AD9864-EB datasheet(35/44 Pages) AD

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AD9864

APPLICATIONS CONSIDERATIONS

data is used, the effective system NF will increase because of the

quantization noise present in the 16-bit data after truncation.

Frequency Planning

The LO frequency (and/or ADC clock frequency) must be chosen

carefully to prevent known internally generated spurs from mixing

down along with the desired signal, thus degrading the SNR per-

formance. The major sources of spurs in the AD9864 are the ADC

clock and digital circuitry operating at 1/3 of fCLK. Thus, the clock

frequency (fCLK) is the most important variable in determining

which LO (and therefore IF) frequencies are viable.

VGA ATTENUATION (dB)

SNR = 90.1dBFS

BW = 50kHz

BW = 150kHz

SNR = 82.9dBFS

BW = 10kHz

SNR = 95.1dBFS

SNR = 103.2dB

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 64. Nominal System Noise Figure and Peak SNR vs. AGCG Setting (fIF =

73.35 MHz, fCLK = 18 MSPS, and 24-bit I/Q Data)

Figure 65 plots the nominal system NF with 16-bit output data

as a function of AGC in both narrow-band and wideband

mode. In wideband mode, the NF curve is virtually unchanged

relative to the 24-bit output data because the output SNR before

truncation is always less than the 96 dB SNR that 16-bit data

can support. However, in narrow-band mode, where the output

SNR approaches or exceeds the SNR that can be supported with

16-bit data, the degradation in system NF is more severe. Fur-

thermore, if the signal processing within the DSP adds noise at

the level of an LSB, the system noise figure can be degraded

even more than Figure 65 shows. For example, this could occur

in a fixed 16-bit DSP whose code is not optimized to process

the AD9864’s 16-bit data with minimal quantization effects. To

limit the quantization effects within the AD9864, the 24-bit

data undergoes noise shaping just prior to 16-bit truncation,

thus reducing the in-band quantization noise by 5 dB (with 2×

oversampling). This explains why 98.8 dBFS SNR performance

is still achievable with 16-bit data in a 10 kHz BW.

Figure 66 plots the measured in-band noise power as a function

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

bandwidth 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 ± fCLK/8.

Also problematic are LO frequencies whose odd order harmon-

ics, 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 relation

(

)

CLK

(12)

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

SNR = 98.8dBFS

BW = 50kHz

BW = 150kHz

SNR = 83dBFS

SNR = 94.1dBFS

BW = 10kHz

SNR = 89.9dBFS

VGA ATTENUATION (dB)

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

CLK

(13)

where n = 1, 2, 3...

Figure 67 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 per-

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

especially the case for particularly narrow-band channels in

Figure 65. Nominal System Noise Figure and Peak SNR vs. AGCG

Setting (fIF = 73.35 MHz, fCLK = 18 MSPS, and 16-bit I/Q Data)

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