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MAX2450CEP

3V, Ultra-Low-Power Quadrature Modulator/Demodulator

Maxim Integrated 

3V, Ultra-Low-Power Quadrature

Modulator/Demodulator

LO_VCC

RL

RL

5k

5k

Q3

TANK

TANK

Q4

Q1

Q2

TO

QUADRATURE

GENERATOR AND

PRESCALER

75

70

65

60

55

50

45

40

35

200

1k

10k

100k

LOAD RESISTANCE (Ω)

Figure 2. Local-Oscillator Equivalent Circuit

Figure 3. Modulator Output Level vs. Load Resistance

_______________Detailed Description

The following sections describe each of the functional

blocks shown in the Functional Diagram. They also refer

to the Typical Application Block Diagram (Figure 1).

Demodulator

The demodulator contains a single-ended-to-differential

converter, two Gilbert-cell multipliers, and two fixed gain

stages. The IF signal should be AC coupled into IF_IN.

Internally, IF_IN is terminated with a 400Ω resistor to

GND and provides a gain of 14dB. This amplified IF sig-

nal is fed into the I and Q mixers for demodulation. The

multipliers mix the IF signal with the quadrature LO sig-

nals, resulting in baseband I and Q signals. The conver-

sion gain of the multipliers is 15dB. These signals are

further amplified by 21dB by the baseband amplifiers.

The baseband I and Q amplifier chains are DC coupled.

Local Oscillator

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

pled differential pair. Figure 2 shows the equivalent

local-oscillator circuit schematic. An external LC reso-

nant tank determines the oscillation frequency, and the

Q of this resonant tank affects the oscillator phase

noise. The oscillation frequency is twice the IF frequen-

cy, so that the quadrature phase generator can use two

latches to generate precise quadrature signals.

The oscillator may be overdriven by an external source.

The source should be AC coupled into TANK/TANK,

and should provide 200mVp-p levels. A choke (typically

2.2µH) is required between TANK and TANK. Differ-

ential input impedance at TANK/TANK is 10kΩ. For sin-

gle-ended drive, connect an AC bypass capacitor

(1000pF) from TANK to GND, and AC couple TANK to

the source.

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. The inphase sig-

nal (at half the local-oscillator frequency) is further

divided by four for the prescaler output.

Prescaler

The prescaler output, PRE_OUT, is buffered and swings

typically 0.35Vp-p with a 10kΩ and 6pF load. It can be

AC-coupled to the input of a frequency synthesizer.

Modulator

The modulator accepts I and Q differential baseband

signals up to 1.35Vp-p with frequencies up to 15MHz,

and upconverts them to the IF frequency. Since these

inputs are biased internally at around 1.5V, I and Q sig-

nals should be capacitively coupled into these high-

impedance ports (the differential input impedance is

approximately 44kΩ). The self-bias design yields very

low on-chip offset, resulting in excellent carrier sup-

_______________________________________________________________________________________ 7

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