10 to 1000 MHz Oscillator circuit with RF2506

Posted on Feb 6, 2014

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 no

10 to 1000 MHz Oscillator circuit with RF2506
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minal) 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 1 µA. 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...

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