LT5503

14

APPLICATIO S I FOR ATIO

AC-Coupled Baseband. Figure 4 shows the simplified

circuit schematic of a high-pass AC-coupled baseband

interface.

5505 F04

18k

18k

0.8pF

0.8pF

0.8pF

0.8pF

Q

I

LT5503

Figure 4. AC-Coupled Baseband Interface

With approximately 18k of differential input resistance,

the suggested minimum AC-coupling capacitor can be

determined using the following equation:

C

f

CPL

C

=

1

(•

• • )

π

signal.

A larger capacitor may be used where the settling time of

charging and discharging the AC-coupling capacitor is not

critical.

DC-Coupled Baseband. The baseband inputs’ internal bias

voltage can be overdriven with an external bias circuit.

This facilitates direct interfacing to a D/A converter for

faster transient response. In this case, the LT5503’s

baseband inputs are DC biased by the converter. The

load on each converter output can be approximated using

into a modulator input:

I

VV

k

INPUT

BIAS

=

−

Ω

14

9

.

Figure 5 shows a simplified circuit schematic for interfac-

ing the LT5503’s baseband inputs to the outputs of a D/A

converter. OIP and OIN are the positive and negative

baseband outputs, respectively, of the converter’s

I-channel. Similarly, OQP and OQN are the positive and

negative baseband outputs, respectively, of the converter’s

Q-channel.

5505 F05

18k

18k

0.8pF

0.8pF

0.8pF

0.8pF

LT5503

OIP

OIN

OQP

OQN

D/A

Figure 5. DC-Coupled Baseband Interface

Modulator RF Input (MODRFIN)

The modulator RF input buffer is driven single-ended. An

internal active balun circuit produces balanced signals to

drive the integrated phase shifter. Limiters following the

phase shifter output accommodate a wide range of

MODRFIN power, resulting in minimal degradation of

modulation gain/phase accuracy performance or carrier

feedthrough. This pin is easily matched to a 50

Ω source

with the simple lowpass network shown in Figure 2. This

pin is internally biased, therefore an AC-coupling capaci-

tor is required.

Modulator VGA (Variable Gain Amp)

The VGA has two digital selection lines to provide a

nominal 0dB, 4.5dB, 9dB and 13.5dB attenuation from the

maximum modulator output power setting. The logic table

is shown below:

GC2

Attenuation

Low

High

GC1

Low

0dB

9dB

High

4.5dB

13.5dB