2.4GHz Monolithic
Voltage-Controlled Oscillators
MAX2750/MAX2751/MAX2752
BYP
0.1µF
MAX2750
MAX2751
MAX2752
GND
FROM
SYNTHESIZER
LOOP FILTER
TUNE
OSCILLATOR
CORE
OUT
OUT TO MIXER/
SYNTHESIZER
220pF
V
CC1
GND
V
CC
SHDN
SHDN
BIAS
V
CC2
220pF
Figure 1. Typical Application Circuit
Detailed Description
Oscillator
The MAX2750/MAX2751/MAX2752 VCOs are imple-
mented as an LC oscillator topology, integrating all of
the tank components on-chip. This fully monolithic
approach provides an extremely easy-to-use VCO,
equivalent to a VCO module. The frequency is con-
trolled by a voltage applied to the TUNE pin, which is
internally connected to the varactor. The VCO core
uses a differential topology to provide a stable frequen-
cy versus supply voltage and improve the immunity to
load variations. In addition, there is a buffer amplifier
following the oscillator core to provide added isolation
from load variations and to boost the output power.
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. The input may incorporate
an extra RC filter stage to reduce high-frequency noise
and spurious 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 in order to
filter the noise present on the voltage source driving the
tuning line.
Output Buffer
The oscillator signal from the core drives an output
buffer amplifier. The amplifier is internally matched to
50Ω including an on-chip DC blocking capacitor. No
external DC blocking capacitor is required, eliminating
the need for any external components. The output
amplifier has its own V
CC
and GND pins to minimize
load-pulling effects. The amplifier boosts the oscillator
signal to a level suitable for driving most RF mixers.
Layout Issues
Always use controlled impedance lines (microstrip,
coplanar waveguide, etc.) for high-frequency signals.
Always place decoupling capacitors as close to the
V
CC
pins as possible; for long V
CC
lines, it may be nec-
essary to add additional decoupling capacitors located
further from the device. Always provide a low-induc-
tance path to ground, and keep GND vias as close to
the device as possible. Thermal reliefs on GND pads
are not recommended.
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