AD9864 datasheet(42/48 Pages) AD

42 / 48 page

Data Sheet

AD9864

Rev. A | Page 41 of 47

INPUT

PRESELECT

FILTER

TUNER

IF CRYSTAL OR

SAW FILTER

VDDA

Σ-∆ ADC

LNA

VCO

ADF42xx

PLL SYN

REFIN

DSP

AD9864

DECIMATION

FILTER

SAMPLE CLOCK

SYNTHESIZER

LOOP

FILTER

VCO

LOOP

FILTER

VDDC

FROM DSP

SYNTH.

VOLTAGE

REFERENCE

SPI

CRYSTAL

OSCILLATOR

IFIN

–16dB

LNA

DAC AGC

FORMATTING/SSI

DOUTA

DOUTB

CLKOUT

CONTROL LOGIC

IF2 =

fCLK/8

Figure 73. Typical Dual Conversion Superheterodyne Application Using the AD9864

Figure 73 shows a typical dual conversion superheterodyne

receiver using the AD9864. An RF tuner is used to select and

downconvert the target signal to a suitable first IF for the

AD9864. A preselect filter may precede the tuner to limit the RF

input to the band of interest. The output of the tuner drives an

IF filter that provides partial suppression of adjacent channels

and interferers that could otherwise limit the dynamic range of

the receiver. Set the conversion gain of the tuner such that the

peak IF input signal level into the AD9864 is no greater than

−18 dBm to prevent clipping. The AD9864 downconverts the

first IF signal to a second IF that is exactly 1/8 of the clock rate

of the Σ-Δ ADC (fCLK/8) to simplify the digital quadrature

demodulation process.

This second IF signal is then digitized by the Σ-Δ ADC,

demodulated into its quadrature I and Q components, filtered

via matching decimation filters, and reformatted to enable a

synchronous serial interface to a DSP. In this example, the LO

and CLK synthesizers of the AD9864 are both enabled,

requiring some additional passive components (for the loop

filters and CLK oscillator of the synthesizer) and a VCO for the

LO synthesizer. Note that not all of the required decoupling

capacitors are shown. See the External Passive Component

Requirements section and Figure 72 for more information on

required external passive components.

The selection of the first IF frequency is often based on the

availability of low cost standard crystal or SAW filters as well as

system frequency planning considerations. In general, crystal

filters are often used for narrow-band radios having channel

bandwidths below 50 kHz with IFs below 120 MHz, while SAW

filters are more suited for channel bandwidths greater than

50 kHz with IFs greater than 70 MHz. The ultimate stop-band

rejection required by the IF filter depends on how much

suppression is required at the AD9864 image band resulting

from downconversion to the second IF. This image band is

offset from the first IF by twice the second IF frequency

(± fCLK/4, depending on high-side or low-side injection).

The selectivity and bandwidth of the IF filter depends on both

the magnitude and frequency offset(s) of the adjacent channel

blocker(s) that could overdrive the input of the AD9864 or

generate in-band intermodulation components. Further

suppression is performed within the AD9864 by its inherent

band-pass response and digital decimation filters. Note that

some applications require additional application-specific

filtering performed in the DSP that follows the AD9864 to

remove the adjacent channel and/or implement a matched filter

for optimum signal detection.

Choose the output data rate of the AD9864, fOUT, to be at least

twice the bandwidth or symbol rate of the desired signal to ensure

that the decimation filters provide a flat pass-band response as

well as to allow for postprocessing by a DSP. After fOUT is

determined, the decimation factor of the digital filters must be

set such that the input clock rate, fCLK, falls between the AD9864

AD9864 Datasheet(PDF) 42 Page - Analog Devices

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