The MAX2622/MAX2623/MAX2624 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.
The oscillator signal from the core drives an output
buffer amplifier. The amplifier is constructed as a com-
mon-emitter stage with an integrated on-chip reactive
output match. No external DC blocking capacitor is
required, eliminating the need for any external compo-
nents. The output amplifier has its own VCC and GND
pins to minimize load-pulling effects. The amplifier
boosts the oscillator signal to a level suitable for driving
most RF mixers.
The tuning input is typically connected to the output of
the PLL loop filter. The loop filter is presumed to pro-
vide an appropriately low-impedance source. It may
incorporate an extra RC filter stage to reduce high-fre-
quency 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 introduced 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.
Always use controlled impedance lines (microstrip,
coplanar waveguide, etc.) for high-frequency signals.
Always place decoupling capacitors as close to the
VCC pins as possible; for long VCC 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.
Monolithic Voltage-Controlled Oscillators
No Connection. Not internally connected.
Oscillator Frequency Tuning Voltage Input. High-impedance input with a voltage input range of 0.4V
(low frequency) to 2.4V (high frequency) adjustment.
Ground Connection for Oscillator and Biasing. Requires a low-inductance connection to the circuit board
Shutdown Logic Input. A high-impedance input logic level low disables the device and reduces supply current
to 0.1µA. A logic level high enables the device.
Ground Connection for Output Buffer. Requires a low-inductance connection to the circuit board
Buffered Oscillator Output
Bias and Oscillator DC Supply Voltage Connection. Bypass with a 220pF capacitor to GND for low noise
and low spurious content performance from the oscillator.
Output Buffer DC Supply Voltage Connection. Bypass with a 220pF capacitor to GND for best high-