AD6635BB Analog Devices Inc, AD6635BB Datasheet - Page 34

• Manufacturer Part Number: AD6635BB
• Description: IC,RF/Baseband Circuit,CMOS,BGA,324PIN,PLASTIC
• Manufacturer: Analog Devices Inc
• Series: AD6635r

Specifications of AD6635BB

• Rohs Status: RoHS non-compliant
• Rf Type: Cellular, CDMA2000, EDGE, GPRS, GSM
• Number Of Mixers: 1
• Current - Supply: 880mA
• Voltage - Supply: 3 V ~ 3.6 V
• Package / Case: 324-BGA
• Frequency: -
• Gain: -
• Noise Figure: -
• Secondary Attributes: -

AD6635
where R is the request signal level and DSL (desired signal
level) is the output signal level that the user desires. So, in the
previous example if the desired signal level is –13.8 dB, the
request level ‘R’ is programmed to be –16.54 dB.
The AGC provides a programmable second order loop filter.
The programmable parameters, gain ‘K’ and pole ‘P,’ com-
pletely define the loop filter characteristics. The error term after
subtracting the request signal level is processed by the loop
filter, G(z). The open loop poles of the second-order loop filter
are 1 and ‘P,’ respectively. The loop filter parameters, pole ‘P’
and gain ‘K,’ allow adjustment of the filter time constant that
determines the window for calculating the peak-to-average ratio.
The open loop transfer function for the filter, including the gain
parameter is given by
If the AGC is properly configured (in terms of offset in Request
level), there are no gains except the filter gain K. Under these
circumstances a closed loop expression for the AGC loop is
possible, and is given by
The gain parameter ‘K,’ and pole ‘P’ are programmable through
registers (0x0E and 0x0F for AGC A and AGC C; 0x16 and
0x17 for AGC B and AGC D) 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 will directly impact the
placement of the closed loop poles and filter characteristics.
These closed loop poles P
nator in the above closed loop 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, the closed loop
poles should be set to meet the time constants required by the
AGC loop. The following relation between time constant and
closed loop poles can be used for this purpose.
where t
P
as given below.
M
time or time constant should be chosen by the user. The sample
rate is the combined sample rate of all the interleaved channels
coming into the AGC/half-band interpolated filters. If two chan-
nels are being used to process one carrier of UMTS at 2 chip
rate, then each channel works at 3.84 MHz, and the combined
1, 2
CIC
. The time constants can also derived from settling times
(CIC decimation is from 1 to 4096) and either the settling
1,2
G
CLOSED
are the time constants corresponding to the poles
P
1
t =
,
P
R
( )
2
2
z
P
=
G z
=
%
1
,
=
ceil
2
( )
(
1
settling time
1
=
+
G z
+
=
È
Í
Í
Î
exp
4
P
(
G z
( )
DSL Offset
1
1
( )
–
–
and P
È
Í
Î
Sample Rate
K
0 094
(
1
.
=
)
–
+
+
1
2
or
P z
Kz
+
are the roots of the denomi-
)
M
(
(
5
K
1
2
–
%
–
CIC
1
1
+
)
– –
˘
˙
˙
˚
+
P
settling time
1
¥
Pz
¥
Kz
–
0 094
3
t
P z
.
K
1
–
,
2
–
)
2
1
)
2
˘
˙
˚
–
1
–
+
4
Pz
P
–
2
–34–
sample rate coming into the half-band interpolated filters is
7.68 MSPS. This rate should be used in the calculation of poles
in the above equation.
The loop filter output corresponds to the signal gain that is
updated by the AGC. Since all computation in the loop filter is
done in logarithmic domain (to the base 2) of the samples, 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. Hence, the applied signal gain is between –48.16 dB
and +48.13 dB in steps of 0.024 dB. The initial value for
signal gain is programmable using the registers 0x0D and 0x15
for AGC A (AGC C) and AGC B (AGC D), respectively.
The products of the gain multiplier are the AGC scaled outputs
in 19-bit representation. These are in turn used as I and Q for
calculating the power and AGC error and loop filtered to pro-
duce signal gain for the next set of samples. These AGC scaled
outputs can be programmed as 4, 5, 6, 7, 8, 10, 12, or 16 bits
using the AGC control word (0x0A, 0x12). The AGC scaled
outputs are truncated to the required bit widths using the clip-
ping circuitry, as shown in the Functional Block Diagram.
Open Loop Gain Setting: If filter gain K occupies only 1 LSB
or 0.0039, then during the multiplication with the error term,
errors of up to 6.02 dB could be truncated. This truncation is
due to the lower bit widths available in the AGC loop. If filter
gain K were the maximum value, truncated errors would be a
less than 0.094 dB (equivalent to 1 LSB of error term represen-
tation). Generally, a small filter gain is used to achieve a large
time constant loop (or slow loops), but in this case, it would
cause large errors to go undetected. Due to this peculiarity, the
designers recommend that if a user wants slow AGC loops, they
should rather use fairly high values for filter gain K and then
use CIC decimation to achieve a slow loop. In this way, the
AGC loop will make large, infrequent gain changes compared
to small and 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
achieved 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 sudden increase or a spike in the signal level. If averag-
ing of four samples is used, the AGC will attack a sudden
increase in signal level more slowly compared to no averaging.
The same would apply to the manner in which the AGC would
attack a sudden decrease in the signal level.
Desired Clipping Level Mode
As noted previously, each AGC can be configured so that the
loop locks on to a desired clipping level or a desired signal level.
The Desired Clipping Level mode can be selected 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, desired clipping level option offers a way to keep from
truncating those signals and still provides an AGC that attacks
quickly and settles to the desired output level. The signal path
for this mode of operation is shown with the dashed arrows in
REV. 0