MAX2511 View Datasheet(PDF) - Maxim Integrated

Part Name

Description

Manufacturer

MAX2511

Low-Voltage IF Transceiver with Limiter and RSSI

Maxim Integrated 

Low-Voltage IF Transceiver

with Limiter and RSSI

Filter Sharing

In half-duplex or TDD applications, the number of exter-

nal filters can be minimized by combining transmit and

receive filter paths (Figure 5).

The 10.7MHz filter that is usually connected to the

TXIN, TXIN ports can be the same filter that is connect-

ed at LIMOUT and LIMOUT. To use the same filter, con-

nect TXIN to LIMOUT, and TXIN to LIMOUT.

The 425MHz SAW filter needed at the RXIN, RXIN ports

and the filter needed at TXOUT and TXOUT can be

shared in a similar manner. The RXIN, RXIN ports must

be DC blocked to prevent the bias voltage needed by

the TXOUT and TXOUT pins from entering the receiver.

When sharing filters in this manner, the transmitter out-

put port (TXOUT, TXOUT) and receiver input port (RXIN,

RXIN) matching networks must be modified. The receiv-

er port’s input impedance must be the parallel combi-

nation of the receiver and transmitter ports in Rx mode.

In this case, the receiver port is active, but the transmit-

ter port adds an additional parasitic impedance. See

the transmitter and receiver-port impedance graphs in

the Typical Operating Characteristics.

When the part is in transmit mode, the RXIN and RXIN

inputs provide back termination for the TXOUT and

TXOUT outputs so that a single IF filter can be connect-

ed (Figure 5). With this technique, the matching network

can be adjusted so the input VSWR is less than 1.5:1 in

Rx mode, and the output VSWR is less than 2:1 in Tx

mode.

Receive IF Filter

The interstage 10.7MHz filter, located between the

MIXOUT pin and the LIMIN pin, is not shared. This filter

prevents the limiter from acting on any undesired sig-

nals that are present at the mixer’s output, such as LO

feedthrough, out-of-band channel leakage, and other

mixer products. This filter is also set up to pass DC bias

voltage from the the VREF pin into the LIMIN and

MIXOUT pins through two filter-termination resistors

(330Ω—see the Typical Operating Circuit for more

information). If the filter can provide a DC shunt path,

such as a transformer-capacitor based filter or some L-C

filters, the two resistors can be combined into one par-

allel, equivalent resistor (165Ω) to reduce component

count (Figure 5—inset).

______________________Layout Issues

A well-designed PC board is an essential part of an RF

circuit. For best performance, pay attention to power-

supply issues, as well as the layout of the matching net-

works and tank circuit.

Power-Supply Layout

For minimizing coupling between different sections of the

chip, the ideal power-supply layout is a star configura-

tion, which has a heavily decoupled central VCC node.

The VCC traces branch out from this node, each going to

one VCC node on the MAX2511. At the end of each of

these traces is a bypass capacitor that is good at the

RF frequency of interest. This arrangement provides

local decoupling at each VCC pin. At high frequency,

any signal leaking from a supply pin sees a relatively

high impedance (formed by the VCC trace impedance)

to the central VCC node, and an even higher imped-

ance to any other supply pin.

Place the VREF decoupling capacitor (0.1µF typ) as

close to the MAX2511 as possible for best interstage fil-

ter performance. Use a high-quality, low-ESR capacitor

for best results.

Matching Network Layout

The TXOUT, TXOUT port requires a bias network that

consists of two inductors to VCC (for differential drive)

and optionally a back-termination resistor for matching

to an external filter. The RXIN, RXIN port also needs an

impedance-matching network. Both networks should be

symmetrical and as close to the chip as possible. See

the Typical Operating Circuit for more details. If you use

a ground-plane PC board, cut out the ground plane

under the matching network components to reduce

parasitic capacitance.

Local-Oscillator Tank Layout

Oscillator-tank circuit layout is critical. Parasitic PC

board capacitance, as well as trace inductance, can

affect oscillation frequency. Keep the tank layout sym-

metrical, tightly packed, and as close to the device as

possible. If a ground-plane PC board is used, the

ground plane should be cut out under the oscillator

components to reduce parasitic capacitance.

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