AD9874EB datasheet(25/40 Pages) AD

25 / 40 page

REV. 0

AD9874

–25–

Once the AD9874 has been tuned, the noise figure degradation

attributed solely to the temperature drift of the LC and RC resona-

tors is minimal. Since the drift of the RC resonator is actually

negligible compared to that of the LC resonator, the external

L and C components’ temperature drift characteristics tend to

dominate. Figure 13d shows the degradation in noise figure as the

product of the LC value is allowed to vary from –12.5% to +12.5%.

Note, the noise figure remains relatively constant over a

±3.5%

range (i.e.,

±35,000 ppm) suggesting that most applications will

not be required to retune over the operating temperature range.

–15

LC ERROR – %

–5

–10

BW = 75kHz

BW = 10kHz

BW = 30kHz

Figure 13d. Typical Noise Figure Degradation from L and

C Component Drift (fCLK = 18 MSPS, fIF = 73.3501 MHz)

DECIMATION FILTER

The decimation filter shown in Figure 14 consists of an fCLK/8

complex mixer and a cascade of three linear phase FIR filters:

DEC1, DEC2, and DEC3. DEC1 downsamples by a factor of 12

using a fourth order comb filter. DEC2 also uses a fourth order

comb filter, but its decimation factor is set by the M field of

decimate-by-4 FIR filter depending on the value of the K Bit within

to either 60

× M or 48 × M for K equal to 0 or 1, respectively.

The output data rate (fOUT) is equal to the modulator clock

frequency (fCLK) divided by the digital filter’s decimation factor.

Due to the transition region associated with the decimation

filter’s frequency response, the decimation factor must be

selected such that fOUT is equal to or greater than twice the signal

bandwidth. This ensures low amplitude ripple in the pass band

along with the ability to provide further application-specific

digital filtering prior to demodulation.

COS

SIN

DATA

FROM

MODULATOR

DEC1

SINC4

FILTER

DEC2

SINC4

FILTER

M + 1

DEC3

FIR

FILTER

COMPLEX

DATA TO

SSI PORT

Figure 14. Decimation Filter Architecture

Figure 15a shows the response of the decimation filter at a decimation

factor of 900 (K = 0, M = 14) and a sampling clock frequency of

18 MHz. In this example, the output data rate (fOUT) is 20 kSPS,

with a usable complex signal bandwidth of 10 kHz centered around

dc. As this figure shows, the first and second alias bands (occurring

at even integer multiples of fOUT/2) have the least attenuation but

provide at least 88 dB of attenuation. Note, signals falling around

frequency offsets that are odd integer multiples of fOUT/2 (i.e.,

10 kHz, 30 kHz, and 50 kHz) will fall back into the transition

band of the digital filter.

FREQUENCY – kHz

–40

–100

030

–20

–60

–80

100

FOLD-

ING

POINT

5.0kHz PASS BAND

–120

–88dB

–101dB

–103dB

Figure 15a. Decimation Filter Frequency Response

for fOUT = 20 kSPS (fCLK = 18 MHz, OSR = 900)

Figure 15b shows the response of the decimation filter with a

decimation factor of 48 and a sampling clock rate of 26 MHz. The

alias attenuation is now at least 94 dB, and this attenuation occurs

for frequencies at the edges of the fourth alias band. The differ-

ence between the alias attenuation characteristics of Figure 15b

and those of Figure 15a is due to the fact that the third decimation

stage decimates by a factor of 5 for Figure 15a versus a factor of 4

for Figure 15b.

–40

–100

–20

–60

–80

–120

FREQUENCY – MHz

1.5

0.5

1.0

2.0

2.5

135.466kHz PASS BAND

–98dB

–115dB

–94dB

Figure 15b. Decimation Filter Frequency Response

for fOUT = 541.666 kSPS (fCLK = 26 MHz, OSR = 48)

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