10 to 1000 MHz Oscillator


Nowadays, it is no longer necessary to use
discrete components to build oscillators. Instead, many manufacturers
provide ready-made voltage-controlled oscillator (VCO)
ICs that need only a few frequency-determining external components. One
example is the RF Micro Devices RF2506. This IC operates with a supply
voltage between 2.7 and 3.6 V (3.3V nominal) and provides a low-noise
oscillator transistor with integrated DC bias setting. In addition, it
has an isolating buffer amplifier that strongly reduces the effects of
load variations (load pulling) on the oscillator. If a voltage less than
0.7V is applied to the power-down input (pin 8), the oscillator is shut
down and the current consumption drops from 9mA to less than 1µA. The VCO is enabled when the voltage on pin 8 is at least +3V.

Connecting the feedback capacitors C1 and C2 to pins 3 (FDBK) and 4 (VTUNE)
transforms the internal transistor into a Colpitts oscillator. A
resonator is also needed; here this consists of C4 and L1, and it is
coupled via C3. Keep the Q factor of the coil as high as possible (by
using an air-core coil, for example), to ensure a low level of phase
noise. Since most applications require a tuneable oscillator, the
varicap diode D1 (BBY40, BBY51, BB804 etc) can be used to adjust the
resonant frequency. The tuning voltage UTune is applied via a high
resistance. The value of the tuning voltage naturally depends on the
desired frequency range and the variable-capacitance diode (D1) that is
used. The table shows a number of suggestions for selecting the
frequency-determining components. If the frequency range is narrow, a
parallel-resonant circuit should be connected between the output pin and
+Vcc, to form the collector load for the output transistor.

This can be built using the same components as the oscillator resonator. With a broadband VCO,
use a HF choke instead, with a value of a few microhenries to a few
nanohenries, depending on the frequency band. In this case C6 is not
needed. The output level of this circuit is –3dBm with an LC load and –7
dBm with a choke load. The table that accompanies the schematic diagram
provides rough indications of component values for various frequencies.
It is intended to provide a starting point for experimentation. The
coupling between the variable-capacitance diode and C5 determines the
tuning range of the VCO. The manufacturer maintains an Internet site at http://www.rfmd.com, where you can find more information about this interesting oscillator IC.

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