RF2504 查看數據表（PDF） - RF Micro Devices

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RF2504

VCO/HIGH-ISOLATION BUFFER AMPLIFIER

RF Micro Devices 

RF2504

Preliminary

Application Notes

The RF2504 has two functional parts: an oscillator and

buffer amplifier. The functional blocks have separate

ground and VCC pins to increase the isolation and

reduce load pulling, one of the key design objectives.

An external resonator is used to add design flexibility,

and the loaded Q of this resonator will affect the perfor-

mance of the resulting oscillator.

To create an oscillation, negative resistance is gener-

ated at pin 2 with a circuit similar to a Colpitts oscillator.

The input impedance at pin 2, measured with a vector

network analyzer, is shown here in the data sheet. In

general, the impedance looks like a negative resis-

tance in series with a capacitor. The negative resis-

tance decays as the frequency increases. An oscillator

is created when an inductive element is placed on pin

2 that is the conjugate of the capacitive reactance. A

greater inductive element will create a lower frequency

of oscillation.

The S11 looking into pin 2 is also shown here in the

data sheet. It has return gain from 500 MHz to 2200

MHz at room temperature. The specified frequency

range of 750 MHz to 1500 MHz defines the region

where the output power is relatively flat. At lower and

higher frequencies, the power will tend to roll off from

the nominal value. The specified frequency range is

conservatively set to ensure oscillation and maintain

performance, but the RF2504 can be used over a

broader frequency range with degraded performance.

12

The overall Q of the external resonator will affect per-

formance. Lower Q means lower power, higher phase

noise, and more load pulling. If the Q is too low, the cir-

cuit will not oscillate. The IC is designed to oscillate

into a resonator with Q>10. The performance is mea-

sured with a microstrip resonator or high quality induc-

tor, which usually has a Q>50. These measurements

define the best performance that can be expected from

the ICs. Lower Q resonators, particularly those includ-

ing a lossy varactor, might have degraded perfor-

mance.

The specified output power is measured into a 50Ω

load. The IC has a high output impedance, and if

desired, output matching can be used to obtain more

power by transforming 50Ω into a higher impedance.

On the RF2504, this could be accomplished by simply

changing the values of the external output inductor and

capacitor.

12-4

Rev A2 010117

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