Datasheet

MAX2754
Applications Information
Tune Input
The tuning input is typically connected to the output of
the PLL loop filter. The loop filter provides an appropri-
ately low-impedance source. Incorporate an extra RC
filter stage to reduce high-frequency noise and spuri-
ous signals. Any excess noise on the tuning input is
directly translated into FM noise, which can degrade
the phase-noise performance of the oscillator.
Therefore, it is important to minimize the noise intro-
duced on the tuning input. A simple RC filter with low
corner frequency is needed during testing to filter the
noise present on the voltage source driving the tuning
line.
Two-Level FSK Applications
The MAX2754 is designed for use in FSK applications
operating in the 2.4GHz to 2.5GHz ISM band. Specifically,
it is targeted for those systems which utilize a direct TX
modulation architecture in which the VCO is directly modu-
lated with the data signal during the transmit (TX) mode.
The VCO in these systems runs at half the RF output
frequency and is used in conjunction with a frequency
doubler to produce the final LO signal for both RX and
TX modes of operation.
Figure 2 shows a typical applications circuit. To com-
pute R1, R2, R3, and R4, determine the modulation
voltage center point (V
MODB
= +1.4V). Compute the
required modulation voltage deviation as follows:
1.2GHz VCO with Linear
Modulation Input
6 _______________________________________________________________________________________
OUTPUT FREQUENCY (MHz)
MODULATION VOLTAGE, V
MOD
(V)
1.0
f
NOM
f
NOM
+ f
MOD
f
NOM
- f
MOD
2.01.5 2.50.5
Figure 1. Modulation Frequency Deviations Characteristics
SHDN
FROM BASEBAND
DIGITAL
CONTROLLER
GND2
OUT
V
CC
V
CC
MOD
GND1
R1
R2
R4
R3
TUNE
FROM PLL
LOOP FILTER
VREG
330nF
OUT TO
MIXER/
SYNTHESIZER
330nF
INVERTED
CMOS
SIGNAL
MAX2754
REG AND BIAS
MOD.
FILTER
OSCILLATOR
CORE
LINEAR
MODULATION
INTERFACE
V
CC
Figure 2. Typical Application Circuit for Two-Level FSK