參數(shù)資料
型號(hào)： AD6652BBC
廠(chǎng)商： ANALOG DEVICES INC
元件分類(lèi)： 通信及網(wǎng)絡(luò)
英文描述： 12-Bit, 65 MSPS IF to Baseband Diversity Receiver
中文描述： SPECIALTY TELECOM CIRCUIT, PBGA256
封裝： 17 X 17 MM, BGA-256
文件頁(yè)數(shù)： 43/76頁(yè)
文件大?。?/td> 1839K
代理商： AD6652BBC
AD6652
representation. Though the user defines the open loop po
and gain
K
, they directly impact the placement of the clo
loop poles and filter characteristics. These closed loop pole
P
2
are the roots of the denominator of the above closed loop
transfer function and are given by
Rev. 0 | Page 43 of 76
le
P
sed
s
P
1
,
2
4
)
1
)
1
,
2
2
1
P
K
P
K
P
P
P
+
+
+
=
Typically the AGC loop performance is defined in terms of its
time constant or settling time. In such a case, set the closed
poles to meet the time constants required by the AGC loop. The
following relation between time constant and closed loop po
can be used for this purpose:
loop
les
=
2
P
exp
M
rate
CIC
τ
×
2
sample
where:
τ
1,2
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:
M
CIC
(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.
If two channels are being used to process one carrier of UMTS
at 2× chip rate, then each channel works at 3.84 MHz and the
combined sample rate coming into the half-band interpolated
filters is 7
the previous equation, if half-band interpolating filters are
bypassed.
les in
les in
t of the signal gain with
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
0 dB and 96.296 dB in steps of 0.024 dB. Initial value for signal
gain is programmable using Register 0x0D for AGC A and
Register 0x15 for AGC B.
The products of the gain multiplier are the AGC scaled outputs,
which have 19-bit representation. These are in turn used as I
and Q for calculating the power and AGC error and loop
filtered to produce signal
AGC scaled outputs can be programmed to have 4-, 5-, 6-, 7-, 8-,
10-, 12-, or 16-bit widths using the AGC control word (0x0A,
0x12). The AGC scaled outputs a
wid hs using the clipping circuitry shown in Figure 51.
n Loop Gain Setting
lter gain K occupies only one LSB or 0.0039, then, during
ultiplication with error term, errors of up t
be uncated. This truncation is due to the lower bit width
lable in the AGC loop. If filter gain K is the maximum value,
cated errors are less than 0.094 dB (equivalent to 1 LSB of
r term representation). Generally, a small filter gain is used
hieve a large time constant loop (or slow loops), but, in this
, it would cause large errors to go undetected. Due to this
liarity, the designers recommend that, if a user wants slow
loops, they should use fairl
gain for the next set of samples. These
re truncated to the required bit
Ope
If fi
the m
o 6.02 dB could
s
avai
trun
erro
to ac
case
pecu
AGC
and then use CIC decimation to achieve a slow loop. In this way,
the AGC loop makes large infrequent gain changes compared
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 slow time constant is still achie
and there is less truncation of errors.
Average Samples Setting
Though it is complicated to express the exact effect of the
number of averaging samples, thinking intuitively, it has a
smoothing effect on the way the AGC loop attacks a sudd
increase or a spike in t
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 sign
Desired Clipping Level Mode
oted previously, each AGC can be configured so that the
locks onto a desired clipping level or a desired signal leve
t desired clippin
individual AGC control words (0x0A, 0x12). For signals that
to exceed the boun
y high values for filter gain K
to
ved
en
he signal level. If averaging of four
al level.
As n
loop
Selec
l.
g level mode by setting Bit 4 of the
tend
desired clipping level option provides a way to keep from
quic
for t
Figu
mod
e.
ds of the peak-to-average ratio, the
l
First, the data from the gain multiplier is truncated to a lower
solution (4, 5, 6, 7, 8, 10, 12, or 16 bits) as set by the AGC
control word. An error term (both I and Q) is generated that is
the difference between the signals before and after truncation.
This term is passed to the complex squared magnitude block,
.68 MSPS. Use this rate in the calculation of po
The loop filter output corresponds to the signal gain that is
updated by the AGC. Because all computation of the samp
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 produc
both the I
tru cating those signals and still provide an AGC that attacks
kly and settles to the desired output level. The signal path
his mode of operation is shown with broken arrows in
re 51, and the operation is similar to the desired signal leve
re
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