AD9864 datasheet(9/44 Pages) AD

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AD9864

Rev. 0 | Page 9 of 44

DEFINITION OF SPECIFICATIONS/TEST METHODS

Single Sideband Noise Figure (SSB NF)

Noise figure (NF) is defined as the degradation in SNR per-

formance (in dB) of an IF input signal after it passes through a

component or system. It can be expressed with the equation

(

)

OUT

IN SNR

SNR

Figure

Noise

log

10 ×

The term SSB is applicable for heterodyne systems containing a

mixer. It indicates that the desired signal spectrum resides on

only one side of the LO frequency (i.e., single sideband); thus a

“noiseless” mixer has a noise figure of 3 dB.

The SSB noise figure of the AD9864 is determined by the equation

(

)

[]

SNR

)

SSB

–

dBm/Hz

174

(

–

log

–

−

where PIN is the input power of an unmodulated carrier, BW is

the noise measurement bandwidth, –174 dBm/Hz is the ther-

mal noise floor at 293K, and SNR is the measured signal-to-

noise ratio in dB of the AD9864.

Note that PIN is set to –85 dBm to minimize any degradation in

measured SNR due to phase noise from the RF and LO signal

generators. The IF frequency, CLK frequency, and decimation

factors are selected to minimize any spurious components

falling within the measurement bandwidth. Note also that a

bandwidth of 10 kHz is used for the data sheet specification. All

references to noise figures within this data sheet imply single

sideband noise figure.

Input Third Order Intercept (IIP3)

IIP3 is a figure of merit used to determine a component’s or

system’s susceptibility to intermodulation distortion (IMD)

from its third order nonlinearities. Two unmodulated carriers

at a specified frequency relationship (f1 and f2) are injected into

a nonlinear system exhibiting third order nonlinearities pro-

ducing IMD components at 2f1 – f2 and 2f2 – f1. IIP3 graphi-

cally represents the extrapolated intersection of the carrier’s

input power with the third order IMD component when plot-

ted in dB. The difference in power (D in dBc) between the two

carriers and the resulting third order IMD components can be

determined from the equation

()

IIP

–

2 ×

Dynamic Range (DR)

Dynamic range is the measure of a small target input signal

(PTARGET) in the presence of a large unwanted interferer signal

(PINTER). Typically, the large signal will cause some unwanted

characteristic of the component or system to degrade, thus mak-

ing it unable to detect the smaller target signal correctly. In the

case of the AD9864, it is often a degradation in noise figure at

increased VGA attenuation settings that limits its dynamic range.

The test method for the AD9864 is as follows. The small target

signal (an unmodulated carrier) is input at the center of the IF

frequency, and its power level (PTARGET) is adjusted to achieve an

SNRTARGET of 6 dB. The power of the signal is then increased by

3 dB prior to injecting the interferer signal. The offset frequency

of the interferer signal is selected so that aliases produced by the

decimation filter’s response as well as phase noise from the LO

(due to reciprocal mixing) do not fall back within the measure-

ment bandwidth. For this reason, an offset of 110 kHz was

selected. The interferer signal (also an unmodulated carrier) is

then injected into the input and its power level is increased to the

point (PINTER) where the target signal SNR is reduced to 6 dB. The

dynamic range is determined with the equation

TARGET

INTER

SNR

–

Note that the AD9864’s AGC is enabled for this test.

IF Input Clip Point

The IF input clip point is defined as the input power that

results in a digital output level 2 dB below full-scale. Unlike

other linear components that typically exhibit a soft compres-

sion (characterized by its 1 dB compression point), an ADC

exhibits a hard compression once its input signal exceeds its

rated maximum input signal range. In the case of the AD9864,

which contains a Σ-∆ ADC, hard compression should be

avoided because it causes severe SNR degradation.

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