AD6654/PCB Analog Devices Inc, AD6654/PCB Datasheet - Page 43

• Manufacturer Part Number: AD6654/PCB
• Description: BOARD EVALUATION FOR AD6654
• Manufacturer: Analog Devices Inc

Specifications of AD6654/PCB

• Module/board Type: Evaluation Board
• For Use With/related Products: AD6554
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

DECIMATING RAM COEFFICIENT FILTER (DRCF)
Following the MRCF is the programmable DRCF FIR filter.
This filter can calculate up to 64 asymmetrical filter taps or up
to 128 symmetrical filter taps. The filter is also capable of a
programmable decimation rate from 1 to 16. A flexible
coefficient offset feature allows loading multiple filters into the
coefficient RAM and changing the filters on the fly. The
decimation phase feature allows a polyphase implementation,
where multiple AD6654 channels are used for processing a
single carrier.
The DRCF filter has 20-bit input and output data and 14-bit
coefficient data. The number of filter taps to calculate is
programmable and is set in the DRCF taps register. The value
of the number of taps minus one is written to this register.
For example, a value of 19 in the register corresponds to
20 filter taps.
The decimation rate is programmable using the 4-bit DRCF
decimation rate word in the DRCF control register. Again, the
value written is the decimation rate minus one.
BYPASS
The DRCF filter can be used in normal operation or bypassed
using the DRCF bypass bit in the DRCF control register. When
the DRCF filter is bypassed, no scaling is applied and the output
of the filter is the same as the input to the DRCF filter.
SCALING
The output of the DRCF filter can be scaled using the 2-bit
DRCF scaling word in the DRCF control register. Table 21 lists
the valid values for the 2-bit word and their corresponding
settings.
Table 21. DRCF Scaling Factor Settings
DRCF Scale Word[1:0]
00
01
10
11
SYMMETRY
The DRCF filter does not require symmetrical filters. However,
if the filter is symmetrical, then the symmetry bit in the DRCF
control register should be set. When this bit is set, only half of
the impulse response needs to be programmed into the DRCF
coefficient memory registers. For example, if the number of
filter taps is equal to 15 or 16 and the filter is symmetrical, then
only eight coefficients need to be written into the coefficient
memory. Because a total of 64 taps can be written into the
memory registers, the DRCF can perform 64 asymmetrical
filter taps or 128 symmetrical filter taps.
Scaling Factor
18.06 dB attenuation
12.04 dB attenuation
6.02 dB attenuation
No scaling, 0 dB
Rev. 0 | Page 43 of 88
COEFFICIENT OFFSET
More than one set of filter coefficients can be loaded into
coefficient RAM at any given time (given sufficient RAM
space). The coefficient offset can be used in this case to access
the two or more different filters. By changing the coefficient
offset, the filter coefficients being accessed can be changed on
the fly. This decimal offset value is programmed in the DRCF
coefficient offset register. When this value is changed during the
calculation of a particular output data sample, the sample
calculation is completed using the old coefficients, and the new
coefficient offset from the next data sample calculation is used.
DECIMATION PHASE
When more than one channel of AD6654 is used to process one
carrier, polyphase implementation of corresponding channels’
DRCF or CRCF is possible using the decimation phase feature.
This feature can be used only under certain conditions. The
decimation phase is programmed using the 4-bit DRCF
decimation phase word of the DRCF control register.
MAXIMUM NUMBER OF TAPS CALCULATED
The output rate of the DRCF filter is given by
where:
f
filter.
M
The DRCF filter consists of two multipliers (one each for the
I and Q paths). Each multiplier, working at the high speed clock
rate (PLL clock), can do one multiply (or one tap) per high
speed clock cycle. Therefore, the maximum number of filter
taps that can be calculated (symmetrical or asymmetrical filter)
is given by
where:
f
the PLL clock multiplier.
f
MRCF
PLLCLK
DRCF
DRCF
Maximum Number of Taps = ceil
is the output rate of the DRCF filter, previously calculated.
is the data rate out of the MRCF filter and into the DRCF
f
is the high speed internal processing clock generated by
is the decimation rate in the DRCF filter.
DRCF
=
M
f
MRCF
DRCF
⎛
⎜
⎜
⎝
f
f
PLLCLK
DRCF
⎞
⎟
⎟
⎠
− 1
AD6654