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LT5571

5571f

BLOCK DIAGRAM

APPLICATIONS INFORMATION

The LT5571 consists of I and Q input differential voltage-

to-current converters, I and Q up-conversion mixers, an RF

signal combiner/balun, an LO quadrature phase generator

and LO buffers.

External I and Q baseband signals are applied to the dif-

ferential baseband input pins, BBPI, BBMI, and BBPQ,

BBMQ. These voltage signals are converted to currents and

translated to RF frequency by means of double-balanced

up-converting mixers. The mixer outputs are combined

in an RF output balun, which also transforms the output

impedance to 50Ω. The center frequency of the resulting

RF signal is equal to the LO signal frequency. The LO input

drives a phase shifter which splits the LO signal into in-

phase and quadrature LO signals. These LO signals are then

applied to on-chip buffers which drive the up-conversion

mixers. Both the LO input and RF output are single-ended,

50Ω-matched and AC-coupled.

Baseband Interface

The baseband inputs (BBPI, BBMI), (BBPQ, BBMQ) present

a differential input impedance of about 90kΩ. At each of

the four baseband inputs, a capacitor of 1.8pF to ground

and a PNP emitter follower is incorporated (see Figure 1),

which limits the baseband bandwidth to approximately

200MHz (–1dB point), if driven by a 50Ω source. The

circuit is optimized for a common mode voltage of 0.5V

which should be externally applied. The baseband input

pins should not be left ﬂoating because the internal PNP’s

base current will pull the common mode voltage higher

than the 0.6V limit. This condition may damage the part.

The PNP’s base current is about 24µA in normal opera-

tion. On the LT5571 demo board, external 50Ω resistors

to ground are added to each baseband input to prevent

this condition and to serve as a termination resistance for

the baseband connections.

It is recommended that the I/Q signals be DC-coupled to

the LT5571. An applied common mode voltage level at the

I and Q inputs of about 0.5V will maximize the LT5571’s

dynamic range. Some I/Q generators allow setting the

common mode voltage independently. For a 0.5V com-

mon mode voltage setting, the common-mode voltage of

those generators must be set to 0.5V to create the desired

0.5V bias, when an external 50Ω is present in the setup

(See Figure 2).

The part should be driven differentially; otherwise, the even-

order distortion products will degrade the overall linearity

severely. Typically, a DAC will be the signal source for the

LT5571. A reconstruction ﬁlter should be placed between

the DAC output and the LT5571’s baseband inputs.

In Figure 3 a typical baseband interface is shown, includ-

ing a ﬁfth-order low-pass ladder ﬁlter. For each baseband

pin, a 0 to 1V swing is developed corresponding to a DAC

output current of 0mA to 20mA. The maximum sinusoidal

single side-band RF output power is about +5.8dBm for

V-I

BALUN

VCC

5571 BD

GND

BBPI

BBMI

BBPQ

BBMQ

GND

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