AD6635BBSLASHPCB datasheet(34/60 Pages) AD

34 / 60 page

REV. 0

–34–

AD6635

ceil

DSL

Offset

()

–

0 094

where R is the request signal level and DSL (desired signal

level) is the output signal level that the user desires. So, in the

previous example if the desired signal level is –13.8 dB, the

request level ‘R’ is programmed to be –16.54 dB.

The AGC provides a programmable second order loop filter.

The programmable parameters, gain ‘K’ and pole ‘P,’ com-

pletely define the loop filter characteristics. The error term after

subtracting the request signal level is processed by the loop

filter, G(z). The open loop poles of the second-order loop filter

are 1 and ‘P,’ respectively. The loop filter parameters, pole ‘P’

and gain ‘K,’ allow adjustment of the filter time constant that

determines the window for calculating the peak-to-average ratio.

The open loop transfer function for the filter, including the gain

parameter is given by

() =

() +

–

––

–

If the AGC is properly configured (in terms of offset in Request

level), there are no gains except the filter gain K. Under these

circumstances a closed loop expression for the AGC loop is

possible, and is given by

KP z

CLOSED

() = ()

() = +() +

–

––

The gain parameter ‘K,’ and pole ‘P’ are programmable through

registers (0x0E and 0x0F for AGC A and AGC C; 0x16 and

0x17 for AGC B and AGC D) from 0 to 0.996 in steps of

0.0039 using 8-bit representation. Though the user defines the

open loop pole ‘P’ and gain ‘K,’ they will directly impact the

placement of the closed loop poles and filter characteristics.

These closed loop poles P1 and P2 are the roots of the denomi-

nator in the above closed loop transfer function and are given by

––

–

()++

()

Typically, the AGC loop performance is defined in terms of its

time constant or settling time. In such a case, the closed loop

poles should be set to meet the time constants required by the

AGC loop. The following relation between time constant and

closed loop poles can be used for this purpose.

Sample Rate

CIC

exp

ÎÍ

˚˙

where

t1,2 are the time constants corresponding to the poles

P1, 2. The time constants can also derived from settling times

as given below.

settling time

MCIC (CIC decimation is from 1 to 4096) and either the settling

time or time constant should be chosen by the user. The sample

rate is the combined sample rate of all the interleaved channels

coming into the AGC/half-band interpolated filters. If two chan-

nels are being used to process one carrier of UMTS at 2

chip

rate, then each channel works at 3.84 MHz, and the combined

sample rate coming into the half-band interpolated filters is

7.68 MSPS. This rate should be used in the calculation of poles

in the above equation.

The loop filter output corresponds to the signal gain that is

updated by the AGC. Since all computation in the loop filter is

done in logarithmic domain (to the base 2) of the samples, the

signal gain is generated using the exponent (power of 2) of the

loop filter output.

The gain multiplier gives the product of the signal gain with both

the I and Q data entering the AGC section. This signal gain is

applied as a coarse 4-bit scaling and then a fine scale 8-bit

multiplier. Hence, the applied signal gain is between –48.16 dB

and +48.13 dB in steps of 0.024 dB. The initial value for

signal gain is programmable using the registers 0x0D and 0x15

for AGC A (AGC C) and AGC B (AGC D), respectively.

The products of the gain multiplier are the AGC scaled outputs

in 19-bit representation. These are in turn used as I and Q for

calculating the power and AGC error and loop filtered to pro-

duce signal gain for the next set of samples. These AGC scaled

outputs can be programmed as 4, 5, 6, 7, 8, 10, 12, or 16 bits

using the AGC control word (0x0A, 0x12). The AGC scaled

outputs are truncated to the required bit widths using the clip-

ping circuitry, as shown in the Functional Block Diagram.

Open Loop Gain Setting: If filter gain K occupies only 1 LSB

or 0.0039, then during the multiplication with the error term,

errors of up to 6.02 dB could be truncated. This truncation is

due to the lower bit widths available in the AGC loop. If filter

gain K were the maximum value, truncated errors would be a

less than 0.094 dB (equivalent to 1 LSB of error term represen-

tation). Generally, a small filter gain is used to achieve a large

time constant loop (or slow loops), but in this case, it would

cause large errors to go undetected. Due to this peculiarity, the

designers recommend that if a user wants slow AGC loops, they

should rather use fairly high values for filter gain K and then

use CIC decimation to achieve a slow loop. In this way, the

AGC loop will make large, infrequent gain changes compared

to small and frequent gain changes, as in the case of a normal

small gain loop filter. However, though the AGC loop makes

large, infrequent gain changes, a slow time constant is still

achieved and there is less truncation of errors.

Average Samples Setting: Though it is complicated to express

the exact effect of the number of averaging samples, thinking

intuitively, it has a smoothing effect on the way the AGC loop

attacks a sudden increase or a spike in the signal level. If averag-

ing of four samples is used, the AGC will attack a sudden

increase in signal level more slowly compared to no averaging.

The same would apply to the manner in which the AGC would

attack a sudden decrease in the signal level.

Desired Clipping Level Mode

As noted previously, each AGC can be configured so that the

loop locks on to a desired clipping level or a desired signal level.

The Desired Clipping Level mode can be selected by setting

Bit 4 of the individual AGC control words (0x0A, 0x12). For

signals that tend to exceed the bounds of the peak-to-average

ratio, desired clipping level option offers a way to keep from

truncating those signals and still provides an AGC that attacks

quickly and settles to the desired output level. The signal path

for this mode of operation is shown with the dashed arrows in

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