AD9874EB datasheet(32/40 Pages) AD

32 / 40 page

REV. 0

–32–

AD9874

Many applications have frequency plans that take advantage of

industry-standard IF frequencies due to the large selection of

low cost crystal or SAW filters. If the selected IF frequency and

ADC clock rate result in a problematic spurious component, an

alternative ADC clock rate should be selected by slightly modi-

fying the decimation factor and CLK synthesizer settings (if

used) such that the output sample rate remains the same. Also,

applications requiring a certain degree of tuning range should

take into consideration the location and magnitude of these spurs

when determining the tuning range as well as optimum IF and

ADC clock frequency.

Figure 23a plots the measured in-band noise power as a function of

the LO frequency for fCLK = 18 MHz and an output signal band-

width of 150 kHz when no signal is present. Any LO frequency

resulting in large spurs should be avoided. As this figure shows,

large spurs result when the LO is fCLK/8 = 2.25 MHz away from a

harmonic of 18 MHz (i.e., n fCLK

± f

CLK/8). Also problematic are

LO frequencies whose odd order harmonics (i.e., m fLO) mix with

harmonics of fCLK to fCLK/8. This spur mechanism is a result of the

mixer being internally driven by a squared-up version of the LO input

consisting of the LO frequency and its odd order harmonics. These

spur frequencies can be calculated from the following relation:

CLK

=±

()

(12)

where m = 1, 3, 5... and n = 1, 2, 3...

A second source of spurs is a large block of digital circuitry that

is clocked at fCLK/3. Problematic LO frequencies associated with

this spur source are given by:

n f

CLK

(13)

where n = 1, 2, 3 ...

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 performance for

a particular LO frequency and fCLK setting. This is especially the

case for very 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, Figure 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

Input 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

± f

IF2_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

± f

SPUR 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] = f CLK/8).

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