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LTC5598IUF-TRPBF

5MHz to 1600MHz High Linearity Direct Quadrature Modulator

Linear Technology 

LTC5598

APPLICATIONS INFORMATION

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 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. 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

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 filter 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 VCMBB = 0.5VDC

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

VCC2 = 5V

VCC1 = 5V

FROM

Q

BUFFER

LTC5598

RF

LOMI

LOPI

BBPI

VCMBB = 0.5VDC

BBMI

30Ω

4pF

4pF

30Ω

GND

Figure 1. Simplified Circuit Schematic

of the LTC5598 (Only I-Half is Drawn)

GNDRF

55682 F01

5598f

9

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