AD6652BC/PCB Analog Devices Inc, AD6652BC/PCB Datasheet - Page 43

• 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

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
transfer function and are given by
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:
where:
τ
exp denotes the inverse of the natural log.
The time constants can also be derived from settling times as
follows:
where:
M
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.
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
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.
1,2
2
CIC
are the roots of the denominator of the above closed loop
are the time constants corresponding to the poles P
(CIC decimation) is from 1 to 4096.
P
P
τ
1
1
=
2 ,
,
P
=
2
.68 MSPS. Use this rate in the calculation of po
2
and Q data entering the AGC section. This signal
%
=
exp
settling
1 (
+
⎡
⎢
⎢ ⎣
4
sample
P
−
time
K
)
M
+
rate
CIC
or
1 (
2
×
5
+
%
τ
P
1
settling
2 ,
−
⎤
⎥
⎥
⎦
K
t of the signal gain with
3
)
2
−
time
4
P
1,2
les in
sed
.
le P
les in
s P
loop
les
Rev. 0 | Page 43 of 76
1
,
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.
Ope
the m
be uncated. This truncation is due to the lower bit width
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
As n
loop
Selec
individual AGC control words (0x0A, 0x12). For signals that
tend
desired clipping level option provides a way to keep from
tru cating those signals and still provide an AGC that attacks
quic
for t
Figu
mod
First, the data from the gain multiplier is truncated to a lower
re
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,
If fi
solution (4, 5, 6, 7, 8, 10, 12, or 16 bits) as set by the AGC
tr
n
t
lable in the AGC loop. If filter gain K is the maximum value,
lter gain K occupies only one LSB or 0.0039, then, during
r term representation). Generally, a small filter gain is used
, it would cause large errors to go undetected. Due to this
kly and settles to the desired output level. The signal path
his mode of operation is shown with broken arrows in
cated errors are less than 0.094 dB (equivalent to 1 LSB of
oted previously, each AGC can be configured so that the
re 51, and the operation is similar to the desired signal leve
n Loop Gain Setting
e.
hieve a large time constant loop (or slow loops), but, in this
liarity, the designers recommend that, if a user wants slow
locks onto a desired clipping level or a desired signal leve
t desired clippin
to exceed the boun
ultiplication with error term, errors of up t
loops, they should use fairl
al level.
g level mode by setting Bit 4 of the
he signal level. If averaging of four
ds of the peak-to-average ratio, the
gain for the next set of samples. These
y high values for filter gain K
re truncated to the required bit
o 6.02 dB could
AD6652
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ved
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to
l.
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