Linear Technology LT5572 User manual

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 945

HIGH LINEARITY DIRECT QUADRATURE MODULATOR

LT5572

DESCRIPTION

Demonstration circuit 945 is an I/Q modulator

featuring the LT

5572. The LT



5572 is a 1.5GHz to

2.5GHz direct I/Q modulator designed for high

performance wireless applications, including wireless

infrastructure. It may also be configured as an image

reject upconverting mixer, by applying 90°phase-

shifted signals to the I and Q inputs.

A high-speed, internally matched LO amplifier drives

two double-balanced mixer cores, allowing the use of

a low power, single-ended LO source. It allows direct

modulation of an RF signal using differential

baseband I and Q signals. The LT



5572 supports

GSM, EDGE, CDMA, CDMA2000, W-CDMA, 64-QAM,

OFDM and other modulation formats.

The I/Q baseband inputs consist of voltage-to-current

converters that in turn drive double-balanced mixers.

The outputs of these mixers are summed and applied

to an on-chip RF transformer, which converts the

differential mixer signals to a 50Ωsingle-ended

output. The four balanced I and Q base-band input

ports are intended for DC coupling from a source with

a common-mode voltage level of about 0.5V. The

differential input impedance of the baseband inputs is

approximately 90KΩ. The input termination

impedance is easily set for a specific application

using external resistors. On the DC945

demonstration circuit, this is set to 100Ωdifferential,

making it ideally suited for current drive applications.

The LO path consists of an LO buffer with single-

ended input, and precision quadrature generators

which produce the LO drive for the mixers.

Demonstration circuit 945 is designed for an RF

output frequency range from 1.5GHz to 2.5GHz.

Design files for this circuit board are available. Call

the LTC factory.

LT is a registered trademark of Linear Technology Corporation.

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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 945

HIGH LINEARITY DIRECT QUADRATURE MODULATOR

Table 1.

Typical Performance Summary (TA= 25°C)

PARAMETER CONDITION (f

= 2MHz, f

=2000MHz) VALUE

Supply Voltage 4.5V to 5.25V

Supply Current V

= 5V, EN = High 120 mA

Maximum Shutdown Current V

= 5V, EN = Low 230 µA

RF Frequency Range 1.5 to 2.5 GHz

Baseband Frequency Range DC to 460 MHz

LO Input Return Loss Z

= 50

Ω,

= 0dBm 15 dB

RF Output Return Loss Z

= 50

Ω

13.5 dB

LO Input Power -10 to +5 dBm

LO Frequency Range 1.5 to 2.5 GHz

Conversion Gain

= -10dBm, P

= 0dBm, defined as P

-2.5dB

Output 3

Order Intercept 2-Tone, P

= -10dBm/Tone,

∆

f = 100KHz, P

= 0dBm

+21.6dBm

Output 2

Order Intercept 2-Tone, P

= -10dBm/Tone,

∆

f = 100KHz, P

= 0dBm

+53.2 dBm

Output 1dB Compression P

= 0dBm +9.3 dBm

LO leakage P

= 0dBm -39.4 dBm

Image Rejection P

= 0dBm -41.2 dBc

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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 945

HIGH LINEARITY DIRECT QUADRATURE MODULATOR

APPLICATION NOTE

RF FREQUENCY RANGE

No tuning is required for

peration over the RF

frequency range of 1.5 GHz to 2.5 GHz.

BASEBAND FREQUENCY RANGE

The baseband frequency range extends from DC to 460

MHz (3dB bandwidth).

LO TO RF LEAKAGE

The LT

5572 offers excellent LO to RF leakage

performance, typically -39.4 dBm.

The LO leakage may be further reduced by the

introduction of small differential DC offsets (V

BBIP

–

BBIM

BBQP

– V

BBQM

) at the

baseband inputs, typically less than 10mV. These DC

offsets may be introduced by applying slightly different

DC bias voltages to each of the four 10KΩresistors

shown in Figure 1.

SIDETONE TO RF LEAKAGE

The LT

5572 also offers very good image rejection

(sidetone suppression) at the RF port. The image

rejection may be further enhanced by the introduction

of small differential phase and amplitude offsets at

the baseband inputs.

TEST EQUIPMENT AND SETUP

Refer to Figure 1 for proper measurement equipment

setup. Before performing measurements on the DUT, it

is very important to evaluate the test system perform-

ance to ensure that: 1) distortion-free input signals are

applied and 2) the spectrum analyzer internal distortion

is minimized. Follow the guidelines below to do this.

Use high performance signal generators with low har-

monic output (>75 dBc) for 2-tone measurements.

The signal generators must provide 2 equal amplitude

outputs in quadrature with one another. High quality

combiners that provide broadband 50 ohm termination

on all ports should be used. The combiners should

have good port-to-port isolation (>30 dB) to prevent

the signal generators from modulating each other and

generating intermodulation products. Attenuators on

the outputs of these generators can also be used to

increase the effective port-to-port isolation.

Spectrum analyzers can produce significant internal

distortion products if they are overdriven. Sufficient

spectrum analyzer input attenuation should be used to

avoid saturating the instrument. A typical input 3

or-

der intercept point for a spectrum analyzer is +40 dBm,

with 20 dB input attenuation applied. This is more

than 10 dB above the intercept point of the DUT, and

should yield accurate 3

order distortion results.

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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 945

HIGH LINEARITY DIRECT QUADRATURE MODULATOR

QUICK START PROCEDURE

Demonstration circuit 945 is easy to set up to evaluate

the performance of the LT

5572. Refer to Figure 1 for

proper measurement equipment setup.

Follow the procedure below:

Connect all test equipment as shown in Figure 1.

Set the DC power supply’s current limit to 150mA,

and adjust output voltage to 5V.

Connect Vcc to the 5V DC supply, and then connect

VCCEN to 5V; the modulator is enabled (on).

Set the baseband bias supply’s output voltage to

0.5V.

Set Signal Generator #1 to provide a 2000MHz,

0dBm, CW signal to the demo board LO input port.

Set the Signal Generators #2 and #3 to provide two

-10dBm CW signals to the combiner ports - one at

2MHz, and the other at 2.1MHz. For both

generators, output B should lead output A by 90

degrees. These 2 signals must be in quadrature to

drive the demo board properly. This is most

conveniently accomplished by using signal

generators with dual outputs with adjustable phase.

An example is the

HP3326A shown in Figure 1.

To measure 3

order distortion and conversion gain,

set the Spectrum Analyzer start and stop

frequencies to 2001.8MHz and 2002.3MHz,

respectively. Sufficient spectrum analyzer input

attenuation should be used to avoid distortion in the

instrument.

The 3

order intercept point is equal to (P

– P

) / 2

+ P

, where P

is the average power level of the two

fundamental output tones at 2002MHz and

2002.1MHz; P

is the average power level of the two

order products at 2001.9MHz and 2002.2MHz.

All units are in dBm.

To measure 2

order distortion, set the Spectrum

Analyzer start and stop frequencies to 2001MHz and

2005MHz, respectively. Sufficient spectrum

analyzer input attenuation should be used to avoid

distortion in the instrument.

10.

The 2

order intercept point is equal to 2*P

– P

where P

is the power level of the fundamental

output tone at 2002MHz, P

is the 2

order product

at 2004.1MHz. All units are in dBm.

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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 945

HIGH LINEARITY DIRECT QUADRATURE MODULATOR

Figure 1.

Proper Measurement Equipment Setup

Dual Output HP3326A (or equivalent)

A (0º) 0°Combiner (Mini-Circuits ZFSC-2-1W or equivalent)

B (90º)



180° divider (Mini-Circuits T2-1T or equivalent)

180º 0º



Baseband bias supply

180º 0º 10Kohm resistor (4PL)

180° divider (Mini-Circuits T2-1T or equivalent)

A (0º)

0°Combiner (Mini-Circuits ZFSC-2-1W or equivalent

)

B (90º)

Dual Output HP3326A (or equivalent)

Spe

trum

Analyzer



Signal Gener

tor

2 A)















Signal Gener

tor







Signal Gener

tor

DC Power Supply



DC Power Supply



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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 945

HIGH LINEARITY DIRECT QUADRATURE MODULATOR

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