MAXIM MAX2605, MAX2609 Technical data

General Description

The MAX2605–MAX2609 are compact, high-performance
intermediate-frequency (IF) voltage-controlled oscillators
(VCOs) designed specifically for demanding portable
wireless communication systems. They combine monolith­ic construction with low-noise, low-power operation in a
tiny 6-pin SOT23 package.

These low-noise VCOs feature an on-chip varactor and
feedback capacitors that eliminate the need for external
tuning elements, making the MAX2605–MAX2609 ideal
for portable systems. Only an external inductor is
required to set the oscillation frequency. In addition, an
integrated differential output buffer is provided for dri­ving a mixer or prescaler. The buffer output is capable
of supplying up to -8dBm (differential) with a simple
power match. It also provides isolation from load
impedance variations.

The MAX2605–MAX2609 operate from a single +2.7V to
+5.5V supply and offer low current consumption. These IF
oscillators can cover the 45MHz to 650MHz frequency
range.

Applications

Cellular and PCS Mobile Phones

2.4GHz ISM Band

902MHz to 928MHz ISM Band

Land Mobile Radio

GPS Receivers

General-Purpose IF Oscillators

Features

♦ Small Size

♦ Integrated Varactor for Tuning

♦ Low Phase Noise

♦ Wide Application Frequency Range

♦ Differential or Single-Ended Outputs

♦ Single +2.7V to +5.5V Supply

♦ Ultra-Small SOT23-6 Package

♦ On-Chip Temperature-Stable Bias

♦ Low-Current Operation

MAX2605–MAX2609

45MHz to 650MHz, Integrated IF

VCOs with Differential Output

________________________________________________________________ Maxim Integrated Products 1

GND

OUT-TUNE

16OUT+

5 V

CC

IND

MAX2605
MAX2606
MAX2607
MAX2608
MAX2609

SOT23-6

TOP VIEW

2

34

19-1673; Rev 0a; 4/02

EVALUATION KIT MANUAL

AVAILABLE

Pin Configuration/

Functional Diagram

Ordering Information

PART

MAX2605

MAX2606

MAX2607 150 to 300

70 to 150

45 to 70

FREQUENCY

RANGE

(MHz)

SUPPLY

CURRENT

(mA)

1.9

2.1

2.1

MAX2608 300 to 500 2.7

PHASE

NOISE

(dBc/Hz)

-117

-112

-107

-100

MAX2609 500 to 650 3.6 -93

Selector Guide

AABE6 SOT23-6-40°C to +85°C

MAX2608EUT-T

AABD

AABC

AABB

TOP

MARK

6 SOT23-6

6 SOT23-6

6 SOT23-6

PIN-

PACKAGE

TEMP. RANGE

-40°C to +85°C

-40°C to +85°C

-40°C to +85°C

MAX2607EUT-T

MAX2606EUT-T

MAX2605EUT-T

PART

AABF6 SOT23-6-40°C to +85°C

MAX2609EUT-T

For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at
1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.

MAX2605–MAX2609

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

2 _______________________________________________________________________________________

ABSOLUTE MAXIMUM RATINGS

DC ELECTRICAL CHARACTERISTICS

(VCC= +2.7V to +5.5V, V

TUNE

= 0.4V to 2.4V, TA= -40°C to +85°C, unless otherwise noted. Typical values are at VCC= +2.75V,

V

TUNE

= 1.5V, and TA= +25°C.) (Note1)

Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.

VCCto GND..............................................................-0.3V to +6V

IND to GND ................................................-0.6V to (V

CC

+ 0.3V)

TUNE to GND .............................................-0.3V to (V

CC

+ 0.3V)

OUT+, OUT- to GND ..................................-0.3V to (V

CC

+ 0.6V)

Continuous Power Dissipation (T

A

= +85°C)

6-Pin SOT23 (derate

8.7mW/°C above +70°C) ...........696mW

Operating Temperature Range ...........................-40°C to +85°C

Junction Temperature......................................................+150°C

Storage Temperature Range .............................-65°C to +150°C

Lead Temperature (soldering, 10s) .................................+300°C

mA

MAX2609

TA= -40°C to +85°C

TA= +25°C

7.5

DC Output Current (Note 3) 0.5 1.0 1.5OUT+ plus OUT-

TA= -40°C to +85°C

TA= +25°C

TA= -40°C to +85°C

TA= +25°C

TA= -40°C to +85°C

TA= +25°C

TA= -40°C to +85°C

TA= +25°C

PARAMETER MIN TYP MAX UNITS

3.5

2.1 3.2

3.0

Supply Current (Note 2)

2.1 2.7

2.7 4.4

5.5

3.6 6.8

TUNE Input Current 0.03 nA

Supply Voltage 2.7 5.5

V

1.9 2.6

2.8

CONDITIONS

MAX2607

MAX2606

MAX2608

MAX2605

mA

45MHz to 650MHz, Integrated IF

VCOs with Differential Output

_______________________________________________________________________________________ 3

AC ELECTRICAL CHARACTERISTICS

(MAX2605–MAX2609 EV kits, VCC= +2.7V to +5.5V, V

TUNE

= 0.4V to 2.4V, TA= -40°C to +85°C, unless otherwise noted. Typical val-

ues are at V

CC

= +2.75V, V

TUNE

= 1.5V, and TA= +25°C.) (Note 1)

MAX2609, QL≥ 40

Peak Tuning Gain

14.5

%/VV

TUNE

= 0.4V to 0.6V step (Note 6)

TA= +25°C

-5.0 +3.8

500 650

MAX2609

MAX2608, QL≥ 40

MAX2607, QL≥ 35

MAX2606, QL≥ 35

MAX2605, QL≥ 35

-107

dBc/Hz

-100

-93

-117

Phase Noise (Note 8)

-112

f

OFFSET

= 100kHz

-4.7 +3.6

TA= +25°C

TA= +25°C

TA= +25°C

TA= +25°C

PARAMETER MIN TYP MAX UNITS

Guaranteed Frequency Limits
(relative to nominal) (Note 5)

-4.1 +3.2

300 500

-4.4 +3.4

-4.6 +3.6

Oscillator Nominal Frequency
Range (Note 4)

45 70

70 150

150 300

CONDITIONS

MAX2606

MAX2605

MAX2608

MAX2607

MAX2608

MAX2605

MAX2606

MAX2607

Single-Ended Output Power
(Note 7)

-10

dBm

TA= -40°C to +85°C

TA= -40°C to +85°C

TA= -40°C to +85°C

TA= -40°C to +85°C

TA= -40°C to +85°C

-2.25 +2.25

-2.5 +2.5

-2.75 +2.75

-2.8 +2.8

-3.0 +3.0

MAX2605–MAX2609

MAX2609

MHz

%

MAX2605–MAX2609

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

4 _______________________________________________________________________________________

AC ELECTRICAL CHARACTERISTICS (continued)

(MAX2605–MAX2609 EV kits, VCC= +2.7V to +5.5V, V

TUNE

= 0.4V to 2.4V, TA= -40°C to +85°C, unless otherwise noted. Typical val-

ues are at V

CC

= +2.75V, V

TUNE

= 1.5V, and TA= +25°C.) (Note 1)

Note 1: Production tested at TA= +25°C. Maximum and minimum over temperature limits are guaranteed by design and

characterization.

Note 2: Supply current is measured while the part is oscillating and inductor Q ≥ Q

MIN

. For MAX2605/MAX2606/MAX2607, Q

MIN

= 35;

for MAX2608/MAX2609, Q

MIN

= 40.

Note 3: The DC output current is the total available output signal current.
Note 4: Application range of the part is achieved using external inductance as specified in Figures 1-5 and shown in Figure 6. The inter-

nal varactors support center frequencies of 45MHz to 650MHz. The center frequency is defined by the value of the external
inductor element, L

F

. The application frequency limits are guaranteed by design and characterization.

Note 5: The guaranteed (tested) limits ƒ

MIN

and ƒ

MAX

are measured at V

TUNE

= 0.4V and V

TUNE

= 2.4V, respectively. Passing

requirements are: ƒ ≤ ƒ

MIN

at V

TUNE

= 0.4 and ƒ ≥ ƒ

MAX

at V

TUNE

= 2.4V. The nominal frequency of oscillation is defined by

the inductor.

Note 6: Describes peak tuning gain, which occurs at V

TUNE

= 0.4V.

Note 7: Measurement at OUT+ or OUT- matched for optimum power transfer into 50Ω load near the center of the operating frequency

range.

Note 8: The phase-noise specifications listed apply to the typical operating circuit shown in Figure 6. Apply over the entire operating

frequency range of the MAX2605–MAX2609.

Note 9: Supply pushing is measured with V

CC

stepped from +2.7V to +3.2V.

1.0

1.5

2.0

3.0

2.5

3.5

4.0

-40 0-20 20 40 60 80

SUPPLY CURRENT

vs. TEMPERATURE

MAX2605/9-01

TEMPERATURE (°C)

SUPPLY CURRENT (mA)

MAX2609

MAX2605

MAX2606MAX2607MAX2608

0

0.02

0.06

0.04

0.08

0.10

-40 0-20 20 40 60 80

TUNE INPUT LEAKAGE CURRENT

vs. TEMPERATURE

MAX2605/9-02

TEMPERATURE (°C)

LEAKAGE CURRENT (nA)

45

50

55

60

65

0 1.00.5 1.5 2.0 2.5 3.0

MAX2605

VCO TUNING CURVE

MAX2605/9-03

V

TUNE

(V)

FREQUENCY (MHz)

Typical Operating Characteristics

(MAX260_ EV kit, VCC= +2.75V, V

TUNE

= 1.4V, TA = +25°C, unless otherwise noted.)

dBcEven-Order Harmonics -30Differential, RL= 50Ω each side

PARAMETER MIN TYP MAX UNITS

60

CONDITIONS

MAX2605

120

MAX2606

480

MAX2608

220

MAX2607

Supply Pushing (Note 9)

720

kHz/V

MAX2609

_______________________________________________________________________________________ 5

MAX2605–MAX2609

45MHz to 650MHz, Integrated IF

VCOs with Differential Output

90

100

110

120

130

0 1.00.5 1.5 2.0 2.5 3.0

MAX2606

VCO TUNING CURVE

MAX2605/9-04

V

TUNE

(V)

FREQUENCY (MHz)

160

180

200

220

240

260

0 1.00.5 1.5 2.0 2.5 3.0

MAX2607

VCO TUNING CURVE

MAX2605/9-05

V

TUNE

(V)

FREQUENCY (MHz)

310

340

370

400

430

0 1.00.5 1.5 2.0 2.5 3.0

MAX2608

VCO TUNING CURVE

MAX2605/9-06

V

TUNE

(V)

FREQUENCY (MHz)

480

520

560

600

640

680

0 1.00.5 1.5 2.0 2.5 3.0

MAX2609

VCO TUNING CURVE

MAX2605/9-07

V

TUNE

(V)

FREQUENCY (MHz)

-50

-40

-30

-20

-10

0

4f

o5fo

2fo3f

o

f

o

6fo7f

o

MAX2605/9-08

FREQUENCY

(dB)

OUTPUT SPECTRUM

NAME FUNCTION

1 IND

Tuning Inductor Port. Connect an inductor from IND to GND to set VCO center frequency (see Oscillation
Frequency).

2 GND Ground. Connect to the ground plane with a low-inductance path.

PIN

3 TUNE Voltage-Control Input for Frequency Tuning. Input voltage range from +0.4V to +2.4V.

4 OUT-

High-Impedance Open-Collector Output. An external pull-up resistor or inductor to VCCis required. Output
power is dependent on external load impedance. OUT- is complementary to OUT+.

6 OUT+

High-Impedance Open-Collector Output. An external pull-up resistor or inductor to VCCis required. Output
power is dependent on external load impedance. OUT+ is complementary to OUT-.

5 V

CC

Supply Voltage Connection. Connect an external bypass capacitor to ground for low noise and low spuri­ous-output content. See Layout Issues for more details.

Pin Description

Typical Operating Characteristics (continued)

(MAX260_ EV kit, VCC= +2.75V, V

TUNE

= 1.4V, TA = +25°C, unless otherwise noted.)

MAX2605–MAX2609

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

6 _______________________________________________________________________________________

Detailed Description

The MAX2605–MAX2609 are low-noise VCOs designed
for fixed/single-frequency IF applications. The core
oscillator circuit is based on the well-known Colpitts
topology. The varactor and feedback capacitors are
integrated on-chip so that only an external inductor is
required to establish the frequency of oscillation and
produce a properly operating VCO. The tuning range,
biasing, startup, etc., are all managed within the IC. This
highly integrated design dramatically simplifies the
parts’ application.

The tuning range is wide enough so that, with the use
of ±2% tolerance inductors, no board-level adjustments
to the oscillation frequency are necessary. Once the
correct inductor value is chosen, the VCO is guaran­teed always to tune to the desired operating frequency.
In addition, with the use of inductors of moderate Q (35
to 40), the VCO achieves excellent phase-noise perfor­mance.

Applications Information

Desired Oscillation Frequency

The desired VCO operating frequency is set by the
value of the external inductance, LF. Figures 1–5 show
the inductance value LFrequired to achieve the desired
oscillation frequency. The inductor value can be taken
directly from these figures. Inductance must be select­ed accurately to ensure proper operation over all con­ditions.

Inductor Implementation

The inductance value required for the desired operat­ing frequency may not necessarily coincide with a stan­dard-value SMT inductor, which typically increases size
in ~1.2x steps. In such cases, the inductance must be
constructed from two inductors, L

F1

and LF2, in order to

achieve the desired inductance value. Choose L

F1

to

be a standard-value inductor with a value just less than

that required for L

F

. Choose LF2to be a standard-value

inductor with a value just less than (LF- LF1). L

F1

should adhere to the minimum Q requirements, but L

F2

may be implemented as a lower-cost, lower-Q, thin-film
SMT inductor. Its lower Q has only a small impact on
the overall Q of the total inductance because it is <20%
of the total inductance. However, the overall Q of L

F1

and LF2must be greater than the minimum inductor Q
(Table 1).

It is also permissible to use PC board traces to provide
a small amount of inductance, thereby adjusting the
total inductance value. On the MAX2608/MAX2609, the
inductance values for LF2are sometimes more exactly
implemented as a PC board trace (shorted to GND),
rather than an SMT inductor. When designing LF with
two inductors, use the simple model in Figure 7 to cal­culate XLand LEQ.

The LFin Figures 1–5 represents an equivalent induc­tance as seen by pin 1 (IND). The equivalent induc­tance corresponds to the inductive reactance
connected to IND at the desired oscillation frequency
(f

NOMINAL

).

LEQ= XL/ (2π f

NOMINAL

) as seen in Figure 8

Design LEQ= LFat the desired f

NOMINAL

. The
MAX2605–MAX2609 are designed to tolerate approxi­mately 0.5pF of external parasitic capacitance at IND.
This parasitic capacitance arises from the pad capaci­tance at the device pin and pads for the inductor.
Additional shunt capacitance is not recommended
because it degrades the tuning range.

Bypass Capacitor on TUNE

The MAX2605–MAX2609’s oscillator design uses a vari­ant of the Colpitts topology, where DC bias for the var­actor is applied via a DC voltage on TUNE and a
ground connection through the external inductor L

F

.

TUNE must also have a high-frequency AC ground for

Table 1. External Inductor LFRange

Table 2. C

BYPASS

Values

403.9 ≤ LF≤ 15500 to 650MAX2609

40

35

35

35

MIN

INDUCTOR

Q

10 ≤ LF≤ 47300 to 500MAX2608

39 ≤ LF≤ 180

150 ≤ LF≤ 820

680 ≤ LF≤ 2200

INDUCTANCE

RANGE

(nH)

FREQUENCY

RANGE

(MHz)

45 to 70

70 to 150

150 to 300MAX2607

MAX2606

MAX2605

PART

≥ 39 pFMAX2609

≥ 100 pFMAX2608

≥ 330 pF

≥ 680 pF

≥ 820 pF

C

BYPASS

MAX2607

MAX2606

MAX2605

DEVICE

MAX2605–MAX2609

45MHz to 650MHz, Integrated IF

VCOs with Differential Output

_______________________________________________________________________________________ 7

MAX2605

REQUIRED INDUCTANCE vs. DESIRED VCO FIXED FREQUENCY

1900

1800

1700

1600

1500

1400

1300

1200

1100

1000

900

800

700

REQUIRED INDUCTANCE (nH)

45

47

49 51 53 55

57

59 61 63 65 67

69

EFFECTIVE INDUCTANCE
FROM IND TO GND

INDUCTOR VALUE
MOUNTED ON EV KIT

MEASUREMENT CONDITIONS

VCC = 2.75V, TA = 25°C,

R

LOAD

= 100Ω||50Ω (100Ω RESISTIVE PULL-UP PARALLELED WITH 50Ω

VNA IMPEDANCE), UNUSED OUTPUT TERMINATED IN 50Ω,

PCB PARASITIC SHUNT CAPACITANCE (IND TO GND) = 0.45pF

THE INDUCTANCE LISTED IS THE PRECISE NOMINAL INDUCTANCE VALUE

REQUIRED FROM IND TO GND IN ORDER TO GUARANTEE THE VCO

CAN TUNE TO THE DESIRED FIXED FREQUENCY, OVER ALL OPERATING

CONDITIONS AND WORST-CASE COMPONENT

VALUES (±2% INDUCTOR AND IC PROCESS VARIATION).

DESIRED VCO FIXED FREQUENCY (MHz)

Figure 1. MAX2605 Required Inductance vs. Desired VCO Fixed Frequency

MAX2605–MAX2609

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

8 _______________________________________________________________________________________

MAX2606

REQUIRED INDUCTANCE vs. DESIRED VCO FIXED FREQUENCY

790

740

690

640

590

540

490

440

390

340

290

240

190

REQUIRED INDUCTANCE (nH)

75

80

85 90 95 100

105

110

115 120 125 130

135

EFFECTIVE INDUCTANCE
FROM IND TO GND

INDUCTOR VALUE
MOUNTED ON EV KIT

140

70 140

145 150

MEASUREMENT CONDITIONS

VCC = 2.75V, TA = 25°C,

R

LOAD

= 100Ω||50Ω (100Ω RESISTIVE PULL-UP PARALLELED WITH 50Ω

VNA IMPEDANCE), UNUSED OUTPUT TERMINATED IN 50Ω,

PCB PARASITIC SHUNT CAPACITANCE (IND TO GND) = 0.45pF

THE INDUCTANCE LISTED IS THE PRECISE NOMINAL INDUCTANCE VALUE

REQUIRED FROM IND TO GND IN ORDER TO GUARANTEE THE VCO

CAN TUNE TO THE DESIRED FIXED FREQUENCY, OVER ALL OPERATING

CONDITIONS AND WORST-CASE COMPONENT

VALUES (±2% INDUCTOR AND IC PROCESS VARIATION).

DESIRED VCO FIXED FREQUENCY (MHz)

Figure 2. MAX2606 Required Inductance vs. Desired VCO Fixed Frequency

MAX2605–MAX2609

45MHz to 650MHz, Integrated IF

VCOs with Differential Output

_______________________________________________________________________________________ 9

MAX2607

REQUIRED INDUCTANCE vs. DESIRED VCO FIXED FREQUENCY

160

150

140

130

120

110

100

90

80

70

60

50

40

REQUIRED INDUCTANCE (nH)

160

170

180 190 200 210 220

230

240 250 260 270 280

INDUCTOR VALUE
MOUNTED ON EV KIT

30

150

290

300

170

EFFECTIVE INDUCTANCE
FROM IND TO GND

MEASUREMENT CONDITIONS

VCC = 2.75V, TA = 25°C,

R

LOAD

= 100Ω||50Ω (100Ω RESISTIVE PULL-UP PARALLELED WITH 50Ω

VNA IMPEDANCE), UNUSED OUTPUT TERMINATED IN 50Ω,

PCB PARASITIC SHUNT CAPACITANCE (IND TO GND) = 0.45pF

THE INDUCTANCE LISTED IS THE PRECISE NOMINAL INDUCTANCE VALUE

REQUIRED FROM IND TO GND IN ORDER TO GUARANTEE THE VCO

CAN TUNE TO THE DESIRED FIXED FREQUENCY, OVER ALL OPERATING

CONDITIONS AND WORST-CASE COMPONENT

VALUES (±2% INDUCTOR AND IC PROCESS VARIATION).

DESIRED VCO FIXED FREQUENCY (MHz)

Figure 3. MAX2607 Required Inductance vs. Desired VCO Fixed Frequency

MAX2605–MAX2609

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

10 ______________________________________________________________________________________

MAX2608

REQUIRED INDUCTANCE vs. DESIRED VCO FIXED FREQUENCY

40.0

38.0

37.0

32.0

29.0

27.0

25.0

22.0

20.0

17.0

15.0

13.0

11.0

REQUIRED INDUCTANCE (nH)

310 320 330 340 350 360 370 380 390 400 410 420 470

9.0
300 480

41.0

420

430

440 450 460 490 500

36.0

35.0

34.0

33.0

31.0

30.0

28.0

26.0

24.0

23.0

21.0

19.0

18.0

16.0

14.0

12.0

10.0

EFFECTIVE INDUCTANCE
FROM IND TO GND

INDUCTOR VALUE
MOUNTED ON EV KIT

MEASUREMENT CONDITIONS

VCC = 2.75V, TA = 25°C,

R

LOAD

= 100Ω||50Ω (100Ω RESISTIVE PULL-UP PARALLELED WITH 50Ω

VNA IMPEDANCE), UNUSED OUTPUT TERMINATED IN 50Ω,

PCB PARASITIC SHUNT CAPACITANCE (IND TO GND) = 0.45pF

THE INDUCTANCE LISTED IS THE PRECISE NOMINAL INDUCTANCE VALUE

REQUIRED FROM IND TO GND IN ORDER TO GUARANTEE THE VCO

CAN TUNE TO THE DESIRED FIXED FREQUENCY, OVER ALL OPERATING

CONDITIONS AND WORST-CASE COMPONENT

VALUES (±2% INDUCTOR AND IC PROCESS VARIATION).

Figure 4. MAX2608 Required Inductance vs. Desired VCO Fixed Frequency

MAX2605–MAX2609

45MHz to 650MHz, Integrated IF

VCOs with Differential Output

______________________________________________________________________________________ 11

MAX2609

REQUIRED INDUCTANCE vs. DESIRED VCO FIXED FREQUENCY

13.5

13.0

11.5

11.0

10.0

9.5

9.0

8.5

7.5

6.5

6.0

5.5

5.0

REQUIRED INDUCTANCE (nH)

510

520

530 540 550 560 570

580

590 600 610 620 630

3.5
500

640

650

14.0

EFFECTIVE INDUCTANCE
FROM IND TO GND

INDUCTOR VALUE
MOUNTED ON EV KIT

12.5

12.0

10.5

8.0

7.0

4.5

4.0

DESIRED VCO FIXED FREQUENCY (MHz)

MEASUREMENT CONDITIONS

VCC = 2.75V, TA = 25°C,

R

LOAD

= 100Ω||50Ω (100Ω RESISTIVE PULL-UP PARALLELED WITH 50Ω

VNA IMPEDANCE), UNUSED OUTPUT TERMINATED IN 50Ω,

PCB PARASITIC SHUNT CAPACITANCE (IND TO GND) = 0.45pF

THE INDUCTANCE LISTED IS THE PRECISE NOMINAL INDUCTANCE VALUE

REQUIRED FROM IND TO GND IN ORDER TO GUARANTEE THE VCO

CAN TUNE TO THE DESIRED FIXED FREQUENCY, OVER ALL OPERATING

CONDITIONS AND WORST-CASE COMPONENT

VALUES (±2% INDUCTOR AND IC PROCESS VARIATION).

Figure 5. MAX2609 Required Inductance vs. Desired VCO Fixed Frequency

MAX2605–MAX2609

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 connect­ed 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; X7R­based capacitors are not suitable. Omitting this capac­itor 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 dif­ferentially; 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 out­put, and works well in applications that operate at lower
frequencies or only require a modest voltage swing.

In Figure 6, Z1 and Z2 are 1kΩ 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 1kΩ
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 net­work 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 VCCto
provide DC bias for the output stage. The series capac­itors are connected from OUT- and OUT+ to the load.
The values for L

MATCH(Z1

and Z2) and C

MATCH(C1

and C2) are chosen according to the operating fre­quency and load impedance. As the output stage is
essentially a high-speed current switch, traditional lin­ear 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
VCCand a voltage swing limit at the output pins OUT­and OUT+.

Layout Considerations

In general, a properly designed PC board is essential
to any RF/microwave circuit or system. Always use con­trolled impedance lines (microstrip, coplanar wave­guide, etc.) on high-frequency signals. Always place
decoupling capacitors as close to the VCCpin as pos­sible. For low phase noise and spurious content, use an
appropriate size decoupling capacitor. For long V

CC

lines, it may be necessary to add additional decoupling
capacitors located further from the device. Always pro­vide 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 sec­tion 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 L

F

and C

BYP

close

to their respective pins. Specifically, place C

BYP

direct-

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

For the higher frequency versions, consider the extra
parasitic inductance and capacitance when determin­ing 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 ground­ed side of the inductor and IC’s GND pin. For best
results, connect the “ground” side to the tuning induc­tor as close to pin 2 as possible. In addition, remove
the ground plane around and under LFand C

BYP

to

minimize the effects of parasitic capacitance.

V diff 2 1mA

1k R

1k R

OUTp p

LOAD

LOAD

−

()

=×

×
+

















Ω

Ω

MAX2605–MAX2609

45MHz to 650MHz, Integrated IF

VCOs with Differential Output

______________________________________________________________________________________ 13

Chip Information

TRANSISTOR COUNT: 158

C1

C3

R

LOAD

R

LOAD

C2

Z2

Z1

OUT-

TUNE

FROM PLL LOOP
FILTER OUTPUT

1

6

OUT+

5

V

CC

L

F

C

BYP

MAX2605
MAX2606
MAX2607
MAX2608
MAX2609

2

34

V

CC

Figure 6. Typical Operating Circuit

MAX2605–MAX2609

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

14 ______________________________________________________________________________________

IND

MAX2605
MAX2606
MAX2607
MAX2608
MAX2609

1

L

F2

C

PAR1

L

F1

C

PAR2

Figure 7. Simple Model of External Inductance

IND

MAX2605
MAX2606
MAX2607
MAX2608
MAX2609

1

LEQ = XL / 2π ƒ

NOMINAL

X

L

Figure 8. Inductive Reactance at Pin 1 (IND)

MAX2605
MAX2606
MAX2607
MAX2608
MAX2609

4

Z

L

V

CC

Γ

Figure 9. Output Matching Network

Package Information

6LSOT.EPS

Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.

Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 _____________________15

© 2002 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

45MHz to 650MHz, Integrated IF

VCOs with Differential Output