AD6654/PCB Analog Devices Inc, AD6654/PCB Datasheet - Page 50

• Manufacturer Part Number: AD6654/PCB
• Description: BOARD EVALUATION FOR AD6654
• Manufacturer: Analog Devices Inc

Specifications of AD6654/PCB

• Module/board Type: Evaluation Board
• For Use With/related Products: AD6554
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

AD6654
DESIRED SIGNAL LEVEL MODE
In this mode of operation, the AGC strives to maintain the
output signal at a programmable set level. The desired signal
level mode is selected by writing Logic 0 into the AGC clipping
error enable bit of the AGC control register. The loop finds the
square (or power) of the incoming complex data signal by
squaring I and Q and adding them.
The AGC loop has an average and decimate block. This average
and decimate operation takes place on power samples and
before the square root operation. This block can be pro-
grammed to average from 1 to 16,384 power samples, and the
decimate section can be programmed to update the AGC once
every 1 to 4,096 samples. The limitation on the averaging
operation is that the number of averaged power samples should
be a multiple of the decimation value (1×, 2×, 3×, or 4×).
The averaging and decimation effectively means that the AGC
can operate over averaged power of 1 to 16,384 output samples.
Updating the AGC once every 1 to 4,096 samples and operating
on average power facilitates the implementation of the loop
filter with slow time constants, where the AGC error converges
slowly and makes infrequent gain adjustments. It is also useful
when the user wants to keep the gain scaling constant over a
frame of data or a stream of symbols.
Due to the limitation that the number of average samples must
be a multiple of the decimation value, only the multiple
numbers 1, 2, 3, or 4 are programmed. This is set using the
AGC average samples word in the AGC average sample register.
These averaged samples are then decimated with decimation
ratios programmable from 1 to 4,096. This decimation ratio is
defined in the 12-bit AGC update decimation register.
The average and decimate operations are tied together and
implemented using a first-order CIC filter and FIFO registers.
Gain and bit growth are associated with CIC filters and depend
on the decimation ratio. To compensate for the gain associated
with these operations, attenuation scaling is provided before the
CIC filter.
This scaling operation accounts for the division associated with
the averaging operation as well as the traditional bit growth in
CIC filters. Because this scaling is implemented as a bit-shift
operation, only coarse scaling is possible. Fine scaling is imple-
mented as an offset in the request level, as explained later in this
section. The attenuation scaling, S
to 14 using a 4-bit CIC scale word in the AGC average samples
register and is given by
S
CIC
=
ceil
[
log
2
(
M
CIC
×
N
AVG
CIC
, is programmable from 0
)
]
Rev. 0 | Page 50 of 88
where:
M
N
multiple of the decimation ratio (1, 2, 3, or 4).
For example, if a decimation ratio M
(decimation of 1,000 and averaging of 3,000 samples), then
the actual gain due to averaging and decimation is 3,000 or
69.54 dB (log
bit-shift operation, only multiples of 6.02 dB attenuations are
possible. S
way, S
compensate for the gain in the average and decimate sections
and, therefore, prevents overflows in the AGC loop. But it is
also evident that the S
difference between gain due to CIC and attenuation provided
by scaling) of up to 6.02 dB. This error should be compensated
for in the request signal level, as explained later in this section.
A logarithm to the Base 2 is applied to the output from the
average and decimate sections. These decimated power samples
are converted to rms signal samples by applying a square root
operation. This square root is implemented using a simple shift
operation in the logarithmic domain. The rms samples obtained
are subtracted from the request signal level R specified in the
AGC desired level register, leaving an error term to be
processed by the loop filter, G(z).
The user sets this programmable Request Signal Level R accord-
ing to the output signal level that is desired. The Request Signal
Level R is programmable from −0 dB to −23.99 dB in steps of
0.094 dB.
The request signal level should also compensate for errors, if
any, due to the CIC scaling, as previously explained in this
section. Therefore, the request signal level is offset by the
amount of error induced in CIC, given by
where Offset is in dB.
Continuing the previous example, this offset is given by
So the request signal level is given by
where:
R is the request signal level.
DSL (desired signal level) is the output signal level that the user
desires.
AVG
CIC
Offset = 10 × log( M
Offset = 72.24 − 69.54 = 2.7 dB
is the decimation ratio (1 to 4,096).
is the number of averaged samples programmed as a
R
CIC
=
scaling always attenuates more than is sufficient to
−
CIC
ceil
2
in this case is 12, corresponding to 72.24 dB. This
(3000)). Because attenuation is implemented as a
⎡
⎢
⎣
(
DSL
. 0
−
094
CIC
Offset
CIC
scaling induces a gain error (the
× N
)
⎤
⎥
⎦
AVG
×
. 0
− S
094
CIC
CIC
dBFS
is 1,000 and N
× 3.01 dB
AVG
is 3