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| 型號： | AD6652XBC |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | 通信及網(wǎng)絡(luò) |
| 英文描述： | SPECIALTY TELECOM CIRCUIT, PBGA256 |
| 封裝： | 17 X 17 MM, MINI, BGA-256 |
| 文件頁數(shù)： | 36/76頁 |
| 文件大小： | 1802K |
| 代理商： | AD6652XBC |
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Preliminary Technical Data
AD6652
Rev. PrC | Page 41 of 76
h
A and
ed
oles P1,
The gain parameter K and pole P are programmable throug
registers (0x0E and 0x0F, respectively, for AGC Channel
Channel B) from 0 to 0.996 in steps of 0.0039 using 8-bit
representation. Though the user defines the open loop pole P
and gain K, they directly impact the placement of the clos
loop poles and filter characteristics. These closed loop p
P2 are the roots of the denominator of the above closed loop
transfer function and are given by
2
4
)
1
(
)
1
(
,
2
1
P
K
P
K
P
+
+
=
Typically the AGC loop pe
time constant or settling ti
rformance is defined in terms of its
e
s
poles to meet the time constants required by the AGC loop. Th
following relation between time constant and closed loop pole
can be used for this purpose:
τ
×
=
2
,
1
2
,
1
exp
rate
sample
M
P
CIC
where:
τ are the time constants corresponding to the poles P .
1,2
exp denotes the inverse of the natural log.
The time constants can also be derived from settling times as
follows:
3
%
5
4
%
2
time
settling
or
time
settling
=
τ
where:
MCIC (CIC decimation) is from 1 to 4096.
settling time or time constant is chosen by the user.
sample rate is the combined sample rate of all the interleaved
channels coming into the AGC/half-band interpolated filters.
0 dB and 96.296 dB in steps of 0.024 dB. Initial value for sig
gain is programmable using Registers 0x0D and 0x15 for AGC
A and AGC B, respectively.
The products of the gain multiplier are the AGC scaled outputs,
which have 19-bit repr
If two channels are being used to process one carrier of UMTS
at 2x chip rate, then each channel works at 3.84 MHz and the
combined sample rate coming into the half-band interpolated
filters is 7.68 Msps. Use this rate in the calculation of poles in
the previous equation, if half-band interpolating filters are
bypassed.
The loop filter output corresponds to the signal gain that is
updated by the AGC. Because all computation of the samples in
the loop filter is done in logarithmic domain (to the Base 2), the
signal gain is generated using the exponent (power of 2) of the
loop filter output.
The gain multiplier gives the product of the signal gain with
both the I and Q data entering the AGC section. This signal
gain is applied as a coarse 4-bit scaling and then a fine scale
8-bit multiplier. Therefore, the applied signal gain is between
nal
esentation. These are in turn used as I
caled outputs can be programmed to have 4-, 5-, 6-, 7-,
ld
e in the AGC loop. If filter gain K is the maximum value,
truncated errors are less than 0.094 dB (equivalent to 1 LSB of
d
is
y should use fairly high values for filter gain K
ay,
w time constant is still achieved
f averaging samples, thinking intuitively, it has a
smoothing effect on the way the AGC loop attacks a sudden
increase or a spike in the signal level. If averaging of 4 samples
is used, the AGC attacks a sudden increase in signal level more
slowly compared to no averaging. The same applies to the
manner in which the AGC attacks a sudden decrease in the
signal level.
Desired Clipping Level Mode
As noted previously, each AGC can be configured so that the
loop locks onto a desired clipping level or a desired signal level.
Select desired clipping level mode by setting Bit 4 of the
individual AGC control words (0x0A, 0x12). For signals that
tend to exceed the bounds of the peak-to-average ratio, the
desired clipping level option provides a way to keep from
truncating those signals and still provide an AGC that attacks
quickly and settles to the desired output level. The signal path
for this mode of operation is shown with broken arrows in
Figure 42, and the operation is similar to the desired signal level
mode.
First, the data from the gain multiplier is truncated to a lower
and Q for calculating the power and AGC error and loop
filtered to produce signal gain for the next set of samples. These
AGC s
8-, 10-, 12-, or 16-bit widths using the AGC control word
(0x0A, 0x12). The AGC scaled outputs are truncated to the
required bit widths using the clipping circuitry shown in
Figure 42.
Open Loop Gain Setting
If filter gain K occupies only one LSB or 0.0039, then, during
the multiplication with error term, errors of up to 6.02 dB cou
be truncated. This truncation is due to the lower bit widths
availabl
error term representation). Generally, a small filter gain is use
to achieve a large time constant loop (or slow loops), but, in th
case, it would cause large errors to go undetected. Due to this
peculiarity, the designers recommend that, if a user wants slow
AGC loops, the
and then use CIC decimation to achieve a slow loop. In this w
the AGC loop makes large infrequent gain changes compared to
small frequent gain changes, as in the case of a normal small-
gain loop filter. However, though the AGC loop makes large
infrequent gain changes, a slo
and there is less truncation of errors.
Average Samples Setting
Though it is complicated to express the exact effect of the
number o
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