ADL5385 데이터 시트보기 (PDF) - Analog Devices
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ADL5385 Datasheet PDF : 26 Pages
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ADL5385
Using AD9777 Auxiliary DAC for Carrier Feedthrough
Nulling
The AD9777 features an auxiliary DAC that can be used to
inject small currents into the differential outputs for each
channel. The auxiliary DAC can produce the small offset
currents necessary to implement the nulling described in the
Carrier Feedthrough Nulling section.
155 Mbps (STM-1) 128 QAM TRANSMITTER
Figure 36 shows how the ADL5385 can be interfaced to the
AD9777 DAC (or any Analog Devices dual DAC with an output
bias level of 0.5 V) to generate a 155 Mbps 128 QAM carrier at
355 MHz. Because the TxDAC output and the IQ modulator inputs
operate at the same bias levels of 0.5 V, a simple dc-coupled
connection can be implemented without any active or passive
level shifting. The bias level and modulator drive level is set by
the 50 Ω ground-referenced resistors and the 100 Ω shunt resistors,
respectively (see the DAC Modulator Interfacing section). A
baseband filter is placed between the bias and signal swing
resistors. This 5-pole Chebychev filter with in-band ripple of
0.1 dB has a corner frequency of 39 MHz.
I
CHANNEL
1/2
AD9777
50Ω LINE
50Ω 67.5pF
50Ω LINE
50Ω 67.5pF
317.4nH
317.4nH
372.5nH
100Ω LINE 0Ω
156.9pF
372.5nH
124.7pF
100Ω LINE 0Ω
156.9pF
124.7pF
200Ω
IBBP
IBBN
Q
CHANNEL
1/2
AD9777
50Ω LINE
50Ω 67.5pF
50Ω LINE
50Ω 67.5pF
317.4nH
317.4nH
ADL5385
372.5nH
100Ω LINE 0Ω
156.9pF
372.5nH
124.7pF
100Ω LINE 0Ω
156.9pF
124.7pF
200Ω
QBBP
QBBN
Figure 36. Recommended DAC-Modulator Interconnect for128 QAM
Transmitter
Figure 37 shows a spectral plot of the 128 QAM spectrum at
a carrier power of −6.3 dBm. Figure 38 shows how EVM
(measured with the analyzer’s internal equalizer both on and
off) and SNR, measured at 55 MHz carrier offset (2.5 times the
carrier bandwidth) varies with output power.
–70
–80
–90
–100
–110
–120
–130
–140
–150
–160
290 300 310 320 330 340 350 360 370 380 390 400 410 420
FREQUENCY (MHz)
Figure 37. Spectral Plot of 128 QAM Transmitter at −6.3 dBm Output Power
Data Sheet
79
0.7
SNR
77
0.6
75
0.5
EVM WITHOUT EQUALIZATION
73
0.4
71
0.3
EVM WITH EQUALIZATION
69
0.2
67
0.1
65
0
–18 –16 –14 –12 –10 –8 –6 –4 –2 0
CARRIER POWER (dBm)
Figure 38. EVM and SNR vs. Output Power for 128 QAM Transmitter
Application
CMTS TRANSMITTER APPLICATION
Because of its broadband operating range from 30 MHz to
2200 MHz, the ADL5385 can be used in direct-launch cable
modem termination systems (CMTS) applications in the 50
MHz to 860 MHz cable band.
The same DAC and DAC-to-modulator interface and filtering
circuit shown in Figure 36 was used in this application. Figure 39
shows a plot of a 4-carrier 256 QAM spectrum at an output
frequency of 485 MHz.
–70
–80
–90
–100
–110
–120
–130
–140
–150
–160
–170
430 440 450 460 470 480 490 500 510 520 530 540
FREQUENCY (MHz)
Figure 39. Spectrum of 4-Carrier 256 QAM CMTS Signal at 485 MHz
Rev. B | Page 18 of 26
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