AD6634BCSLASHPCB datasheet(30/52 Pages) AD

30 / 52 page

REV. 0

–30–

AD6634

operation. The rms samples so obtained are subtracted 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 output signal level desired. The request signal level, R,

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

The request signal level should also compensate for error, if any,

due to the CIC scaling as explained previously. Therefore, the

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

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, Offset = 72.24 – 69.54 = 2.7 dB. The

request signal level is given by,

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. 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, completely 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 below.

() =

() +

–

––

–

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 regis-

ters (0x0E and 0x0F, respectively, for AGC channel A and B) 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 character-

istics. These closed loop poles P1, 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 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

where

are the time constants corresponding to the poles

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

as given below,

Setting 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, 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, if half-band interpolating filters are bypassed.

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. Thus 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 the registers 0x0D and 0x15 for

AGC A and AGC B, respectively.

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

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 gain for next set of samples. These 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 are truncated to required bit widths using

the clipping circuitry as shown in the block diagram.

Open Loop Gain Setting

If filter gain K occupies only one LSB, or 0.0039, during the

multiplication with error term, errors of up to 6.02 dB could be

truncated. This truncation is due to the lower bit widths avail-

able 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 representation). 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 that they 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

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 lesser truncation of errors.

Average Samples Setting

Though it is complicated to express the exact effect of the num-

ber of averaging samples, thinking intuitively it has a smoothing

effect on the way the AGC loop attacks a sudden increase or a

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