MAX2511EEI Maxim Integrated Products, MAX2511EEI Datasheet - Page 13

• Manufacturer Part Number: MAX2511EEI
• Description: RF Transceiver IF Txr w/Limitr RSSI
• Manufacturer: Maxim Integrated Products

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.
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 V
MIXOUT pins through two filter-termination resistors
(330 —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 (165 ) to reduce component
count (Figure 5—inset).
______________________________________________________________________________________
REF
pin into the LIMIN and
Receive IF Filter
Filter Sharing
Low-Voltage IF Transceiver
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.
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 V
The V
one V
these traces is a bypass capacitor that is good at the
RF frequency of interest. This arrangement provides
local decoupling at each V
any signal leaking from a supply pin sees a relatively
high impedance (formed by the V
to the central V
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.
The TXOUT, TXOUT port requires a bias network that
consists of two inductors to V
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.
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.
______________________Layout Issues
with Limiter and RSSI
CC
CC
traces branch out from this node, each going to
node on the MAX2511. At the end of each of
CC
Local-Oscillator Tank Layout
node, and an even higher imped-
Matching Network Layout
CC
Power-Supply Layout
CC
pin. At high frequency,
CC
(for differential drive)
trace impedance)
CC
node.
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