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AD6652

Rev. 0 | Page 41 of 76

gain

GC reject

nly

he AGC strives to maintain a constant mean

multiplier.

uncation of bits below the output range. Overflow is caused by

pping errors when the output signal exceeds the output range.

Modulation error occurs when the output gain varies during the

reception of data.

Set the desired signal level based on the probability-density

function of the signal, so that the errors due to underflow and

overflow are balanced. Set the gain and damping values of the

loop filter so that the AGC is fast enough to track long-term

amplitude variations of the signal that might cause excessive

underflow or overflow, but slow enough to avoid excessive loss

of amplitude information due to the modulation of the signal.

AGC LOOP

The AGC loop is implemented using a log-linear architecture. It

performs four basic operations: power calculation, error calcu-

lation, loop filtering, and gain multiplication. The AGC can be

configured to operate in one of the following modes:

• Desired signal level mode

• Desired clipping level mode as set by Bit 4 of AGC control

word (0x0A, 0x12)

The AGC adjusts the gain of the incoming data according to

how far its level is from the desired signal level or desired

clipping level, depending on the mode of operation selected.

AUTOMATIC GAIN CONTROL

The AD6652 is equipped with two independent automatic

control (AGC) loops for direct interface with a Rake receive

Each AGC circuit has 96 dB of range. It is important that the

decimating filters of the AD6652 preceding the A

Two datapaths to the AGC loop are provided: one before the

clipping circuitry and one after the clipping circuitry, as show

in Figure 51. For desired signal level mode, only the I/Q pat

before the clipping is used. For desired clipping level mode, the

difference of the I/Q signals before and after the clipping

circuitry is used.

undesired signals, so that each AGC loop is operating on o

the carrier of interest and carriers at other frequencies do not

affect the ranging of the loop.

The AGC compresses the 23-bit complex output from the

interpolating half-band filter into a programmable word size o

4 to 8, 10, 12, or 16 bits. Because the small signals from the

lower bits are pushed into higher bits by adding gain, the

clipping of the lower bits does not compromise the SNR of t

signal of interest. T

CLIP

23 BITS

CLIP

MEAN SQUARE (I + jQ)

GAIN

MULTIPLIER

PROGRAMM

USED ONLY FOR

DESIRED

CLIPPING LEVEL

MODE

–

output power despite input signal fluctuations. This permits

operation in environments where the dynamic range of the

signal exceeds the dynamic range of the output resolution.

The AGCs and the interpolation filters need not be linked

together. Either can be selected without the other. The AGC

section can be bypassed, if desired, by setting Bit 0 of the AGC

control word. When bypassed, the I/Q data is still clipped to a

desired number of bits, and a constant gain can be provided

through the AGC gain

AVERAGE 1 – 16384 SAMPLES

DECIMATE 1 – 4096 SAMPLES

SQUARE ROOT

× z–1

1 – (1 + P)

× z–1 + P × z–2

ERROR

K GAIN

P POLE

–

R DESIRED

ABLE

BIT WIDTH

LOG2(X)

DESIRED SIGNAL LEVEL MODE

In this mode of operation, the AGC strives to maintain the

output signal at a programmable set level. This mode of opera-

tion is selected by writing AGC control word (0x0A:4, and

0x12:4) to Logic 0. First, the loop finds the square (or power) of

the incoming complex data signal by squaring I and Q and

adding them. This operation is implemented in exponential

domain using 2x.

The AGC loop has average and decimate blocks that operate on

power samples before the square root operation, as shown in

Figure 51. The average block can be programmed to average

1 to 16,384 power samples, and the decimate block can be pro-

grammed to update the AGC once every 1 to 4096 samples. The

limitations on the averaging operation are that the number of

averaged power samples must be an integer multiple of the

decimation value, and the only allowable multiple values are

1, 2, 3, or 4.

The averaging and decimation effectively mean that the AGC

can operate over averaged power of 1 to 16,384 output samples.

The choice of updating the AGC once every 1 to 4096 samples

and operating on average power facilitates the implementation

of a loop filter with slow time constants, where the AGC error

converges slowly and makes infrequent gain adjustments. It

would also be useful where the user wants to keep the gain

scaling constant over a frame of data (or a stream of symbols).

Figure 51. Block Diagram of the AGC

Three sources of error can be introduced by the AGC function

underflow, overflow, and modulation. Underflow is caused by

cli

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