Understanding VCO concepts

A voltage controlled oscillator circuit can be modelled as the combination of an amplifier with gain and a frequency dependent feedback loop as illustrated in Figure 1.

The general expression is    which states that the system will oscillate, provided βA = 1. At the frequency of oscillation, the total phase shift around the loop must be 360° and the magnitude of the open loop gain must be unity.

The common emitter circuit provides 180° phase shift. If the circuit is used with feedback from collector to base, the feedback circuit must provide additional 180° phase shift.

If a common base circuit is used, there is no phase shift between the emitter and collector signals, the feedback circuit must provide either 0° or full 360° phase shift.

Although the model shown in Figure 1 can be used to analyse and determine the necessary and sufficient conditions for oscillation, it is easier to use the model shown in Figure 2 where the analysis is performed in terms of a negative resistance concept.

Figure 1: An oscillator may be modelled as the combination of an amplifier and a feedback loop.

Figure 2: An oscillator may be modelled using a negative resistance concept.

This is based on the concept that a tuned circuit, once excited, will oscillate continuously if there is no resistive element present to dissipate the energy.

It is the function of the amplifier to generate the negative resistance or maintain oscillation by supplying an amount of energy equal to that dissipated. The selection of the circuit topology is dictated by:

• Frequency of oscillation;
• Tuning range;
• Choice of transistor;
• Type of resonator.

A bipolar transistor with capacitances between the base and emitter, and the emitter and ground, can be used to generate a negative resistance. Examples of these circuits are shown in Figure 3.

Figure 3: Form of oscillator circuit discussed (RF equivalent).

An inductive resonator may be used in any parallel resonant oscillator circuit such as a Colpitts or a Clapp. The resonator operates at a point where it resonates with the load capacitance.

The circuit of Figure 3 is preferred for high stability oscillator circuits because of the ease with which the resonator may be isolated from the load.

Consider the circuit in Figure 4(a). The design equations are derived by replacing the circuit by its equivalent circuit. This is illustrated in Figure 4(b).

Figure 4(a) and 4(b): Oscillator design evaluation for CLAPP circuit.

The input impedance seen to the right of the dotted line in Figure 4(b) is given by

The quantity (gm/² C₁/C₂) is negative indicating a negative resistance component and therefore to maintain oscillations we must have

where r is the series resistance of the resonator.

The frequency of oscillation is given by

The ratio C1/C2 is selected to be greater than 1 so that the circuit has sufficient loop gain for start-up at the lowest operating temperature. A VCO is obtained by replacing a fixed capacitor such as C3 with a varactor.

The bipolar transistor with an inductive reactance between the base and the ground can also generate a negative resistance (see Figure 5). This topology is preferred at higher frequencies. The oscillator is obtained by adding a tank circuit as shown in Figure 6.

Figure 5: Common base circuit configured for generating negative resistance.

Figure 6: Common base circuit configured as an oscillator.

There are two basic types of varactors: abrupt and hyperabrupt. The abrupt tuning diodes will provide a very high Q and will also operate over a very wide tuning voltage range (0 to 60). This diode provides the best phase noise performance because of its high quality factor.

The diodes, because of their linear voltage vs capacitance characteristic, will provide a much more linear tuning characteristic than the abrupt diodes. These are the best choice for wideband tuning VCOs.

An octave tuning range can be covered in less than 20 V tuning range.

Their disadvantage is that they have a much lower Q and therefore provide a phase noise characteristic higher than that provided by the abrupt types.

For a varactor diode, the capacitance is related to the bias voltage by the following equation:

where A is a constant, V_R is the applied reverse bias voltage, and Φ is the built-in potential which is 0.7 V for silicon diodes, and 1.2 V for gallium arsenide. For the analysis that follows, we may write

where A is the capacitance of the diode when V is one volt and n is a number between 0.3 and 0.6, but can be as high as two for an abrupt junction. The tank circuit of a typical VCO has a parallel tuned circuit consisting of an inductance L, a fixed capacitance Cf and a varactor diode C. The frequency of oscillation can be written as

Let ω_o be the angular frequency of the unmodulated carrier and V_o and C_o be the corresponding values of V and C. Then from Equation (3) we have

If V_o is modulated by a small voltage V, the carrier will be deviated by a small frequency δω.

Substituting for L and rearranging, we have

The oscillator tuning sensitivity K can then be written as