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The AGC Loop

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

contains four basic operations: power calculation, error calcula-

tion, loop filtering, and gain multiplication.

The AGC can be configured to operate in one of two modes:

Desired Signal Level mode or 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 it is from a

given desired signal level or desired clipping level, depending on

the mode of operation selected. Two data paths to the AGC

loop are provided: one before the clipping circuitry and one

after the clipping circuitry, as shown in Figure 32. For Desired

Signal Level mode, only the I/Q path from before the clipping is

used. For Desired Clipping Level mode, the difference of the I/Q

signals from before and after the clipping circuitry is used.

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 operation

is selected by putting a value of zero in Bit 4 of AGC control

word (0x0A, 0x12). 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 (power of 2).

The AGC loop has an average and decimate block. This average

and decimate operation takes place on power samples and before

the square root operation. This block can be programmed to

average 1–16384 power samples and the decimate section can

be programmed to update the AGC once every 1–4096 samples.

The limitation on the averaging operation is that the number of

averaged power samples should be a multiple of the decimation

value (1, 2, 3, or 4 times).

The averaging and decimation effectively means the AGC can

operate over averaged power of 1–16384 output samples. The

choice of updating the AGC once every 1–4096 samples and

operating on average power facilitates the implementation of a

loop filter with slow time constants, where the AGC error con-

verges slowly and makes infrequent gain adjustments. It would

also be useful in scenarios where the user wants to keep the gain

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

CLIP

23 BITS

CLIP

MEAN SQUARE (I + jQ)

AVERAGE 1–16384 SAMPLES

DECIMATE 1–4096 SAMPLES

SQUARE ROOT

USED ONLY FOR

DESIRED

CLIPPING LEVEL

MODE

GAIN

MULTIPLIER

–

POWER OF 2

Kz –1

1 – (1 + P)z–1 + Pz –2

ERROR

'K' GAIN

'P' POLE

–

'R' DESIRED

PROGRAMMABLE

BIT WIDTH

LOG2(X)

Figure 32. Block Diagram of the AGC

Due to the limitation on the number of average samples being a

multiple of the decimation value, only the multiple number 1, 2,

3, or 4 is programmed. This number is programmed in Bits 1, 0

of the 0x10 and 0x18 registers. These averaged samples are then

decimated with decimation ratios programmable from 1 to

4096. This decimation ratio is defined in the 12-bit registers

0x11 and 0x19.

The average and decimate operations are tied together and

implemented using a first-order CIC filter and some FIFO

registers. There is a gain and bit growth associated with CIC

filters, which depend on the decimation ratio. To compensate

for the gain associated with these operations, attenuation scaling

is provided before the CIC filter.

This scaling operation accounts for the division associated with

the averaging operation as well as the traditional bit growth in

CIC filters. Since this scaling is implemented as a bit shift

operation, only coarse scaling is possible. Fine scaling is imple-

mented as an offset in the Request Level explained later. The

attenuation scaling (SCIC) is programmable from 0 to 14 using

four bits of the 0x10 and 0x18 registers and is given by:

ceil

CIC

AVG

=¥

()

[]

log

where MCIC is the decimation ratio (1–4096) and NAVG is the

number of averaged samples programmed as a multiple of the

decimation ratio (1, 2, 3, or 4).

For example if a decimation ratio, MCIC, is 1000, and NAVG is

selected to be 3 (decimation of 1000 and averaging of 3000

samples), the actual gain due to averaging and decimation is

3000 or 69.54 dB (= 20

log 3000). Since attenuation is

implemented as a bit shift operation, only multiples of 6.02 dB

attenuations are possible. SCIC in this case is 12, corresponding

to 72.24 dB. This way SCIC scaling always attenuates more

than sufficiently to compensate for the gain changes in the aver-

age and decimate sections, and hence prevents overflows in the

AGC loop. But it is also evident that the CIC scaling is intro-

ducing a gain error (difference between gain due to CIC and

attenuation provided) of up to 6.02 dB. This error should be

compensated for in the Request signal level as explained below.

Logarithm to the base 2 is applied to the output from the aver-

age and decimate section. These decimated power samples (in

logarithmic domain) are converted to rms signal samples by

applying a square root. This square root is implemented using a

simple shift operation. The rms samples so obtained are sub-

tracted from the request signal level ‘R’ specified in registers

(0x0B, 0x14) leaving an error term to be processed by the loop

filter, G(z).

The user sets this programmable request signal level ‘R’ according

to the desired output signal level. The request signal level ‘R’

is programmable from 0 to –23.99 dB in steps of 0.094 dB. The

request signal level should also compensate for any error due to

the CIC scaling as explained previously. Hence, the request

signal level is offset by the amount of error induced in the CIC

given by

Offset

CIC

AVG

CIC

=¥

()

6 02

log

–

where, the offset is in dB.

Continuing with the previous example, this offset is given by

72.24 – 69.54 = 2.7 dB. So the request signal level is given by

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