Detailed Description

Internally, the MAX2150 includes a broadband I/Q

modulator, internally matched broadband output driver

amplifier, fine-resolution fractional-N frequency synthe-

sizer, an LO buffer amplifier, and an on-chip low-noise

crystal oscillator circuit.

A simple 3-wire interface is provided for synthesizer

programming and device configuration and control.

Independent hardware and software power-down con-

trol of the I/Q modulator, frequency synthesizer, and LO

buffer amplifier is provided, as well as the ability to shut

down the entire chip.

I/Q Modulator

The MAX2150 modulator is composed of a pair of

matched double-balanced mixers, a broadband pas-

sive LO quadrature generator, and a summing amplifi-

er. The mixers accept differential I/Q baseband signals

that directly modulate the internal 0

° and 90° LO sig-

nals applied to the I/Q mixers. An external LO source

drives an internal LO quadrature generator that shifts

the phase of the LO signal applied to the Q mixer by

90

° relative to the LO signal applied to the I-channel

mixer. The modulated output of the I/Q mixers is

summed together, and the undesired sideband is sup-

pressed.

The I+, I-, Q+, and Q- input ports feature high-linearity

buffer amplifiers with a typical -3dB bandwidth of

75MHz and accept differential input voltages up to

input common-mode requirement of 1.6V. For single-

ended operation, bypass the I and Q ports to ground.

See the Typical Application Circuit for recommended

component values.

The broadband output driver amplifier is matched on

chip across the entire operating frequency range and

requires an output DC-blocking capacitor. For optimum

performance, the output match can be improved with

simple L-section and/or PI-section matching networks.

Always ensure that DC blocking is provided, because

internal bias voltages are present at this output.

The modulator can be shut down with both hardware

(pin 1) and software (TE bit). This mode is useful for

quiet synthesizer programming or to mute the RF out-

put signal. The hardware pin and software bits must be

set to logic-1 to enable the modulator. If the hardware

pin or software bit is set to logic-0, or if both are set to

logic-0, the modulator is disabled.

LO Buffer Amplifier

The broadband buffer amplifier output is internally

matched and requires a DC-blocking capacitor to iso-

late on-chip bias voltages. Power-down of the LO buffer

can be controlled by both BUFEN (pin 23), as well as

BUFEN by software by setting the BUFEN (BE) bit

through the 3-wire interface. The hardware pin and the

software bit must be a logic-1 to enable the buffer. If

the hardware or software bit is set to logic-0, the LO

buffer is disabled.

Frequency Synthesizer

The MAX2150 features an internal 28-bit sigma-delta

frequency synthesizer. This architecture enables the

use of very high (30MHz) comparison frequencies,

which significantly reduces the in-loop phase noise as

a result of reduced division ratios. The high comparison

frequency also allows significantly increased PLL

bandwidths for very fast switching speed applications.

Divider Programming

The MAX2150 frequency programming is determined

as follows. The overall division ratio (D) has an integer

value (N), as well as a fractional component (F):

The N and F values are encoded as straight binary

numbers. Determination of these values is illustrated by

the following example:

Then:

D = 1721.125 / 20 = 86.05625

Therefore:

Wideband I/Q Modulator with Sigma-Delta

Fractional-N Synthesizer

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9

Pin Description (continued)

PIN

NAME

FUNCTION

26, 27

I-, I+

Differential I-Channel Baseband Inputs to the Modulator. These pins connect directly to the bases of a

differential pair and require an external common-mode bias voltage of 1.6V.

28

VCC_RF

Supply Voltage Input for RF Circuits. Bypass as close to pin as possible. The bypass capacitor should

not share ground vias with other branches.

—

Exposed pad

Ground