Layout Issues

A properly designed PC board is an essential part of any

RF/microwave circuit. On high-frequency inputs and out-

puts, use controlled-impedance lines and keep them as

short as possible to minimize losses and radiation. At

high frequencies, trace lengths that are on the order of

λ/10 or longer can act as antennas.

Keeping the traces short also reduces parasitic induc-

tance. Generally, 1in of a PC board trace adds about

20nH of parasitic inductance. The parasitic inductance

can have a dramatic effect on the effective inductance.

For example, a 0.5in trace connecting a 100nH inductor

adds an extra 10nH of inductance or 10%.

To reduce the parasitic inductance, use wider traces

and a solid ground or power plane below the signal

traces. Also, use low-inductance connections to ground

on all GND pins, and place decoupling capacitors

close to all VDD connections.

The EV kit PC board can serve as a reference design

for laying out a board using the MAX1479.

Detailed Description

Power-Down Control

The MAX1479 can be controlled externally using the

ENABLE connector. The IC draws approximately 0.2nA

(at room temperature) in shutdown mode. Jumper JU1

is used to control this mode. The shunt can be placed

between pins 1 and 2 to enable the device. Remove

the JU1 shunt for external control. See Table 1 for

jumper function descriptions.

Data Input

The MAX1479 EV kit transmits ASK and FSK data with

data rates of up to 100kbps (ASK) or 20kbps (FSK).

JU2 controls whether the MAX1479 transmits the ASK

carrier frequency (or the FSK high frequency), turns off

the PA (or transmits the FSK low frequency), or trans-

mits an external data stream. See Table 1.

REF_IN External Frequency Input

For applications where the correct frequency crystal is

not available, it is possible to directly inject an external

frequency through the REF_IN SMA (not provided).

Connect the SMA to a low-phase-noise generator. The

addition of C18 and C19 is necessary (use 0.01µF

capacitors).

Battery Operation

The MAX1479 EV kit can be powered by an external

power supply or by the supplied 3V coin-cell battery. Set

jumper JU3 to connect pins 2 and 3 for battery operation.

RF Output

The MAX1479 EV kit includes two SMA connectors for

RF output. RF_OUT is a standard SMA and is used to

connect the PA output to test equipment. Output is

matched to 50

Ω. ANTENNA_OUT is a reverse polarity

SMA and is used to connect to the 1/4-wave whip

antenna (not supplied). Note that resistor R2 (0

Ω)

needs to be added.

Modulation Mode Input

Jumper JU4 sets the mode of transmission. Set jumper

JU4 to connect pins 1 and 2 for FSK mode, 2 and 3 for

ASK mode.

FSK Frequency Deviation

The FSK deviation is set by jumpers JU5 through JU7.

The maximum deviation depends on the PC board para-

sitics. The EV kit max is around 50kHz (315MHz). If very

large FSK frequency deviations are desired, use a crystal

with a larger motional capacitance and/or reduce PC

board parasitic capacitances. One way to reduce para-

sitic capacitances on the EV kit is to remove C14, C15,

and C16 and move the crystal closer to the IC.

MAX1479 Evaluation Kit

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Table 1. JU1 Through JU4 Jumpers

Function

JUMPER

STATE

FUNCTION

1-2

RF carrier transmit enable

2-3

Power-down mode

JU1

N.C.

External power-down control

1-2

RF carrier transmit mode (ASK),

FSK high frequency (FSK)

2-3

PA off, PLL ON (ASK),

FSK low frequency (FSK)

JU2

N.C.

External data transmit

1-2

External supply operation

JU3

2-3

Battery operation

1-2

FSK mode

JU4

2-3

ASK mode