MAX2608EUT-T datasheet(12/15 Pages) MAXIM | 45MHz to 650MHz, Integrated IF VCOs with Differential Output

Part No.	MAX2608EUT-T

Description	45MHz to 650MHz, Integrated IF VCOs with Differential Output

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Manufacturer	MAXIM [Maxim Integrated Products]

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12 / 15 page

45MHz to 650MHz, Integrated IF

VCOs with Differential Output

12

______________________________________________________________________________________

the cathode of the varactor. This is accomplished

through the use of a simple bypass capacitor connect-

ed from TUNE to ground. The value of this capacitor

should be greater than or equal to the values listed in

Table 2.

This capacitor provides an AC “short” to ground for the

internal node of the varactor. It is acceptable to select

the next-largest standard-value capacitor. Use a

capacitor with a low-loss dielectric such as NPO; X7R-

based capacitors are not suitable. Omitting this capac-

itor would affect the tuning characteristics of the

MAX2605–MAX2609. Proper operation of the VCOs

requires the use of this bypass capacitor.

The MAX2605–MAX2609 VCO is designed to tune over

the full tuning range with a voltage range of 0.4V to

2.4V applied to TUNE. This voltage typically originates

from the output of the phase-locked (PLL) loop filter.

Output Interface

The MAX2605–MAX2609 VCO includes a differential

output amplifier after the oscillator core. The amplifier

stage provides valuable isolation and offers a flexible

interface to the IF stages, such as a mixer and PLL

prescaler. The output can be taken single ended or dif-

ferentially; however, the maximum output power and

lowest harmonic output are achieved in the differential

output mode.

Both outputs (OUT- and OUT+) are open-collector

types and require a pull-up element to VCC; this can be

either resistive or inductive. A resistor pull-up is the

most straightforward method of interfacing to the out-

put, and works well in applications that operate at lower

frequencies or only require a modest voltage swing.

In Figure 6, Z1 and Z2 are 1k

Ω pull-up resistors that are

connected from OUT+ and OUT- to VCC, respectively.

These resistors provide DC bias for the output amplifier

and are the maximum value permitted with compliance

to the output voltage swing limits. In addition, the 1k

Ω

resistors maximize the swing at the load. DC-blocking

capacitors are connected from OUT- and OUT+ to the

load. If the load driven is primarily resistive and the

VCO operating frequency is below the -3dB bandwidth

of the output network, then the peak-to-peak differential

signal amplitude is approximately:

To optimize the output voltage swing or the output

power, use a reactive power match. The matching net-

work is a simple shunt-inductor series-capacitor circuit,

as shown in Figure 6. The inductors are connected

from OUT- and OUT+ (in place of resistors) to VCC to

provide DC bias for the output stage. The series capac-

itors are connected from OUT- and OUT+ to the load.

The values for LMATCH (Z1 and Z2) and CMATCH (C1

and C2) are chosen according to the operating fre-

quency and load impedance. As the output stage is

essentially a high-speed current switch, traditional lin-

ear impedance using techniques with [S] parameters

do not apply. To achieve a reactive power match, start

with the component values provided in the EV kit, and

adjust values experimentally.

In general, the differential output may be applied in any

manner, as would conventional differential outputs. The

only constraints are the need for a pull-up element to

VCC and a voltage swing limit at the output pins OUT-

and OUT+.

Layout Considerations

In general, a properly designed PC board is essential

to any RF/microwave circuit or system. Always use con-

trolled impedance lines (microstrip, coplanar wave-

guide, etc.) on high-frequency signals. Always place

decoupling capacitors as close to the VCC pin as pos-

sible. For low phase noise and spurious content, use an

appropriate size decoupling capacitor. For long VCC

lines, it may be necessary to add additional decoupling

capacitors located further from the device. Always pro-

vide a low-inductance path to ground. Keep the GND

vias as close to the device as possible. In addition, the

VCO should be placed as far away from the noisy sec-

tion of a larger system, such as a switching regulator or

digital circuits. Use star topology to separate the

ground returns.

The resonator tank circuit (LF) is critical in determining

the VCO’s performance. For best performance, use

high-Q components and choose values carefully. To

minimize the effects of parasitic elements, which

degrade circuit performance, place LF and CBYP close

to their respective pins. Specifically, place CBYP direct-

ly across pins 2 (GND) and 3 (TUNE).

For the higher frequency versions, consider the extra

parasitic inductance and capacitance when determin-

ing the oscillation frequency. Be sure to account for the

following: PC board pad capacitance at IND, PC board

pad capacitance at the junction of two series inductors,

series inductance of any PC board traces, and the

inductance in the ground return path from the ground-

ed side of the inductor and IC’s GND pin. For best

results, connect the “ground” side to the tuning induc-

tor as close to pin 2 as possible. In addition, remove

the ground plane around and under LF and CBYP to

minimize the effects of parasitic capacitance.

V

diff

2

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1k

R

1k

R

OUTp p

LOAD

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