10MHz to 1050MHz Integrated

RF Oscillator with Buffered Outputs

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7

_______________Detailed Description

Oscillator

The oscillator is a common-collector, negative-

resistance type that uses the IC’s internal parasitic ele-

ments to create a negative resistance at the base-

emitter port. The transistor oscillator has been opti-

mized for low-noise operation. Base and emitter leads

are provided as external connections for a feedback

capacitor and resonator. A resonant circuit, tuned to

the appropriate frequency and connected to the base

lead, will cause oscillation. Varactor diodes may be

used in the resonant circuit to create a voltage-con-

trolled oscillator (VCO). The oscillator is internally

biased to an optimal operating point, and the base and

emitter leads need to be capacitively coupled due to

the bias voltages present.

Output Buffers

The output buffers (OUT and OUT) are an open-

collector, differential-pair configuration and provide

load isolation to the oscillator. The outputs can be used

differentially to drive an integrated circuit mixer.

Alternatively, isolation is provided between the buffer

outputs when one output drives a mixer (either upcon-

version or downconversion) and the other output drives

a prescaler. The isolation in this configuration prevents

prescaler noise from corrupting the oscillator signal’s

spectral purity.

A logic-controlled SHDN pin turns off all bias to the IC

when pulled low.

__________Applications Information

Design Principles

At the frequency of interest, the MAX2620 portion of

Figure 2 shows the one-port circuit model for the TANK

pin (test port in Figure 1).

For the circuit to oscillate at a desired frequency, the res-

onant tank circuit connected to TANK must present an

impedance that is a complement to the network

(Figure 2). This resonant tank circuit must have a positive

real component that is a maximum of one-half the magni-

tude of the negative real part of the oscillator device, as

well as a reactive component that is opposite in sign to

the reactive component of the oscillator device.

LESS THAN 1/2

TANK

RESONANT

TANK

OSCILLATOR

DEVICE

Figure 2. Simplified Oscillator Circuit Model

MAX2620

1

2

VCC1

TANK

FDBK

SHDN

3

4

8

OUT

VCC2

OUT

GND

7

6

5

TEST PORT

BIAS

SUPPLY

C13*

1000pF

C2*

1000pF

ON

OFF

1000pF

1000pF

1000pF

10

Ω

10

Ω

L3*

220nH

51

Ω

C3*

2.7pF

C4*

1pF

1000pF

1000pF

C10*

1000pF

*AT 10MHz, CHANGE TO:

C3 = C4 = 270pF

L3 = 10

µH

C2 = C10 = C13 = 0.01

µF

OUT

OUT

Figure 1. 900MHz Test Circuit