GC5018 Datasheet, PDF (37/115 Pages) Texas Instruments – 8-CHANNEL WIDEBAND RECEIVER

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GC5018
8-CHANNEL WIDEBAND RECEIVER
SLWS169 – MAY 2005
Separate programmable D values allow the user to set different attack and decay time constants, and to
set shorter time constants for when the signal falls too low (equal to zero), or is too high (saturates). The
magnitude is considered to be consistently equal to zero by using a 4-bit counter that counts up every
time the 8-bit magnitude value is zero, and counts down otherwise. If the counter’s value exceeds a user
specified threshold, then Dabv is used. Similarly the magnitude is considered too high by using a counter
that counts up when the magnitude is maximum, and counts down otherwise. If this counter exceeds
another user specified threshold, then Dsat is used.
As an example, if the AGC’s current gain at a particular moment in time is 5.123, and the magnitude of the
signal is greater than zero, but less than the user-programmed threshold. Step size Dblw will be used to
modify the gain for the next sample. This represents the AGC attack profile. If Dblw is set to a value of 5,
then the gain for the next sample will be 5.123 + 5.123 x 2^–5 = 5.123 + 0.160 = 5.283. If the signal’s
magnitude is still less than the user-programmed threshold, then the gain for the next sample will be 5.283
+ 5.283 x 2^–5 = 5.283 + 0.165 = 5.448. This continues until the signal’s magnitude exceeds the
user-programmed threshold. When the magnitude exceeds threshold (but is not saturated), then step size
Dabv is automatically employed as a size rather than Dblw.
The AGC converges linearly in dB with a step size of 40log(1+2^-D) when the error is greater than 12 dB
(i.e. the gain is off by 12 dB or more). Within 6 dB the behavior is approximately a exponential decay with
a time constant of 2^(D-0.5) samples.
The suggested value of D is 5 or 6 when the error is greater than 12dB (i.e., in the fast range detected by
consistently zero or saturated data). This gives a step size of 0.5 or 0.25 dB per sample.
The suggested value when the gain is off by less than 12 dB is D=10, giving a exponential time constant
for delay of around 724 samples (63% decay every 724 samples).
AGC GAIN ERROR
7
6
5
4
3
2
1
D=3
0
1
10
100
D=18
1000 10000 100000 1000000
SAMPLES
D=3
D=4
D=5
D=6
D=7
D=8
D=9
D=10
D=11
D=12
D=13
D=14
D=15
D=16
D=17
D=18
The AGC noise once the AGC has converged is a random error of amplitude ±2^-D relative to the RMS
signal level. This means that the error level is –6xD dB below the signal RMS level. At D=10 (–60 dB) the
error is negligible. The plot above shows the AGC response for vales of D ranging from 3 to 18. Error dB
represents the distance the signal level is from the desired target threshold.
The AGC is also subject to user specified upper and lower adjustment limits. The AGC stops incrementing
the gain if the adjustment exceeds Amax. It stops decrementing the gain if the adjustment is less than
Amin.
RECEIVE DIGITAL SIGNAL PROCESSING
37