Detailed Description

Oscillator

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.

Output Buffer

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-

pins to minimize load-pulling effects. The amplifier

boosts the oscillator signal to a level suitable for driving

most RF mixers.

Applications Information

Tune Input

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.

Layout Issues

Always use controlled impedance lines (microstrip,

coplanar waveguide, etc.) for high-frequency signals.

Always place decoupling capacitors as close to the

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

4

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NAME

FUNCTION

1

N.C.

No Connection. Not internally connected.

2

TUNE

Oscillator Frequency Tuning Voltage Input. High-impedance input with a voltage input range of 0.4V

(low frequency) to 2.4V (high frequency) adjustment.

PIN

3

GND

Ground Connection for Oscillator and Biasing. Requires a low-inductance connection to the circuit board

ground plane.

4

SHDN

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.

8

GND

Ground Connection for Output Buffer. Requires a low-inductance connection to the circuit board

ground plane.

7

OUT

Buffered Oscillator Output

6

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.

5

Output Buffer DC Supply Voltage Connection. Bypass with a 220pF capacitor to GND for best high-

frequency performance.

Pin Description