lt5571 Linear Technology Corporation, lt5571 Datasheet - Page 8

• Manufacturer Part Number: lt5571
• Description: 620mhz - 1100mhz High Linearity Direct Quadrature Modulator
• Manufacturer: Linear Technology Corporation

BLOCK DIAGRAM
APPLICATIONS INFORMATION
LT5571
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
8
BBMQ
BBPQ
BBMI
BBPI
14
16
7
5
2
V-I
V-I
4
GND
6
9
90°
0°
8
V
LO
CC
3
pins should not be left fl oating 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 fi lter 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 fi fth-order low-pass ladder fi lter. 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
13
10
BALUN
12
GND
15
17
5571 BD
11
1
RF
EN
5571f