參數(shù)資料
型號： AD9874ABSTZ
廠商： ANALOG DEVICES INC
元件分類： 通信及網(wǎng)絡(luò)
英文描述： SPECIALTY TELECOM CIRCUIT, PQFP48
封裝： PLASTIC, MS-026BBC, LQFP-48
文件頁數(shù)： 27/40頁
文件大?。?/td> 1682K
代理商： AD9874ABSTZ
REV. A
AD9874
–33–
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 perfor-
mance for a particular LO frequency and fCLK setting. This is
especially the case for particularly 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, Fig-
ure 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
IN-B
AND
PO
WER
–
dBFS
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.
90
50
0
–40
–20
–80
–60
100
IF FREQUENCY – MHz
dBFS
–120
–100
60
70
80
D =
f
CLK/4 = 4.5MHz
DESIRED
RESPONSES
Figure 25. Response of AD9874 to a –20 dBm 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,
fIF2_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
fSPUR 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] = fCLK/8).
IN-B
AND
PO
WER
–
dBFS
–60
–70
–80
–90
–50
0
250
300
LO FREQUENCY – MHz
200
150
100
50
Figure 23b. Same as Figure 23a Excluding LO Frequencies Known to Produce Large In-Band Spurs
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