AD9864 datasheet(15/48 Pages) AD

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AD9864

Data Sheet

Rev. A | Page 14 of 47

TERMINOLOGY

Single Sideband Noise Figure (SSB NF)

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

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

a component or system. It can be expressed with the equation

Noise Figure = 10 × log(SNRIN/SNROUT)

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 (that is, single sideband); therefore,

a noiseless mixer has a noise figure of 3 dB.

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

SSB NF = PIN − [10 × log(BW)] − (−174 dBm/Hz) − SNR

where:

PIN is the input power of an unmodulated carrier.

BW is the noise measurement bandwidth.

−174 dBm/Hz is the thermal noise floor at 293 K.

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 the susceptibility of a

component or system 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 producing

IMD components at 2f1 − f2 and 2f2 − f1. IIP3 graphically

represents the extrapolated intersection of the carrier’s input

power with the third-order IMD component when plotted 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

D = 2 × (IIP3 − PIN)

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 causes some unwanted

characteristic of the component or system to degrade, thus

making it unable to detect the smaller target signal correctly.

For 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 response of the decimation filter, as well as phase noise from

the LO (due to reciprocal mixing), do not fall back within the

measurement 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

DR = PINTER − PTARGET + SNRTARGET

Note that the AGC of the AD9864 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 compression

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

a hard compression when its input signal exceeds its rated

maximum input signal range. For the AD9864, which contains

a Σ-∆ ADC, hard compression must be avoided because it

causes severe SNR degradation.

AD9864 Datasheet(PDF) 15 Page - Analog Devices

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