AD6649EBZ datasheet(22/40 Pages) AD

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AD6649

Rev. 0 | Page 22 of 40

DIGITAL PROCESSING

The AD6649 includes a digital processing section that provides

filtering. This digital processing section includes a numerically

controlled oscillator (NCO), a selectable FIR filter (high perfor-

mance or low latency), and a second coarse NCO (fS/4 fixed

value) for output frequency translation (complex to real). These

blocks can be configured in several modes to implement a

signal processing function. Refer to Figure 1 for the functional

block diagram of the AD6649.

NUMERICALLY CONTROLLED OSCILLATOR (NCO)

Frequency translation is accomplished with an NCO shared

between the two channels. Amplitude and phase dither can be

enabled on chip to improve the noise and spurious performance of

the NCO.

Because the filtering prevents usage of part of the Nyquist

spectrum, a means is needed to translate the sampled input

spectrum into the usable range of the decimation filter. To

achieve this, a 32-bit, tuning, complex NCO is provided. This

NCO/mixer allows the input spectrum to be tuned to dc, where

it can be effectively filtered by the subsequent filter blocks to

prevent aliasing.

When using the low latency FIR, the NCO must be tuned to fS/4

(0x40000000). This prevents unwanted aliases from falling back

into the band of interest.

NCO AND FIR FILTER MODES

The NCO and FIR blocks can be used in two modes depending on

the bandwidth and latency requirement of the application. The two

modes of operation of these blocks are summarized in Table 11.

Table 11. Signal Path Modes

Mode

FIR

Output

Bandwidth at

245.76 MSPS

Fixed-Frequency NCO,

95 MHz FIR Filter

Low latency

(default)

95 MHz

Tunable-Frequency NCO,

100 MHz FIR Filter

High performance

99.5 MHz

Two fixed-coefficient FIR filters provide filtering capability. A

low latency FIR or a high performance FIR can be selected. It

removes the negative frequency images to avoid aliasing negative

frequencies for real outputs. Figure 38, Figure 39, and Figure 40

show the progression of a 95 MHz bandwidth signal through the

filter stages when using the fixed-frequency NCO and 95 MHz FIR

filter with a sample rate of 245.76 MSPS. The tunable-frequency

NCO can be used instead and operates in a similar fashion. In

these modes, the output is centered at 61.44 MHz, assuming a

245.76 MSPS sample rate.

fS/4 FIXED-FREQUENCY NCO

A fixed-frequency fS/4 NCO is provided to translate the filtered,

decimated signal from dc to fS/4 to allow a real output. The fS/4

NCO is required in all operation modes because complex

output from the part is not supported.

REAL ADC INPUT

122.88

61.44

108.94

13.94

–61.44

–108.94

–13.94

Figure 38. Example AD6649 Real 95 MHz Bandwidth Input Signal Centered at

61.44 MHz (fADC = 245.76 MHz)

COMPLEX ADC OUTPUT/NCO OUTPUT

–122.88

122.88

–75.38

75.38

–47.5

47.5

Figure 39. Example AD6649 95 MHz Bandwidth Input Signal Tuned to DC

Using the NCO (NCO Frequency = 61.44 MHz)

TUNED NCO OUTPUT

122.88

13.94

108.94

61.44

Figure 40. Example AD6649 95 MHz Bandwidth Output Signal Tuned to fS/4

(NCO Frequency = 61.44 MHz)

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