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AD9864

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 (%)

010

–10

–5

BW = 75kHz

BW = 10kHz

BW = 30kHz

Figure 50. 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 51 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 Register 0x07. Thus, the composite decimation factor

can be set 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 fre-

quency (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 sig-

nal 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

Σ-∆

MODULATOR

DEC1

SINC4

FILTER

DEC2

SINC4

FILTER

M + 1

DEC3

FIR

FILTER

COMPLEX

DATA TO

SSI PORT

Figure 51. Decimation Filter Architecture

Figure 52 shows the response of the decimation filter at a deci-

mation factor of 900 (K = 0, M = 14) and a sampling clock fre-

quency 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 that 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

120

–88dB

–101dB

–103dB

Figure 52. Decimation Filter Frequency Response for

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

Figure 53 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 at least 94 dB and occurs for frequen-

cies at the edges of the fourth alias band. The difference

between the alias attenuation characteristics of Figure 52 and

those of Figure 53 is due to the fact that the third decimation

stage decimates by a factor of 5 for Figure 52 compared with a

factor of 4 for Figure 53.

–40

–100

–20

–60

–80

–120

FREQUENCY (MHz)

EQUENCY

1.5

0.5

1.0

2.0

2.5

±135.466kHz PASS BAND

–98dB

–115dB

–94dB

Figure 53. Decimation Filter Frequency Response for

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

Figure 54 and Figure 55 show expanded views of the pass band

for the two possible configurations of the third decimation fil-

ter. When decimating by 60n (K = 0), the pass-band gain varia-

tion is 1.2 dB; when decimating by 48n (K = 1), the pass-band

gain variation is 0.9 dB. Normalization of full scale at band

center is accurate to within 0.14 dB across all decimation

modes. Figure 56 and Figure 57 show the folded frequency

response of the decimator for K = 0 and K = 1, respectively.

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