AD9874ABST datasheet(33/40 Pages) AD

33 / 40 page

REV. A

AD9874

–33–

Figure 23b shows that omitting the LO frequencies given by

Equation 12 for m = 1, 3, and 5 and by Equation 13 accounts

for most of the spurs. Some of the remaining low level spurs can

be attributed to coupling from the SSI digital output. As a

result, users are also advised to optimize the output bit rate

(fCLKOUT via the SSIORD register) and the digital output driver

strength to achieve the lowest spurious and noise figure perfor-

mance for a particular LO frequency and fCLK setting. This is

especially the case for particularly narrow-band channels in

which low level spurs can degrade the AD9874’s sensitivity

performance.

Despite the many spurs, sweet spots in the LO frequency are

generally wide enough to accommodate the maximum signal

bandwidth of the AD9874. As evidence of this property, Fig-

ure 24 shows that the in-band noise is quite constant for LO

frequencies ranging from 70 MHz to 71 MHz.

70.5

70.0

–50

–70

–60

–90

–80

71.0

LO FREQUENCY – MHz

Figure 24. Expanded View from 70 MHz to 71 MHz

Spurious Responses

The spectral purity of the LO (including its phase noise) is an

important consideration since LO spurs can mix with undesired

signals present at the AD9874’s IFIN input to produce an in-band

response. To demonstrate the low LO spur level introduced within

the AD9874, Figure 25 plots the demodulated output power as a

function of the input IF frequency for an LO frequency of

71.1 MHz and a clock frequency of 18 MHz.

–40

–20

–80

–60

100

IF FREQUENCY – MHz

–120

–100

D =

CLK/4 = 4.5MHz

DESIRED

RESPONSES

Figure 25. Response of AD9874 to a –20 dBm IF

Input when fLO = 71.1 MHz

The two large –10 dBFS spikes near the center of the plot are

the desired responses at fLO,

fIF2_ADC, where fIF2_ADC = fCLK/8,

i.e., at 68.85 MHz and 73.35 MHz. LO spurs at fLO

fSPUR

would result in spurious responses at offsets of

fSPUR around the

desired responses. Close-in spurs of this kind are not visible on

the plot, but small spurious responses at fLO

fIF2_ADC

fCLK, i.e.,

at 50.85 MHz, 55.35 MHz, 86.85 MHz, and 91.35 MHz, are

visible at the –90 dBFS level. This data indicates that the AD9874

does an excellent job of preserving the purity of the LO signal.

Figure 25 can also be used to gauge how well the AD9874

rejects undesired signals. For example, the half-IF response (at

69.975 MHz and 72.225 MHz) is approximately –100 dBFS,

giving a selectivity of 90 dB for this spurious response. The

largest spurious response at approximately –70 dBFS occurs

with input frequencies of 70.35 MHz and 71.85 MHz. These

spurs result from third order nonlinearity in the signal path

(i.e., abs [3

fLO – 3

fIF_Input] = fCLK/8).

–60

–70

–80

–90

–50

250

300

LO FREQUENCY – MHz

200

150

100

Figure 23b. Same as Figure 23a Excluding LO Frequencies Known to Produce Large In-Band Spurs

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