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AD6655

Rev. 0 | Page 37 of 84

DIGITAL DOWNCONVERTER

The AD6655 includes a digital processing section that provides

filtering and reduces the output data rate. This digital processing

section includes a numerically controlled oscillator (NCO),

a half-band decimating filter, an FIR filter, and a second coarse

NCO (fADC/8 fixed value) for output frequency translation. Each

of these processing blocks (except the decimating half-band

filter) has control lines that allow it to be independently enabled

and disabled to provide the desired processing function. The

digital downconverter can be configured to output either real data

or complex output data. These blocks can be configured in five

recommended combinations to implement different signal

processing functions.

DOWNCONVERTER MODES

Table 18 details the recommended downconverter modes of

operation in the AD6655.

Table 18. Downconverter Modes

Mode

NCO/Filter

Output Type

Half-band filter only

Real

Half-band filter and FIR filter

Real

NCO and half-band filter

Complex

NCO, half-band filter, and FIR filter

Complex

NCO, half-band filter, FIR filter, and

fADC/8 NCO

Real

NUMERICALLY CONTROLLED OSCILLATOR (NCO)

Frequency translation is accomplished with an NCO. Each of

the two processing channels shares a common NCO. Amplitude

and phase dither can be enabled on chip to improve the noise and

spurious performance of the NCO. A phase offset word is available

to create a known phase relationship between multiple AD6655s.

Because the decimation filter prevents usage of half 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, fine 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.

HALF-BAND DECIMATING FILTER AND FIR FILTER

The goal of the AD6655 digital filter block is to allow the sample

rate to be reduced by a factor of 2 while rejecting aliases that fall

into the band of interest. The half-band filter is designed to operate

as either a low-pass or high-pass filter and to provide greater

than 100 dB of alias protection for 22% of the input rate of the

structure. For an ADC sample rate of 150 MSPS, this provides

a maximum usable bandwidth of 16.5 MHz when using the filter

in real mode (NCO bypassed) or a maximum usable bandwidth

of 33.0 MHz when using the filter in the complex mode (NCO

enabled).

The optional fixed-coefficient FIR filter provides additional

filtering capability to sharpen the half-band roll-off to enhance

the alias protection. It removes the negative frequency images

to avoid aliasing negative frequencies for real outputs.

fADC/8 FIXED-FREQUENCY NCO

A fixed fADC/8 NCO is provided to translate the filtered, decimated

signal from dc to fADC/8 to allow a real output. Figure 68 to

Figure 71 show an example of a 20 MHz input as it is processed

by the blocks of the AD6655.

–50

–24

–14

–4

Figure 68. Example AD6655 Real 20 MHz Bandwidth Input Signal Centered at

14 MHz (fADC = 100 MHz)

–50

–38

–28

–18 –10

Figure 69. Example AD6655 20 MHz Bandwidth Input Signal Tuned to DC

Using the NCO (NCO Frequency = 14 MHz)

–50

–38

–28

–18 –10

Figure 70. Example AD6655 20 MHz Bandwidth Input Signal wth the

Negative Image Filtered by the Half-Band and FIR Filters

–50

0.25

22.5

12.5

Figure 71. Example AD6655 20 MHz Bandwidth Input Signal Tuned to

fADC/8 for Real Output

AD6655 Datasheet(PDF) 37 Page - Analog Devices

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