AD6634BCSLASHPCB datasheet(28/52 Pages) AD

28 / 52 page

REV. 0

–28–

AD6634

For RCF Scale of 0, Scaling Factor is equal to –18.06 dB, and for

maximum RCF Scale of 15, Scaling Factor is equal to 72.25 dB.

If Bit 7 is set, the same exponent will be used for both the real

and imaginary (I and Q) outputs. The exponent used will be the

one that prevents numeric overflow at the expense of small

signal accuracy. However, this is seldom a problem as small

numbers would represent 0 regardless of the exponent used.

Bit 8 is the RCF bank select bit used to program the register. When

this bit is 0, the lowest block of 128 is selected (taps 0 through

127). When high, the highest block is selected (taps 128 through

255). It should be noted that while the chip is computing filters,

tap 127 is adjacent to 128 and there are no paging issues.

Bit 9 selects where the input to each RCF originates. If Bit 9 is clear,

the RCF input comes from the CIC5 normally associated with

the RCF. If, however, the bit is set, the input comes from CIC5

channel 1. The only exception is channel 1, which uses the output

of CIC5 channel 0 as its alternate. Using this feature, each RCF

can operate on its own channel data or be paired with the RCF

of channel 1. The RCF of channel 1 can also be paired with chan-

nel 0. This control bit is used with polyphase distributed filtering.

If Bit 10 is clear, the AD6634 channel operates in normal mode.

However, if Bit 10 is set, the RCF is bypassed to Channel BIST.

See the BIST (Built-In Self-Test) section for more details.

INTERPOLATING HALF-BAND FILTERS

The AD6634 has two interpolating half-band finite impulse

response filters that immediately precede the two digital AGCs

and follow the four RCF channel outputs. Each interpolating

half-band takes 16-bit I and 16-bit Q data from the preceding

RCF and outputs 16-bit I and 16-bit Q to the AGC. The half-band

and AGC operate independently of each other, so the AGC can

be bypassed, in which case the output of the half-band is sent

directly to the output data port. The half-bands also operate

independently of each other—either one can be enabled or

disabled. The control register for half-band A is at address 0x08

and for half-band B, address 0x09.

Half-band filters also perform the function of interleaving data

from various RCF channel outputs prior to the actual function

of interpolation. This interleaving of data is allowed even when

the actual function of Half-band filter is bypassed. This feature

allows for the usage of multiple channels (implementing a

polyphase filter) on the AD6634 to process a single carrier. Either

RCF phase decimation or start hold-off counter for the channels

is used to appropriately phase the channels. For example, if two

channels of AD6634 are used to process one CDMA2000 carrier,

RCF filters for both the channels should be 180

° out of phase.

This can be done using RCF phase decimation or an appropriate

start hold-off counter followed by appropriate NCO phase offsets.

Half-band A can listen to all four channels: 0, 1, 2, and 3; channel

0 and 1; or only channel 0. Half-band B can listen to channels 2

and 3, or only channel 2. Each half-band interleaves the channels

specified in its control register and interpolates by 2 on the

combined data from those channels. For one channel running at

twice the chip rate, the half-band can be used to output channel

data at 43 the chip rate.

With respect to the chip rate, the frequency response of the

interpolating half-band FIR is shown in Figure 32.

MULTIPLES OF CHIP RATE

1.0

2.0

3.0

3.5

4.0

–20

–40

–60

–70

–80

0.5

1.5

2.5

–10

–30

–50

SPECTRUM OF HALF-BAND

fSAMP

fCHIP

fSAMP

fCHIP

Figure 32. Interpolating Half-Band Frequency Response

The SNR of the interpolating half-band is around –149.6 dB.

The highest error spurs due to fixed-point arithmetic are around

–172.9 dB. The coefficients of the 13-tap interpolating half-band

FIR are given in the Table VII.

Table VII. Half-Band Coefficients

–66

309

512

309

–66

AUTOMATIC GAIN CONTROL

The AD6634 is equipped with two independent automatic gain

control (AGC) loops for direct interface with a RAKE receiver.

Each AGC circuit has 96 dB of range. It is important that the

decimating filters of the AD6634 preceding the AGC reject

undesired signals, so that each AGC loop is only operating on

the carrier of interest and carriers at other frequencies do not

affect the ranging of the loop.

The AGC compresses the 23-bit complex output from the inter-

polating half-band filter into a programmable word size of 4–8,

10, 12, or 16 bits. Since the small signals from the lower bits are

pushed into higher bits by adding gain, the clipping of the lower

bits does not compromise the SNR of the signal of interest. The

AGC maintains a constant mean power on the output despite the

level of the signal of interest, allowing operation in environments

where the dynamic range of the signal exceeds the dynamic range

of the output resolution.

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