ADL5385

Rev. 0 | Page 16 of 24

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 34 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.

ADL5385

1/2

AD9777

I

CHANNEL

50Ω

67.5pF

50Ω

67.5pF

156.9pF

156.9pF

124.7pF

124.7pF

50Ω LINE

50Ω LINE

100Ω LINE

0Ω

100Ω LINE

317.4nH

372.5nH

317.4nH

372.5nH

0Ω

200Ω

IBBP

IBBN

1/2

AD9777

Q

CHANNEL

50Ω

67.5pF

50Ω

67.5pF

156.9pF

156.9pF

124.7pF

124.7pF

50Ω LINE

50Ω LINE

100Ω LINE

0Ω

100Ω LINE

317.4nH

372.5nH

317.4nH

372.5nH

0Ω

200Ω

QBBP

QBBN

Figure 34. Recommended DAC-Modulator Interconnect for128 QAM

Transmitter

Figure 35 shows a spectral plot of the 128 QAM spectrum at

a carrier power of −6.3 dBm. Figure 36 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

420

410

390 400

380

370

360

350

340

330

320

310

300

FREQUENCY (MHz)

Figure 35. Spectral Plot of 128 QAM Transmitter at −6.3 dBm Output Power

79

65

–18

0

CARRIER POWER (dBm)

77

75

73

71

69

67

0.7

0

0.6

0.5

0.4

0.3

0.2

0.1

–2

–4

–6

–8

–10

–12

–14

–16

SNR

EVM WITHOUT EQUALIZATION

EVM WITH EQUALIZATION

Figure 36. EVM and SNR vs. Output Power for 128 QAM Transmitter

Application

CMTS TRANSMITTER APPLICATION

Because of its broadband operating range from 50 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 34 was used in this application. Figure 37

shows a plot of a 4-carrier 256 QAM spectrum at an output

frequency of 485 MHz. Figure 38 shows how adjacent channel

power (measured at 750 KHz, 5.25 MHz, and 12 MHz offset

from the last carrier) and modulation error ratio (MER) vary

with carrier power.

–70

–80

–90

–100

–110

–120

–130

–140

–150

–160

–170

430

540

530

520

510

500

490

480

470

460

450

440

FREQUENCY (MHz)

Figure 37. Spectrum of 4-Carrier 256 QAM CMTS Signal at 485 MHz