lt5572 Linear Technology Corporation, lt5572 Datasheet - Page 8

• Manufacturer Part Number: lt5572
• Description: 1.5ghz To 2.5ghz High Linearity Direct Quadrature Modulator
• Manufacturer: Linear Technology Corporation

BLOCK DIAGRA
APPLICATIO S I FOR ATIO
LT5572
The LT5572 consists of I and Q input differential voltage-
to-current converters, I and Q up-conversion mixers, an
RF output 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 in-
put 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) and (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 –1dB bandwidth to
approximately 250MHz. The circuit is optimized for an
externally applied common mode voltage of 0.5V. The
baseband input pins should not be left fl oating because
8
U
U
W
BBMQ
BBPQ
BBMI
BBPI
W
14
16
7
5
2
V-I
V-I
4
GND
U
6
90°
9
0°
8
V
LO
CC
3
the internal PNP’s base current will pull the common mode
voltage higher than the 0.6V limit. This may damage the part
if continued indefi nitely. The PNP’s base current is about
20µA in normal operation. On the LT5572 demo board,
external 50Ω resistors to ground are included at each
baseband input to prevent this condition and to serve as
a termination resistance for the baseband connections.
The I/Q input signals to the LT5572 should be DC coupled
with an applied common mode voltage level of about
0.5V in order to bias the LT5572 at its optimum operating
point. 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 0.5V
(See Figure 2).
The baseband inputs should be driven differentially; oth-
erwise, the even-order distortion products will degrade
the overall linearity severely. Typically, a DAC will be the
signal source for the LT5572. Reconstruction fi lters should
be placed between the DAC outputs and the LT5572’s
baseband inputs.
In Figure 3, a typical baseband interface is shown including
a 5th-order lowpass ladder fi lter for reconstruction. For each
baseband pin, a 0V to 1V swing is developed corresponding
to a DAC output current of 0mA to 20mA. The maximum
sinusoidal single sideband RF output power at 2.14GHz is
about +6.2dBm for full 0V to 1V swing on each baseband
13
10
BALUN
12
GND
15
17
5572 BD
11
1
RF
EN
5572f