GC5018 Datasheet, PDF (36/115 Pages) Texas Instruments – 8-CHANNEL WIDEBAND RECEIVER

GC5018
8-CHANNEL WIDEBAND RECEIVER
SLWS169 – MAY 2005
3.2.10 DDC AGC
www.ti.com
I, Q 18
12 integer &
12 fractional
24
limit &
round
18
I, Q outputs (up to 25−bits in AGC bypass mode)
threshold zero mask ucnt
8
4
8
ocnt
4
dblw dabv dzro dsat
44
44
8
magnitude
2
compare
under/over
detect
2
shift select
Freeze
(from register bit
Freeze
(from sync source)
Gain 24
clear
amax amin
16
16
29
29
shift
accumulate
24
limit
S=+/−1, D=4−bit shift ⁵
min limit
max limit
gain adjust
The GC5018 automatic gain control circuit is shown above. The basic operation of the circuit is to multiply
the 18 bit input data from the PFIR by a 24-bit gain word that represents a gain or attenuation in the range
of 0 to 4096. The gain format is mixed integer and fraction. The 12-bit integer allows the gain to be
boosted by up to factor of 4096 (72 dB). The 12-bit fractional part allows the gain to be adjusted up or
down in steps of one part in 4096, or approximately 0.002 dB. If the integer portion is zero, then the circuit
attenuates the signal. The gain adjusted output data is saturated to full scale and then rounded to
between 4 and 18 bits in steps of one bit.
The AGC portion of the circuit is used to automatically adjust the gain so that the median magnitude of the
output data matches a target value, which is performed by comparing the magnitude of the output data
with a target threshold. If the magnitude is greater than the threshold, then the gain is decreased,
otherwise it is increased. The gain is adjusted as: G(t) = G + A(t), where G is the default, user supplied
gain value, and A(t) is the time varying adjustment. A(t) is updated as A(t) = A(t) + G(t)xSx2^–D , where S=1
if the magnitude is less than the threshold and is –1 if the magnitude exceeds the threshold, and where D
sets the adjustment step size. Note that the adjustment is a fraction of the current gain. This is designed to
set the AGC noise level to a known and acceptable level while keeping the AGC convergence and
tracking rate constant, independent of the gain level. The AGC noise will be equal to ±2^–D and the AGC
attack and decay rate will be exponential, with a time constant equal to 2^–D. Hence, the AGC will increase
or decrease by 0.63 times G(t) in 2^D updates.
If one assumes the data is random with a Gaussian distribution, which is valid for UMTS if more than 12
users with different codes have been overlaid, then the relationship between the RMS level and the
median is MEDIAN = 0.6745xRMS, hence the threshold should be set to 0.6745 times the desired RMS
level.
The gain step size can be set using four different values of D, each of which is a 4 bit integer. D can range
from 3 to 18. The user can specify values of D for different situations, i.e., when the signal magnitude is
below the user-specified threshold (Dblw), is above the threshold (Dabv), is consistently equal to zero
(Dzro) or is consistently equal to maximum (Dsat). It is important to note that D represents a gain step
size. Smaller values of D represent larger gain steps. The definition of equal to zero is any number when
masked by zero_mask is considered to be zero. This permits consistently very small amplitude signals to
have their gain increased rapidly.
36
RECEIVE DIGITAL SIGNAL PROCESSING