Preliminary
12-4
RF2504
Rev A2 010117
12
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 intopin 2is alsoshown 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.
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 50
Ω
load. The IC has a high output impedance, and if
desired, output matching can be used to obtain more
power by transforming 50
Ω into a higher impedance.
On the RF2504, this could be accomplished by simply
changing the values of the external output inductor and
capacitor.
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