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| 型號(hào)： | AD6652BBC |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | 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 |
| 文件頁數(shù)： | 42/76頁 |
| 文件大?。?/td> | 1839K |
| 代理商： | AD6652BBC |
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AD6652
Because the number of average samples must be an integer
multiple of the decimation value, only the multiple number 1, 2,
3, or 4 is programmed. This number is programmed in Ou
Port Control Registers 0x10:1–0 and 0x18:1–0. These averaged
samples are then decimated with decimation ratios programm
ble from 1 to 4096. This 12-bit decimation ratio is defined in
Registers 0x11 and 0x19.
Rev. 0 | Page 42 of 76
tput
a-
gisters.
bit growth associated with CIC filters depends on
the decimation ratio. To compensate for the gain associated
with these operations, attenuation scaling is provided before the
CIC filter.
unts for the division associated with
in
t
operation, only coarse scaling is possible. Fine scaling is imple-
secti
14 u
iste
reg
rs, and is given by
ort control
wher
o 4096).
amples programmed as a
or 4).
g up to t
he next whole
im
athCad-
ber.
xample, if a
ted to be 3 (decimation of 1000 and av
amples), then the actual gain due to averaging and decimation
3000 or 69.54 dB ( = log
2
(3000)). Because attenuation is
ift operation, only multiples of 6.02 dB
implemented as a bit sh
attenuations are possible.
S
CIC
, in this case, is 12 correspondin
to 72.24 dB. This way,
S
CIC
scaling always attenuates more than
sufficient to compensate for the gain changes in average and
decimate sections and, therefore, prevents overflows in the AGC
loop. But it is also evident that the CIC scaling is inducing a
gain error (difference between gain due to CIC and attenuation
provided) of up to 6.02 dB. This error should be compensated
for in the request signal level, as explained below.
ation ratio
M
CIC
is 1
decim
d
N
avg
is
g of 3000
g
is
utput from the aver-
appl
simp
tracted from the request signal level,
R
, specified in Registers
(0x0B, 0x14), leaving an error term to be processed by the loop
filter,
G(z)
.
ing a
b-
rogrammable request signal level,
R
, according to the
t signal level desired. The request signal level
R
is
outpu
programmable from 0 to 23.99 dB in steps of 0.094 dB. The
est signal level should also compensate for error, i
e CIC scaling, as explained previously. Therefore, th
est signal leve
CI given by
requ
to th
requ
f any, due
e
l is offset by the amount of error induced in
C,
Set this p
The average and decimate operations are linked together and
implemented using a first-order CIC filter and FIFO re
The gain and
This scaling operation acco
the veraging operation as well as the traditional bit growth
CI filters. Because this scaling is implemented as a bit shif
me ted as an offset in the request level, explained later in this
on. The attenuation scaling, S
CIC
, is programmable from 0 to
sing four bits of 0x10 and 0x18 of the output p
)]
(
[log
2
avg
CIC
CIC
N
M
ceil
S
×
=
e:
M
CIC
is the decimation ratio (1 t
N
avg
is the number of averaged s
multiple of dec
Ceil
is M
num
ation ratio (1, 2, 3,
speak for roundin
For e
selec
s
is
000 an
eragin
Logarithm to the Base 2 is applied to the o
age and decimate section. These decimated power samples (in
log rithmic domain) are converted to rms signal samples by
ying a square root. This square root is implemented us
le shift operation. The rms samples so obtained are su
02
.
)
(
log
20
10
×
×
×
=
CIC
avg
CIC
S
N
M
Offset
wh re the offset is in dB.
C ntinuing with the previous example, this offset is given by
h
Offset
= 72.24 69.54 = 2.7 dB
So t e request signal level is given by
094
.
)
(
×
094
.
=
Offset
DSL
ceil
R
e:
wher
DSL
desir
Therefore, in the previous example, if the desired signal level is
13.8 dB, the request signal level,
R
, is programmed to be
54 dB.
16.
The AGC provides a programmable second-order loop filter.
The programmable parameters, gain
K
and pole
P
, completely
e the loop filter characteristics. The error term after
racting the request signal level is p
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
K
, allow adjustment of the filter time constant, which
determines the window for calculating the peak-to-averag
.
defin
subt
rocessed by the loop
gain
e
ratio
The
para
R
is the request signal level.
(desired signal level) is the output signal level that the user
es.
open loop transfer function for the filter, including the gain
meter is as follows:
2
1
)
1
1
+
+
Pz
z
P
If the AGC is properly configured (in terms of offset in request
level), then 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
1
Kz
)
(
=
z
G
2
1
1
)
1
(
1
)
(
1
)
z
(
G
)
(
+
+
=
+
=
Pz
z
P
K
Kz
z
G
z
G
closed
The gain parameter
K
and pole
P
are programmable through
registers (0x0E and 0x0F, respectively, for AGC Channel A and
Channel B) from 0 to 0.996 in steps of 0.0039 using 8-bit
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