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LT5528EUF

1.5GHz to 2.4GHz High Linearity Direct Quadrature Modulator

Linear Technology 

LT5528

APPLICATIO S I FOR ATIO

and 12pF to ground is incorporated (see Figure 1), which

limits the baseband bandwidth to approximately 330MHz

(–1dB point). The common-mode voltage is about 0.52V

and is approximately constant over temperature.

It is important that the applied common-mode voltage level

of the I and Q inputs is about 0.52V in order to properly

bias the LT5528. Some I/Q test generators allow setting

the common-mode voltage independently. In this case, the

common-mode voltage of those generators must be set

to 0.26V to match the LT5528 internal bias, because for

DC signals, there is no –6dB source-load voltage division

(see Figure 2).

50Ω

–+ 0.52VDC

GENERATOR

0.26VDC

50Ω

50Ω

45Ω

–+ 0.52VDC

GENERATOR

0.52VDC

0.52VDC –+

LT5528

5528 F02

Figure 2. DC Voltage Levels for a Generator Programmed at

0.26VDC for a 50Ω Load and the LT5528 as a Load

It is recommended that the part be driven differentially;

otherwise, the even-order distortion products will de-

grade the overall linearity severely. Typically, a DAC will

be the signal source for the LT5528. To prevent aliasing,

a filter should be placed between the DAC output and the

LT5528’s baseband inputs. In Figure 3, an example interface

schematic shows a commonly used DAC output interface

followed by a passive 5th order ladder filter. The DAC in

this example sources a current from 0mA to 20mA. The

interface may be DC coupled. This allows adjustment of

the DAC’s differential output current to minimize the LO

feed-through. Optionally, transformer T1 can be inserted

to improve the current balance in the BBPI and BBMI pins.

This will improve the second-order distortion performance

(OIP2).

The maximum single sideband CW RF output power at

2GHz using 20mA drive to both I and Q channels with the

configuration shown in Figure 3 is about –2.5dBm. The

maximum CW output power can be increased by con-

necting resistors R5 and R6 to –5V instead of GND, and

changing their values to 550Ω. In that case, the maximum

single sideband CW RF output power at 2GHz will be about

2.3dBm. In addition, the ladder filter component values

require adjustment for a higher source impedance.

VCC = 5V

RF = –2.5dBm, MAX

BALUN

C

LOMI

LT5528

LOPI

0.5V

0mA TO 20mA

R5, 50Ω

DAC

C1

0mA TO 20mA GND R6, 50Ω

L1A

C2

L1B

L2A

C3

L2B

OPTIONAL

BBPI

T1

1:1

BBMI

R1

45Ω

CM

R3

VREF = 0.52V

R2

45Ω

R4

0.5V

GND

5528 F03

Figure 3. LT5528 5th Order Filtered Baseband Interface with Common DAC (Only I-Channel is Shown)

5528f

9

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