AD8349 datasheet(18/28 Pages) AD

18 / 28 page

AD8349

Rev. 0 | Page 18 of 28

BASEBAND DAC INTERFACE

The recommended baseband input swing and bias levels of the

AD8349’s differential baseband inputs allow for direct

connection to most baseband DACs without the need for any

external active components. Typically these DACs have a

differential full-scale output current from 0 mA to 20 mA on

each differential output. These currents can be easily converted

to voltages using ground-referenced shunt resistors. Most

baseband DACs for transmit chains are designed with two

DACs in a single package.

AD9777 INTERFACE

The AD977x family of dual DACs is well suited to driving the

baseband inputs of the AD8349. The AD9777 is a dual 16-bit

DAC that can generate either a baseband output or a complex

IF using the device’s complex modulator.

The basic interface between the AD9777’s IOUT outputs and the

AD8349’s differential baseband inputs is shown in Figure 51.

The Resistors R1 and R2 set the dc bias level, and R3 sets the

amplitude of the baseband input voltage swing.

OPTIONAL

LOW-PASS

FILTER

R1Q

R2Q

R3Q

IOUTB2

IOUTA2

QBBN

QBBP

IOUTB1

IOUTA1

IBBN

IBBP

AD9777

AD8349

OPTIONAL

LOW-PASS

FILTER

R1I

R2I

R3I

Figure 51. Basic AD9777 to AD8349 Interface

0.15

0.30

0.45

0.60

0.75

0.90

1.05

1.20

1.35

1.50

R3 (

Ω)

100

1.103

Figure 52. Relationship Between R3 in Figure 51 and Peak

Baseband Input Voltage.

BIASING AND FILTERING

A value of 40 Ω on R1 and R2 in Figure 51 will generate the

required 400 mV dc bias. Note that this is independent of the

value of R3. Figure 52 shows the relationship between the value

of R3 and the peak baseband input voltage with the 40 Ω

resistors in place. From Figure 52, it can be seen that a value of

240 Ω will provide a peak-to-peak swing of approximately

1.2 V p-p differential into the AD8349’s baseband inputs.

The closest available resistor values are 40.2 Ω and 240 Ω, and

these values were used in the characterization of the AD8349

when the DAC was used as a signal source.

When using a DAC, low-pass image reject filters are typically

used to eliminate images that are produced by the DAC. They

provide the added benefit of eliminating broadband noise that

might feed into the modulator from the DAC.

Figure 53 shows a single sideband spectrum at 2140 MHz. The

baseband sine and cosine signals come from the digital output

of a Rohde & Schwarz AMIQ arbitrary waveform generator.

These signals drive the AD9777 dual DAC, which in turn drives

the AD8349’s baseband inputs. Note that the AD9777’s complex

modulator is not being used.

Due to offset voltages, internal device mismatch, and imperfect

quadrature over the AD8349’s operating range, the SSB

spectrum has a number of undesirable components such as LO

feedthrough and undesired sideband leakage. When the

AD8349 is driven by a modulated baseband signal, (e.g. 8-PSK,

GMSK, QPSK, or QAM), these nonidealities will manifest

themselves as degraded error vector magnitude (EVM), and

degraded spectral purity.

–90

–80

–70

–60

–50

–40

–30

–20

–10

SSB = 1.7dBm

LO = –44.5dBm

USB = –52dBc

THIRD HARMONIC = –36.8dBc

CENTER 2.14GHz

SPAN 10MHz

Figure 53. AD8349 Single Sideband Spectrum at 2140 MHz

REDUCING UNDESIRED SIDEBAND LEAKAGE

Undesired sideband leakage is the result of phase and amplitude

imbalances between the I and Q channel baseband signals.

Therefore, to reduce the undesired sideband leakage, the

amplitude and phase of the baseband signals have to be

matched at the mixer cores. Because of mismatches in the

AD8349 Datasheet(PDF) 18 Page - Analog Devices

AD8349 Datasheet(HTML) 18 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

Datasheet Download

Link URL