AD9874BST AD [Analog Devices], AD9874BST Datasheet - Page 6

• Manufacturer Part Number: AD9874BST
• Description: IF Digitizing Subsystem
• Manufacturer: AD [Analog Devices]

AD9874
DEFINITION OF SPECIFICATIONS/TEST METHODS
Single-Sideband Noise Figure (SSB NF)
Noise figure (NF) is defined as the degradation in SNR perfor-
mance (in dB) of an IF input signal after it passes through a
component or system. It can be expressed with the following
equation:
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 AD9874’s SSB noise figure is determined by the following
equation:
where P
the noise measurement bandwidth, –174 dBm/Hz is the thermal
noise floor at 293°K, and SNR is the measured signal-to-noise
ratio in dB of the AD9874.
Note, P
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, a bandwidth
of 10 kHz is used for the data sheet specification on Page 2.
Refer to Figures 22a and 22b for an indication of how NF varies
with BW. Also, refer to the TPCs to see how NF is affected by
different operating conditions. 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 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 (f
system exhibiting third order nonlinearities producing IMD
components at 2f
the extrapolated intersection of the carrier’s input power with
the third order IMD component when plotted in dB. The dif-
ference in power (D in dBc) between the two carriers and the
resulting third order IMD components can be determined from
the following equation:
SSB NF
Noise Figure
D
= ×
2
IN
IN
is set to –85 dBm to minimize any degradation in
=
is the input power of an unmodulated carrier, BW is
(
IIP
P
IN
=
3
10
–
1
– {
– f
P
×
10
IN
2
LOG SNR
)
and 2f
×
1
LOG BW
and f
(
2
– f
(
2
) are injected into a nonlinear
1
IN
. IIP3 graphically represents
SNR
)} –
174
OUT
dBm Hz
)
–
SNR
–6–
Dynamic Range (DR)
Dynamic range is the measure of a small target input signal
(P
(P
characteristic of the component or system to degrade, thus making
it unable to detect the smaller target signal correctly. In the case of
the AD9874, it is often a degradation in Noise Figure at increased
VGA attenuation settings that limits its dynamic range (refer to
TPC 15a, 15b, and 15c).
The test method for the AD9874 is as follows. The small target
signal (an unmodulated carrier) is input at the center of the IF
frequency and its power level (P
SNR
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 (P
The dynamic range is determined from the following equation:
Note, the AD9874’s AGC is enabled for this test.
IF Input Clip Point
The IF input clip point is defined to be 2 dB below the input
power level (P
ADC. Unlike other linear components that typically exhibit a
“soft” compression (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
AD9874, which contains a - ADC, “hard” compression
should be avoided since it causes severe SNR degradation.
TARGET
INTER
DR
TARGET
=
). Typically, the large signal will cause some unwanted
INTER
P
) in the presence of a large unwanted interferer signal
INTER
of 6 dB. The power of the signal is then increased by
) where the target signal SNR is reduced to 6 dB.
IN
–
), resulting in the “clipping” of the AD9874’s
P
TARGET
+
SNR
TARGET
TARGET
) is adjusted to achieve an
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