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AD6636

Rev. 0 | Page 66 of 72

<7:5>: AGC Word Length Control Bits. These bits define the

word length of the AGC output. The output word can be 4 to 8,

10, 12, or 16 bits wide. Table 41 shows the possible selections.

Table 41. AGC Word Length Control Bits

AGC Control Bits <7:5>

Output Word Length (Bits)

000

001

010

011

100

101

110

111

<4>: AGC Mode Bit. When this bit is set, the AGC operates to

maintain a desired signal level. When this bit is cleared, it

operates to maintain a constant clipping level. See the

Automatic Gain Control section for details about these modes.

<3>: AGC Sync Now Bit. This bit is used to synchronize a

particular AGC irrespective of the channel through the

programming ports (microport or serial port). When this bit is

set, the AGC block updates a new output sample (RSSI sample)

and starts working toward a new update sample.

<2>: Initialize on Sync Bit. This bit is used to determine whether

or not the AGC should initialize on a sync. When this bit is set,

during a synchronization the CIC filter is cleared and new

values for CIC decimation, number of averaging samples, CIC

scale, signal gain Gs’ gain K, and pole parameter P are loaded.

When Bit <2> = 0, the above-mentioned parameters are not

updated, and the CIC filter is not cleared. In both cases an AGC

update sample is output from the CIC filter and the decimator

starts operating towards the next output sample whenever a

sync occurs.

<1>: First Sync Only. This bit is used to ignore repetitive

synchronization signals. In some applications, the synchroniza-

tion signal occurs periodically. If this bit is cleared, each

synchronization request resynchronizes the AGC. If this bit is

set, only the first occurrence causes the AGC to synchronize

and updates the AGC gain values periodically, depending on the

decimation factor of the AGC CIC filter.

<0>: AGC Bypass Bit. When this bit is set, the AGC section is

bypassed. The N-bit representation from the interpolating half-

band filters is still reduced to a lower bit width representation as

set by Bits <7:5> of the AGC control register. A truncation at the

output of the AGC accomplishes this task.

AGC Hold-Off Register <15:0>

The AGC hold-off counter is loaded with the value written to

this address when either a soft sync or pin sync comes into the

channel. The counter begins counting down. When it reaches 1,

a sync is sent to the AGC. This sync might or might not

initialize the AGC, as defined by the control word. The AGC

loop is updated with a new sample from the CIC filter whenever

a sync occurs. If this register is Logic 1, the AGC is updated

immediately when the sync occurs. If this register Logic 0, the

AGC cannot be synchronized.

AGC Update Decimation <11:0>

This 12-bit register sets the AGC decimation ratio from 1 to

4096. An appropriate scaling factor should be set to avoid loss of

bits. The decimation ratio is given by the decimal value of the

AGC update decimation<11:0> register contents plus 1, that is,

12’0x000 describes a decimation ratio of 1, and 12’0xFFF

describes a decimation ratio of 4096.

AGC Signal Gain <11:0>

This register is used to set the initial value for a signal gain used

in the gain multiplier. This 12-bit value sets the initial signal

gain in the range of 0 dB and 96.296 dB in steps of 0.024 dB.

Initial signal gain (SG) in dB should be converted to a register

setting using the following formula:

⎥

⎦

⎤

⎢

⎣

⎡

×256

)

(

log

AGC Error Threshold <11:0>

This 12-bit register is the comparison value used to determine

which loop gain value (K1 or K2) to use for optimum operation.

When the magnitude-of-error signal is less than the AGC error

threshold value, then K1 is used; otherwise, K2 is used. The word

format of the AGC error threshold register is four bits to the left

of the binary point and eight bits to the right. See the Automatic

Gain Control section for details.

⎥

⎦

⎤

⎢

⎣

⎡

×256

)

(

log

Threshold

Error

AGC Average Samples <5:0>

This 6-bit register contains the scale used for the CIC filter and

the number of power samples to be averaged before being sent

to the CIC filter.

<5:2>: CIC Scale. This 4-bit word defines the scale used for the

CIC filter. Each increment of this word increases the CIC scale

by 6.02 dB.

<1:0>: Number of AGC Average Samples. This defines the

number of samples to be averaged before they are sent to the

CIC decimating filter. See Table 42.

Table 42. Number of AGC Average Samples

AGC Average Samples <1:0>

Number of Samples Taken

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