ISL5416 Datasheet

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ISL5416

the number of passes in the computation. A listing the

accuracy and gain is provided below.

TABLE 5. AGC MAGNITUDE COMPUTATION ACCURACY AND

GAIN

PASSES

2

3

4

8

ERROR +/- (dB)

0.48

0.13

0.03

0.0001

GAIN

1.581

1.630

1.642

1.647

96

84

72

60

dB

48

36

24

12

0

0

32768

65536

98304

131072

Code

163840

196608

229376 262144

location or by the SYNCInX if enabled in IWA *000. The AGC

can be set to a fixed gain either by setting the both upper

and lower gain limits to the desired gain or by setting the

loop gain to zero and programming the accumulator directly.

The bit weighting for the AGC loop is provided in Table 86.

ISL5416 AGC Forward Gain Response

With maximum gain and with full scale I and Q inputs equal

to ~+/-1.0, the maximum output from the computation is

1.414 * 1.647 = 2.329. The error detector subtracts the

magnitude from the programmable AGC Threshold value.

The AGC Threshold value is set in IWA register *009h and

should be programmed to K times the desired magnitude of

the I/Q where K is the gain of the magnitude computation.

Two adjustment/settling modes are provided in the ISL5416.

In the mean settling mode, the loop adjusts the gain so that

the average magnitude is equal to the programmed set point.

In this mode, the error is scaled by the loop gain and

integrated to compute the forward gain. The loop settles to

the final value asymptotically because the size of the

adjustment decreases as the error decreases. The initial

settling from large errors is fast, but the final pull in is slower.

After the loop has settled, the small adjustment size causes

minimal AM distortion of the signal. The other settling mode

is the median mode. In this mode, the sign of the error is

used increase or decrease the gain by a fixed amount. The

amount of the adjustment is programmed by the loop gain.

The loop settles to the point where there are an equal

number of samples above and below the set point. The loop

settling is roughly linear in dB, but after the loop has settled,

the step size remains the same, so the amount of AM

distortion may be objectionable. The ISL5416 provides two

programmable loop gains, each with a separate attack and

decay settling. The micro-processor can control the loop

gain, or the AGC counters can select the loop gain, so a

large loop gain can be used for initial settling and a smaller

one for tracking. The counters can also select the settling

mode, so the median mode can be used at the beginning of

each time slot and the mean mode used after the initial

settling.

The AGC loop filter is an accumulator (integrator). The

output of the accumulator is the forward gain word that

controls the barrel shifter and multiplier, closing the loop.

There are programmable limits on the accumulator range to

minimize settling time by restricting the AGC to only that

portion of the 96 dB range that is needed. The accumulator

can be loaded by the microprocessor. The gain load is

double buffered-the gain is first loaded into a holding register

by the uP. The gain is then transferred from the holding

register to the accumulator by a write to a special address

17

FIGURE 5A. ISL5416 AGC FORWARD GAIN RESPONSE

I

ISL5416 AGC Forward Gain Response

Magnified View

6

5

4

dB

3

2

1

0

0

2048

4096

6144

8192

Code

10240

12288

14336

16384

FIGURE 5B. ISL5416 AGC FORWARD GAIN RESPONSE

MAGNIFIED VIEW (ACTUAL AND IDEAL LINEAR

IN dB)

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