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AD6635

INTERPOLATING HALF-BAND FILTERS

The AD6635 has four interpolating half-band FIR filters that

immediately precede the four digital AGCs and immediately

follow the RCF channel outputs. Each interpolating half-band

takes I and Q data from the preceding RCF and outputs I and

Q data to the AGC. The half-band filters and AGC operate

independently of each other, so the AGC can be bypassed, in

which case the output of the half-band filter is sent directly to

the output data port. The half-band filters also operate indepen-

dently of each other––any one can be enabled or disabled using

the Half-Band Control registers.

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 the half-band filter is bypassed. This

feature allows for the usage of multiple channels (implementing

a polyphase filter) on the AD6635 to process a single carrier.

Either RCF phase decimation or a start holdoff counter for the

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

if two channels of AD6635 are used to process one cdma2000

carrier, RCF filters for both channels should be 180

∞ out of

phase. This can be done using RCF phase decimation or an

appropriate start holdoff counter followed by appropriate NCO

phase offsets.

Half-band filter A can listen to either Channels 0 to 3, Channels

0 and 1, or only Channel 0. Half-band filter B can listen to

Channels 2 and 3 or to only Channel 2. Each half-band filter

interleaves the channels specified in its control register. The

interleaved data so combined is interpolated by 2. The inter-

leaving function can be used independently of the interpolating

function, in which case the half-band filter is bypassed using the

Half-Band Control registers. When the half-band filter is bypassed,

the interleaving function is still performed. For one channel

running at twice the chip rate, the half-band can be used to

output channel data at 4

¥ the chip rate.

In Figure 31, the frequency response of the interpolating half-band

FIR filter is shown in the graph with respect to the chip rate.

FREQUENCY IN 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

fSAMP

fCHIP

Figure 31. Interpolating Half-Band Filter

Frequency Response

The SNR of the interpolating half-band filter is approximately

149.6 dB. The highest error spurs due to fixed-point arithmetic

are around –172.9 dB. The coefficients of the 13-tap interpolat-

ing half-band FIR filter are given in the Table VII.

Table VII. Half-Band Coefficients

–66

309

512

309

–66

AUTOMATIC GAIN CONTROL

The AD6635 is equipped with four 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 AD6635 preceding the AGC reject

undesired signals so that each AGC loop is operating on only

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 environ-

ments where the dynamic range of the signal exceeds the

dynamic range of the output resolution.

The AGC and the interpolation filters are not tied together, and

any one or both of them can be selected without the other. The

AGC section can be bypassed if desired by setting Bit 0 of the

AGC control word. When bypassed, the I/Q data is passed to

the output port after clipping to 16-bit I/Q data.

There are three sources of error introduced by the AGC func-

tion: underflow, overflow, and modulation. Underflow is caused

by truncation of bits below the output range. Overflow is caused

by clipping errors when the output signal exceeds the output

range. Modulation error occurs when the output gain varies

during the reception of a data.

The desired signal level should be set based on the probability

density function of the signal so that the errors due to underflow

and overflow are balanced. The gain and damping values of the

loop filter should be set so that the AGC is fast enough to track

long term amplitude variations of the signal that might cause exces-

sive underflow or overflow, but slow enough to avoid excessive loss

of amplitude information due to the modulation of the signal.

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