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參數(shù)資料
| 型號(hào)： | AD6636PCB |
| 廠商： | Analog Devices, Inc. |
| 元件分類： | 數(shù)字上／下變頻 |
| 英文描述： | 150 MSPS Wideband Digital Down-Converter (DDC) |
| 中文描述： | 150MSPS的寬帶數(shù)字下變頻器（DDC） |
| 文件頁數(shù)： | 35/72頁 |
| 文件大?。?/td> | 1629K |
| 代理商： | AD6636PCB |
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AD6636
CHANNEL RAM COEFFICIENT FILTER (CRCF)
Following the DRCF is the programmable decimating CRCF
FIR filter. The only difference between the DRCF and CRCF
filters is the coefficient bit width. The DRCF has 14-bit
coefficients, while the DRCF has 20-bit coefficients.
Rev. 0 | Page 35 of 72
This filter can calculate up to 64 asymmetrical filter taps or up
to 128 symmetrical filter taps. The filter is capable of a
programmable decimation rate from 1 to 16. The flexible
coefficient offset feature allows loading multiple filters into the
coefficient RAM and changing the filters on the fly. The
decimation phase feature allows for a polyphase implementa-
tion in which multiple AD6636 channels are used to process a
single carrier.
The CRCF 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 CRCF 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
CRCF decimation rate word in the CRCF control register.
Again, the value written is the decimation rate minus one.
Bypass
The CRCF filter can be used in normal operation or bypassed
using the CRCF bypass bit in the CRCF control register. When
the CRCF filter is bypassed, no scaling is applied and the output
of the filter is the same as the input to the CRCF filter.
Scaling
The output of the CRCF filter can be scaled using the 2-bit
CRCF scaling word in the CRCF control register. Table 20
shows the valid values for the 2-bit word and the corresponding
settings. | ∑
COEFF
| is the sum of all coefficients (in normalized
form) used to calculate the FIR filter.
Table 20. CRCF Scaling Factor Settings
CRCF Scale Word [1:0]
00
01
10
11
Scaling Factor
18.06 dB attenuation
12.04 dB attenuation
6.02 dB attenuation
No scaling, 0 dB
Symmetry
The CRCF filter does not require symmetrical filters. However,
if the filter is symmetrical, then the symmetry bit in the CRCF
control register should be set. When this bit is set, only half the
impulse response needs to be programmed into the CRCF
coefficient memory registers. For example, if the number of
filter taps is equal to 15 or 16 and the filter is symmetric, 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 CRCF can perform 64 asymmetrical filter
taps or 128 symmetrical filter taps.
Coefficient Offset
More than one set of filter coefficients can be loaded into the
coefficient RAM at any 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 CRCF coefficient
offset register. When this value is changed during the calcula-
tion of a particular output data sample, the sample calculation is
completed using the old coefficients and the new coefficient
offset is brought into effect from the next data sample
calculation.
Decimation Phase
When more than one channel of the AD6636 is used to process
one carrier, polyphase implementation of the 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 CRCF decimation phase word of the CRCF control
register.
Maximum Number of Taps Calculated
The output rate of the CRCF filter is given by
CRCF
DRCF
M
CRCF
f
f
=
where:
f
DRCF
is the data rate out of the DRCF filter and into the CRCF
filter.
M
CRCF
is the decimation rate in the CRCF filter.
The CRCF 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 multiply (or tap once). Therefore, the
maximum number of filter taps that can be calculated
(symmetrical or asymmetrical filter) is given by
1
=
CRCF
PLLCLK
f
f
ceil
Taps
of
Number
Maximum
where:
f
PLLCLK
is the high speed internal processing clock generated by
the PLL clock multiplier.
f
CRCF
is the output rate of the CRCF filter as calculated
previously.
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