zeros on the ends, then the
starting point can be adjusted to form slight delays in when the
filter is computed with reference to the high speed clock. This
allows for vernier adjustment of the symbol timing. Course
adjustments can be made with the RCF decimation phase.
The output rate of this filter is determined by the output rate o
the CIC5 stage and M
f
taps
SAMPR
− 1)
ficient address RCF
=
f
M
en both sides of the impulse res
SAMP
ents need not be symmetric, and the
RCF
data from the CIC5 into a 160 × 40 RAM.
taps
RCF
, can be e
5
, as follows:
Impulse Response
h(0)
h(1)
h(2)
ven o
OFF
+ N
r odd. If the coefficients
taps
ts is only 128 words
− 1 is reached.
ponse must be
Data
N(0) oldest
N(1)
N(2) newest
es,
the
g
his
in
f