AD6652BBC datasheet(43/76 Pages) AD

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AD6652

Rev. 0 | Page 43 of 76

sed

P1,

representation. Though the user defines the open loop po

and gain

K, they directly impact the placement of the clo

loop poles and filter characteristics. These closed loop pole

P2 are the roots of the denominator of 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, set the closed

poles to meet the time constants required by the AGC loop. The

following relation between time constant and closed loop po

can be used for this purpose:

loop

les

⎥

⎤

⎢

⎡

exp

CIC

⎦

⎢⎣

rate

sample

where:

τ1,2 are the time constants corresponding to the poles P1,2.

exp denotes the inverse of the natural log.

The time constants can also be derived from settling times as

follows:

time

settling

time

settling

where:

MCIC (CIC decimation) is from 1 to 4096.

settling time or time constant is chosen by the user.

sample rate is the combined sample rate of all the interleaved

channels coming into the AGC/half-band interpolated filters.

If two channels 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

les in

the previous equation, if half-band interpolating filters are

les in

t of the signal gain with

and Q data entering the AGC section. This signal

The products of the gain multiplier are the AGC scaled outputs,

gain for the next set of samples. These

re truncated to the required bit

Ope

If fi

the m

o 6.02 dB could

avai

trun

erro

to ac

case

pecu

AGC

y high values for filter gain K

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

ved

he signal level. If averaging of four

al level.

As n

loop

Selec

g level mode by setting Bit 4 of the

tend

ds of the peak-to-average ratio, the

desired clipping level option provides a way to keep from

quic

for t

Figu

mod

First, the data from the gain multiplier is truncated to a lower

solution (4, 5, 6, 7, 8, 10, 12, or 16 bits) as set by the AGC

control word. An error term (both I and Q) is generated that is

the difference between the signals before and after truncation.

This term is passed to the complex squared magnitude block,

.68 MSPS. Use this rate in the calculation of po

bypassed.

The loop filter output corresponds to the signal gain that is

updated by the AGC. Because all computation of the samp

the loop filter is done in logarithmic domain (to the base 2), the

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

loop filter output.

The gain multiplier gives the produc

both the I

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

8-bit multiplier. Therefore, the applied signal gain is between

0 dB and 96.296 dB in steps of 0.024 dB. Initial value for signal

gain is programmable using Register 0x0D for AGC A and

which have 19-bit representation. These are in turn used as I

and Q for calculating the power and AGC error and loop

filtered to produce signal

AGC scaled outputs can be programmed to have 4-, 5-, 6-, 7-, 8-,

10-, 12-, or 16-bit widths using the AGC control word (0x0A,

0x12). The AGC scaled outputs a

wid hs using the clipping circuitry shown in Figure 51.

n Loop Gain Setting

lter gain K occupies only one LSB or 0.0039, then, during

ultiplication with error term, errors of up t

be uncated. This truncation is due to the lower bit width

lable in the AGC loop. If filter gain K is the maximum value,

cated errors are less than 0.094 dB (equivalent to 1 LSB of

r term representation). Generally, a small filter gain is used

hieve a large time constant loop (or slow loops), but, in this

, it would cause large errors to go undetected. Due to this

liarity, the designers recommend that, if a user wants slow

loops, they should use fairl

the AGC loop makes large infrequent gain changes compared

small 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 achie

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 sudd

increase or a spike in t

samples is used, the AGC attacks a sudden increase in signal

level more slowly compared to no averaging. The same applies

to the manner in which the AGC attacks a sudden decrease in

the sign

Desired Clipping Level Mode

oted previously, each AGC can be configured so that the

locks onto a desired clipping level or a desired signal leve

t desired clippin

individual AGC control words (0x0A, 0x12). For signals that

to exceed the boun

tru cating those signals and still provide an AGC that attacks

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

his mode of operation is shown with broken arrows in

re 51, and the operation is similar to the desired signal leve

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