LTC5598
9
5598f
The LTC5598 consists of I and Q input differential voltage-
to-current converters, I and Q up-conversion mixers, an
RF output buffer, an LO quadrature phase generator and
LO buffers.
External I and Q baseband signals are applied to the
differential 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 buffer, 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. In most applications, the LOP input is driven by
the LO source via an optional matching network, while
the LOM input is terminated with 50Ω to RF ground via
a similar optional matching network. The RF output is
single-ended and internally 50Ω matched.
Baseband Interface
The circuit is optimized for a common mode voltage of
0.5V which should be externally applied. The baseband
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. In shut-down mode, it is recommended to have
a termination to ground or to a 0.5V source with a value
lower than 1kΩ. The PNP’s base current is about –68μA
in normal operation.
The baseband inputs (BBPI, BBMI, BBPQ, BBMQ) present
a single-ended input impedance of about –7.4kΩ each.
Because of the negative input impedance, it is important
to keep the source resistance at each baseband input low
enough such that the parallel value remains positive vs
baseband frequency. At each of the four baseband inputs, a
capacitor of 4pF in series with 30Ω is connected to ground.
This is in parallel with a PNP emitter follower (see Figure 1).
The baseband bandwidth depends on the source impedance.
For a 25Ω source impedance, the baseband bandwidth
(–1dB) is about 300MHz. If a 5.6nH series inductor is
APPLICATIONS INFORMATION
inserted in each of the four baseband connections, the
–1dB baseband bandwidth increases to about 800MHz.
It is recommended to include the baseband input impedance
in the baseband lowpass fi lter design. The input impedance
of each baseband input is given in Table 1.
Table 1. Single-Ended BB Port Input Impedance vs Frequency
for EN = High and V
CMBB
= 0.5V
DC
FREQUENCY
(MHz)
BB INPUT
IMPEDANCE
REFLECTION COEFFICIENT
MAG ANGLE
0.1 –10578 – j263 1.01 –0.02
1 –8436 – j1930 1.011 –0.15
2 –6340 – j3143 1.013 –0.36
4 –3672 – j3712 1.014 –0.78
8 –1644 – j2833 1.015 –1.51
16 –527 – j1765 1.016 –2.98
30 –177 – j1015 1.017 –5.48
60 –45.2 – j514 1.017 –11
100 –13.2 – j306 1.014 –18.5
140 –0.2 – j219 1 –25.7
200 4.5 – j151 0.982 –36.6
300 10.4 – j99.4 0.921 –52.9
400 12.3 – j72.4 0.854 –68.2
500 14.7 – j57.5 0.780 –79.9
600 15.5 – j46.3 0.720 –91.4
The baseband inputs should be driven differentially;
otherwise, the even-order distortion products may degrade
the overall linearity performance. Typically, a DAC will
Figure 1. Simplifi ed Circuit Schematic
of the LTC5598 (Only I-Half is Drawn)
BBPI
BBMI
GND
LOMI LOPI
GNDRF
RF
FROM
Q
55682 F01
30Ω
30Ω
4pF
4pF
LTC5598
V
CC1
= 5V
V
CMBB
= 0.5V
DC
V
CC2
= 5V
BUFFER
