GC5016 Datasheet, PDF (24/88 Pages) Texas Instruments – WIDEBAND QUAD DIGITAL DOWN CONVERTER/ UP CONVERTER

GC5016
SLWS142G − JANUARY 2003 − REVISED NOVEMBER 2005
www.ti.com
Valid
DELAY
DELAY
Valid
Data ²⁰
In
28
20 Data
Plus
8 Overflow
7 Integer
19 _Plus
12 Fractional
agc_rnd
5
LS20
20
ROUND
MS16
OVERFLOW
MS8
MAGNITUDE
LS8
8
Data Out
(Upper 20 − agc_rnd bits are valid,
lower bits are cleared)
agc_Dzro
agc_zero_cnt ^agc_Dadv
agc_thresh agc_sat_cnt
agc_Dblw
agc_Dsat
8
4
4
44
44
2
2
5
COMPARE
UNDER/OVER
SHIFT SELECT
DETECT
S=±1, D=4-Bit Shift
19
Gain
7 Integer
Plus
12 Fractional
Valid
agc_freeze
G(t)=Gain+A(t) ¹⁹
7 Integer
Plus
12 Fractional
A(t)=Gain Adjust
Sync
agc_max agc_min
SHIFT
24
CLR
MS16
Sign Plus
ACCUMULATE
7 Integer A(t+1)=A(t)+S 2−(D+3)G(t)
Plus
16 Fractional
16
16
LIMIT
*agc_min < A(t) <
agc_max
7 Integer
Plus
9 Fractional
Under Limit
Over Limit
19
Figure 12. GC5016 AGC Circuit
The AGC portion of the circuit is used to change the adaptive gain so that the median magnitude of the output data
matches a target value. The magnitude of the gain-adjusted (manual + adaptive) output data is compared to a target
threshold. If the magnitude is greater than the threshold, the gain is decreased. If not, it is increased. The gain is
adjusted as:
G(t) = G + A(t)
A(t) = A(t) + G(t) x S x 2^−(D+3)
where G is the default, user supplied gain value, and A(t) is the time varying adjustment, 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.
Because the adjustment is a fraction of the current gain, one can show that the AGC noise is an amplitude jitter in
the data output equal to ±(data output) x 2^−(D+3). This means that the AGC noise is always 6 x (D+3) dB below the
output signal’s power level. The AGC attack and decay rate is exponential with a time constant equal to 2^(D+1.75)
complex samples. This means the AGC covers to within 63% of the required gain change in one time constant and
to within 98% of the change in the four time constants.
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 = 0.6745
x RMS.
Hence the threshold should be set to 0.6745 times the desired RMS level.
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