LT5503(RevA) データシートの表示（PDF） - Linear Technology

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LT5503
(Rev.:RevA)

1.2GHz to 2.7GHz Direct IQ Modulator and Mixer

Linear Technology 

LT5503

Applications Information

AC-Coupled Baseband. Figure 3 shows the simplified circuit

schematic of a high-pass AC-coupled baseband interface.

CCPL

I

CCPL

IB

CCPL

Q

CCPL

QB

BI+ LT5503

18k 0.8pF

BI–

0.8pF

BQ+

0.8pF

18k

BQ–

0.8pF

5505 F03

Figure 3. AC-Coupled Baseband Interface

With approximately 18k of differential input resistance,

the suggested minimum AC-coupling capacitor can be

determined using the following equation:

CCPL

=

(18

•

1

103 •

π

•

fC )

where fC is the 3dB cut-off frequency of the baseband

input 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 optimal VBIAS

is 1.4V, independent of VCC. In general, the maximum

VBIAS should be less than VCC – 0.4V. The DC load on

each converter output can be approximated using the

following equation where IINPUT is the current flowing

into a modulator input:

IINPUT

=

VBIAS − 1.4V

9kΩ

Figure 4 shows a simplified circuit schematic for in-

terfacing 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.

OIP

OIN

D/A

OQP

OQN

IINPUT BI+ LT5503

IINPUT BI–

18k 0.8pF

IINPUT BQ+

0.8pF

IINPUT BQ–

0.8pF

18k

0.8pF

5505 F04

Figure 4. 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 50Ω source

with the simple lowpass network shown in Figure 1. 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:

ATTENUATION

GC1

Low

High

LOW

0dB

4.5dB

GC2

HIGH

9dB

13.5dB

5503fa

14

For more information www.linear.com/LT5503

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