AD6636PCB datasheet(32/72 Pages) AD

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AD6636

Rev. 0 | Page 32 of 72

The filter input sample rate is the same as the FIR2 filter output

rate and is given by one of the following equations:

fHB2 = fFIR2 = fHB1, if HB1 is bypassed

fHB2 = fFIR2 =

HB1

, if HB1 is not bypassed

where:

fFIR1 is the input rate of the FIR1 filter.

fHB1 is the input rate of the HB1 filter.

The input to the filter has a maximum of 75 MHz. The

maximum output rate when not bypassed is 37.5 MHz.

The filter has a ripple of 0.00075 dB and rejection of 81 dB. For

an alias rejection of 81 dB, the alias-protected bandwidth is 33%

of the filter input sample rate. The bandwidth of the filter for a

ripple of 0.00075 dB is the same as alias-protected bandwidth,

due to the nature of half-band filters. The 3 dB bandwidth of

this filter is 47% of the filter input sample rate. For example, if

the sample rate into the filter is 25 MHz, then the alias-

protected bandwidth of the HB2 filter is 8.25 MHz (33% of

25 MHz). If the bandwidth of the required carrier is greater

than 8.25 MHz, then HB2 might not be useful.

FRACTION OF HB2 INPUT SAMPLE RATE

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0.01

–19.99

–29.99

–9.99

–39.99

–49.99

–60.00

–70.00

–80.00

–90.00

–100.00

–110.00

–120.00

–90

0.66

0.34

FIR2 + HB2

RESPONSE

Figure 36. Composite Response of FIR1 and HB1 filters to Their Input Rates

INTERMEDIATE DATA ROUTER

Following the FIR-HB cascade filters is the intermediate data

router. This data router consists of muxes that allow the I and Q

data from any channel front end (input port + NCO + CIC +

FIR-HB) to be processed by any channel back end (MRCF +

DRCF + CRCF). The choice of channel front end is made by

programming a 3-bit MRCF data select word in the MRCF

control register. The valid values for this word and their

corresponding settings are listed in Table 17.

Table 17. Data Router Select Settings

MRCF Data Select [2:0]

Data Source

000

Channel 0

001

Channel 1

010

Channel 2

011

Channel 3

1x0

Channel 4

1x1

Channel 5

Allowing different channel back ends to select different channel

front ends is useful in the polyphase implementation of filters.

When multiple AD6636 channels are used to process a single

carrier, a single-channel front end feeds more than one channel

back end. After processing through the channel back ends (RCF

filters), the data is interleaved back from all the polyphased

channels.

MONO-RATE RAM COEFFICIENT FILTER (MRCF)

The MRCF is a programmable sum-of-products FIR filter. This

filter block comes after the first data router and before the

DRCF and CRCF programmable filters. It consists of a

maximum of eight taps with 6-bit programmable coefficients.

Note that this block does not decimate and is used as a helper

filter for the DRCF and CRCF filters that follow in the signal

chain.

The number of filter taps that are to be calculated is program-

mable using the 3-bit number-of-taps word in the MRCF

control register of the channel under consideration. The 3-bit

word programmed is one less than the number of filter taps.

The coefficients themselves are programmed in eight MRCF

coefficient memory registers for individual channels. The input

and output data to the block are both 20-bit.

Symmetry

Though the MRCF filter does not require symmetrical filters, if

the filter is symmetrical, then the symmetry bit in the MRCF

control register should be set. When this bit is set, only half of

the impulse response needs to be programmed into the MRCF

coefficient memory registers. For example, if the number of

filter taps is equal to five or six and the filter is symmetrical,

then only three coefficients need to be written into the

coefficient memory. For both symmetrical and asymmetrical

filters, the number of filter taps is limited to eight.

Clock Rate

The MRCF filter runs on an internal high speed PLL clock. This

clock rate can be as high as 200 MHz. If the half clock rate bit in

the MRCF control register is set, then only half the PLL clock

rate is used (maximum of 100 MHz). This results in power

savings, but can only be used if certain conditions are met.

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