3V, Ultra-Low-Power

Quadrature Modulator

_______________________________________________________________________________________

5

_______________Detailed Description

The MAX2452 quadrature modulator integrates several

important RF functions on a single chip. It includes dif-

ferential buffers for the baseband inputs, two double-

balanced mixers, a local oscillator, a quadrature phase

generator, and a divide-by-8 prescaler. The prescaler

simplifies the implementation of a phase-locked loop.

Each of the functional blocks (shown in the

Functional

Diagram) is described in detail in the following sec-

tions.

Local Oscillator

The local-oscillator section is formed by an emitter-cou-

pled differential pair. Figure 1 shows the equivalent

local-oscillator circuit schematic. An external LC reso-

nant tank determines the oscillation frequency, and the

Q of this resonant tank determines the phase noise of

the oscillator. The oscillation frequency is twice the IF

frequency, so that the quadrature phase generator can

use two latches to generate precise quadrature signals.

Quadrature Phase Generator

The quadrature phase generator uses two latches to

divide the local-oscillator frequency by two, and gener-

ates two precise quadrature signals. Internal limiting

amplifiers shape the signals to approximate square

waves to drive the Gilbert-cell mixers in the modulator.

The inphase signal (at half the local-oscillator frequen-

cy) is further divided by four for an external phase-

locked loop.

Modulator

The modulator accepts up to 1.35Vp-p I and Q differen-

tial baseband signals up to 15MHz, and upconverts

them to higher-frequency IF signals. Since these inputs

are biased internally at around 1.5V, you can improve

carrier suppression by externally capacitively coupling

the signals into these high-impedance ports (the differ-

ential input impedance is approximately 44k

Ω). The

self-bias design is for very low on-chip offset, resulting

in excellent carrier suppression. The IF output is

designed to drive a high impedance (>20k

Ω), such as

an IF buffer or an upconverter mixer.

Prescaler

The prescaler output, PRE_OUT, is buffered and

swings typically 0.35Vp-p with a 10k

Ω and 6pF load. It

can be AC-coupled to the input of a frequency synthe-

sizer.

Master Bias

During normal operation, ENABLE should remain above

master bias and reduces the circuit current to 2µA. The

master bias section includes a bandgap reference

generator and a PTAT (Proportional To Absolute

Temperature) current generator.

TANK

TANK

Q1

Q2

Q3

Q4

5k

5k

TO

DIVIDE-BY-2

Figure 1. Local-Oscillator Equivalent Circuit