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AD6652

Rev. 0 | Page 40 of 76

POLATING HALF-BAND FILTERS

nd follow the four

it Q to the AGC. The half-band and AGC

opera e independently of each other, so the AGC can be

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

to the output data port. The half-band filters also

er one can be enabled

at Address

f the

ethods:

hannels of the AD6652 are used to process

hannel 2. Each half-band interleaves

INTER

The AD6652 has two interpolating half-band FIR filters th

immediately precede the two digital AGCs a

RCF channel outputs. Each interpolating half-band takes

16-bit I and 16-bit Q data from the preceding RCF and output

16-bit I and 16-b

bypassed

directly

operate independently of each other—eith

or disabled. The control register for Half-Band A is

0x08 and for Half-Band B is at Address 0x09.

Half-band filters also perform the function of interleaving data

from various RCF channel outputs prior to the actual function

of interpolation. Interleaving of data is allowed even when the

half-band filter is bypassed. This allows the implementation of

polyphase filter by combining the processing power of multiple

channels to act upon a single carrier. This is accomplished by

appropriate phasing of the processing channels using one o

following m

• RCF phase decimation

• Start hold-off counter

For example, if two c

one CDMA2000 carrier, RCF filters for both the channels

should be 180° out of phase. This can be done using RCF phas

decimation or an appropriate start hold-off counter followed

appropriate NCO phase offsets.

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

3; Channel 0 and 1; or only Channel 0. Half-band B can listen t

Channels 2 and 3, or only C

the channels specified in its control register and interpolates by

two 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 four times the chip rate. The frequency

response of the interpolating half-band FIR is shown in

Figure 50.

MULTIPLES OF CHIP RATE

1.0

2.0

3.0

3.5

4.0

–70

–80

0.5

1.5

2.5

fSAMP

SPECTRUM OF HALF-BAND

–20

–40

–10

–30

–50

–60

fSAMP

fCHIP

Figure 50. 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 Table 18.

Table 18. Half-Band Coefficients

−66

309

512

309

−66

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