High-Dynamic-Range, Direct Up-/Downconversion

1500MHz to 2300MHz Quadrature Mod/Demod

________________________________________________________________________________________________

11

on the I and the Q inputs results in a -17dBm input

power level delivered to the I and Q internal 50Ω termi-

nations. This results in an RF output power of -26.6dBm.

External Diplexer

LO leakage at the RF port can be nulled to a level less

than -80dBm by introducing DC offsets at the I and Q

ports. However, this null at the RF port can be compro-

mised by an improperly terminated I/Q IF interface. Care

must be taken to match the I/Q ports to the driving DAC

term may leak back into the modulator’s I/Q input port

where it can mix with the internal LO signal to produce

additional LO leakage at the RF output. This leakage

effectively counteracts against the LO nulling. In addi-

tion, the LO signal reflected at the I/Q IF port produces a

residual DC term that can disturb the nulling condition.

As demonstrated in Figure 3, providing an RC termination

on each of the I+, I-, Q+, Q- ports reduces the amount of

LO leakage present at the RF port under varying temper-

ature, LO frequency, and baseband termination condi-

tions. See the

Typical Operating Characteristics for

details. Note that the resistor value is chosen to be 50Ω

with a corner frequency 1 / (2πRC) selected to adequate-

ness of the baseband response at the highest baseband

I+/I- and Q+/Q- effectively see the RC networks and thus

become terminated in 25Ω (R/2). The RC network pro-

RF Demodulator

The MAX2023 can also be used as an RF demodulator,

downconverting an RF input signal directly to base-

band. The single-ended RF input accepts signals from

1500MHz to 2300MHz with power levels up to +30dBm.

The passive mixer architecture produces a conversion

loss of typically 9.5dB. The downconverter is optimized

for high linearity and excellent noise performance, typi-

cally with a +38dBm IIP3, an input P1dB of +29.7dBm,

and a 9.6dB noise figure.

A wide I/Q port bandwidth allows the port to be used as

an image-reject mixer for downconversion to a quadra-

ture IF frequency.

The RF and LO inputs are internally matched to 50Ω.

Thus, no matching components are required, and only

DC-blocking capacitors are needed for interfacing.

Power Scaling with Changes

to the Bias Resistors

Bias currents for the LO buffers are optimized by fine

tuning resistors R1, R2, and R3. Maxim recommends

using ±1%-tolerant resistors; however, standard ±5%

values can be used if the ±1% components are not

readily available. The resistor values shown in the

Typical Application Circuit were chosen to provide

peak performance for the entire 1500MHz to 2300MHz

band. If desired, the current can be backed off from

this nominal value by choosing different values for R1,

R2, and R3. Contact the factory for additional details.

Layout Considerations

A properly designed PC board is an essential part of

any RF/microwave circuit. Keep RF signal lines as short

as possible to reduce losses, radiation, and induc-

tance. For the best performance, route the ground pin

traces directly to the exposed paddle under the pack-

age. The PC board exposed paddle MUST be connect-

ed to the ground plane of the PC board. It is suggested

that multiple vias be used to connect this paddle to the

lower level ground planes. This method provides a

good RF/thermal conduction path for the device. Solder

the exposed paddle on the bottom of the device pack-

age to the PC board. The MAX2023 evaluation kit can

be used as a reference for board layout. Gerber files

are available upon request at www.maxim-ic.com.

RF MODULATOR

LO

50

Ω

50

Ω

L = 11nH

C = 2.2pF

∑

L = 11nH

I

Q

50

Ω

50

Ω

C = 2.2pF

C = 2.2pF

0

°

90

°

MAX2023

Figure 3. Diplexer Network Recommended for DCS 1800/

PCS 1900 EDGE Transmitter Applications