AD9864-EBZ datasheet(40/48 Pages) AD

40 / 48 page

Data Sheet

AD9864

Rev. A | Page 39 of 47

–60

–70

–80

–90

–50

250

300

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150

100

LO FREQUENCY (MHz)

Figure 69. Total In-Band Noise + Spur Power with No Signal Applied as a Function of the LO Frequency (fCLK = 18 MHz and Output Signal Bandwidth = 150 kHz)

–60

–70

–80

–90

–50

250

300

LO FREQUENCY (MHz)

200

150

100

Figure 70. Same as Figure 69 Excluding LO Frequencies Known to Produce Large In-Band Spurs

Spurious Responses

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

important consideration because LO spurs can mix with undesired

signals present at the AD9864 IFIN input to produce an in-band

response. To demonstrate the low LO spur level introduced

within the AD9864, Figure 71 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.

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, that is,

at 68.85 MHz and 73.35 MHz. LO spurs at fLO ± fSPUR result in

spurious responses at offsets of ± fSPUR around the desired

responses. Close-in spurs of this kind are not visible on the plot;

however, small spurious responses at fLO ± fIF2_ADC ± fCLK (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 AD9864 does

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

Figure 71 can also be used to gauge how well the AD9864

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

(that is, abs [3 × fLO − 3 × fIF_INPUT] = fCLK/8).

–40

–20

–80

–60

100

IF FREQUENCY (MHz)

–120

–100

D =

fCLK/4 = 4.5MHz

DESIRED

RESPONSES

Figure 71. Response of AD9864 to a −20 dBm IF Input when fLO = 71.1 MHz

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