Many circuit arrangements can be used to produce oscillation. The following several circuits are all known as variable-frequency oscillators (VFOs), because their frequencies are adjustable over a wide range.
The Armstrong Circuit
An Armstrong oscillator using an N-channel JFET. This is a common-source amplifier with positive feedback through a tuned circuit.
A common emitter or common source class A amplifier can be made to oscillate by coupling the output back to the input through a transformer that reverses the phase of the fed-back signal. The schematic diagram of above figure shows a common-source amplifier whose drain circuit is coupled to the gate circuit by means of a transformer. The frequency is controlled by a capacitor in series with the secondary winding. The inductance of the secondary, along with the capacitance, forms a resonant circuit that passes energy easily at one frequency while attenuating the energy at other frequencies. This circuit is known as an Armstrong oscillator. A bipolar transistor can be used in place of the JFET, as long as the device is biased for class A amplification.
The Hartley Circuit
A Hartley oscillator using a PNP bipolar transistor. The Hartley circuit can be recognized by the tapped inductor in the tuned LC circuit.
Another method of obtaining controlled feedback at RF is shown in above figure. In this circuit, a PNP bipolar transistor is used. The circuit uses a single coil with a tap on the windings to provide the feedback. A variable capacitor in parallel with the coil determines the oscillating frequency, and allows for frequency adjustment. This circuit is called a Hartley oscillator.
In the Hartley circuit, as well as in most other RF oscillator circuits, it is important to use only the minimum amount of feedback necessary to get oscillation. The amount of feedback is controlled by the position of the coil tap. The circuit shown in above figure uses about 25 percent of its amplifier power to produce feedback. The other 75 percent of the power can be used as output. Oscillators usually produce less than 1 W of RF power output. If more power is needed, the signal can be boosted by one or more stages of amplification.
The Colpitts Circuit
A Colpitts oscillator using a P-channel JFET. The Colpitts circuit can be recognized by the split capacitance in the tuned LC circuit.
The capacitance can be tapped, instead of the inductance, in the tuned circuit of an RF oscillator. Such a circuit is called a Colpitts oscillator, and a P-channel JFET version is diagrammed in above figure. The amount of feedback is controlled by the ratio of the capacitances. A variable inductor provides for frequency adjustment. This is a matter of convenience, because it can be difficult to find a dual variable capacitor that maintains the correct ratio of capacitances throughout its tuning range.
Using fixed capacitors eliminates this problem, and it costs less, too!
Unfortunately, finding a good variable inductor for use in a Colpitts oscillator can be just about as hard as obtaining a suitable dual-gang variable capacitor. A permeability-tuned coil can be used, but ferromagnetic cores impair the frequency stability of an RF oscillator. A roller inductor can be employed, but these are bulky and expensive. An inductor with several switch-selectable taps can be used, but this does not allow for continuous frequency adjustment. Despite these shortcomings, the Colpitts circuit offers exceptional stability and reliability when properly designed, and is preferred by some engineers for this reason.
The Clapp Circuit
A series-tuned Colpitts oscillator, also known as a Clapp oscillator. This circuit uses an NPN bipolar transistor.
A variation of the Colpitts oscillator makes use of series resonance, instead of parallel resonance, in the tuned circuit. Otherwise, the circuit is basically the same as the parallel-tuned Colpitts oscillator. Above figure is a schematic diagram of a series-tuned Colpitts oscillator circuit that uses an NPN bipolar transistor. This circuit is also known as a Clapp oscillator. Its frequency won’t change much when high-quality components are used. The Clapp oscillator is a reliable circuit. It isn’t hard to get it to oscillate and keep it going. Another advantage of the Clapp circuit is that it allows the use of a variable capacitor for frequency control, while accomplishing feedback through a capacitive voltage divider.
Getting the Output
In the Hartley, Colpitts, and Clapp oscillators just described and shown in (The Hartley Circuit figure) through just above figure, the output is taken from the emitter or source, not from the collector or drain. There’s a reason for this. The output of an oscillator can be taken from the collector or drain, just as is done in a common emitter or common-source amplifier to get maximum gain. But in an oscillator, stability is more important than gain. The stability of an oscillator is better when the output is taken from the emitter or source, as compared with taking it from the collector or drain. Variations in the load impedance are less likely to affect the frequency of oscillation, and a sudden decrease in load impedance is less likely to cause the circuit to stop oscillating.
To prevent the output signal from being short-circuited to ground, an RF choke (RFC) is connected in series with the emitter or source in the Colpitts and Clapp oscillator circuits. The choke lets dc pass while blocking ac (just the opposite of a blocking capacitor). Typical values for RF chokes range from about 100 μH at high frequencies, such as 15 MHz, to 10 mH at low frequencies, such as 150 kHz.
The Voltage-Controlled Oscillator
The frequency of a VFO can be adjusted by means of a varactor diode in the tuned LC circuit. Recall that a varactor, also called a varicap, is a semiconductor diode that works as a variable capacitor when it is reverse-biased. The capacitance depends on the reverse-bias voltage. The greater this voltage, the lower the value of the capacitance.
The Hartley and Clapp oscillator circuits lend themselves well to varactor-diode frequency control. The varactor is placed in series or parallel with the tuning capacitor, and is isolated for dc by blocking capacitors. Varactors are cheaper than variable capacitors or inductors. They’re also less bulky. These are the chief advantages of a VCO over an old-fashioned LC tuned VFO.
At ultrahigh frequencies (UHF) and microwave radio frequencies, certain types of diodes can be used as oscillators.
A Pierce oscillator circuit using an N-channel JFET.
Quartz crystals can be used in place of tuned LC circuits in RF oscillators, as long as it isn’t necessary to change the frequency often. Crystal oscillators offer frequency stability far superior to that of LC tuned VFOs.
There are several ways that crystals can be connected in bipolar or FET circuits to get oscillation. One common circuit is the Pierce oscillator. An N-channel JFET and quartz crystal are connected in a Pierce configuration as shown in the schematic diagram of above figure. The crystal frequency can be varied somewhat (by about 0.1 percent, or 1 part in 1000) by means of an inductor or capacitor in parallel with the crystal. But the frequency is determined mainly by the thickness of the quartz wafer, and by the angle at which it is cut from the original quartz sample.
Crystals change in frequency as the temperature changes. But they are far more stable than LC circuits, most of the time. Some crystal oscillators are housed in temperature-controlled chambers called crystal ovens. In this environment, crystals maintain their frequency so well that they are sometimes used as frequency standards against which other oscillators are calibrated.
The Phase-Locked Loop
One type of oscillator that combines the flexibility of a VFO with the stability of a crystal oscillator is known as a phase-locked loop (PLL). This makes use of a circuit called a frequency synthesizer. The heart of the PLL is a VCO. The output of this oscillator passes through a programmable multiplier/divider, a digital circuit that divides and/or multiplies the VCO frequency by integral (whole-number) values chosen by the operator. As a result, the output frequency can be any rationalnumber multiple of the crystal frequency. A well-designed PLL circuit can be tuned in small digital increments over a wide range of frequencies.
The output frequency of the multiplier/divider is locked, by means of a phase comparator, to the signal from a crystal-controlled reference oscillator. As long as the output from the multiplier/divider is exactly on the reference oscillator frequency, the two signals are in phase, and the output of the phase comparator is 0 V dc. If the VCO frequency begins to drift, the output frequency of the multiplier/divider will drift, too (although at a different rate). But even a frequency change of less than 1 Hz causes the phase comparator to produce a dc error voltage. This error voltage is either positive or negative, depending on whether the VCO has drifted higher or lower in frequency. The error voltage is applied to a varactor, causing the VCO frequency to change in a direction opposite to that of the drift. This forms a dc feedback circuit that maintains the VCO frequency at a precise value. It is a loop circuit that locks the VCO onto a particular frequency by means of phase sensing, hence the term phase-locked loop (PLL).
Block diagram of a phase-locked loop (PLL).
The key to the stability of the PLL lies in the fact that the reference oscillator is crystalcontrolled. A block diagram of a PLL circuit is shown in above figure. When you hear that a radio receiver, transmitter, or transceiver is synthesized, it usually means that the frequency is determined by a PLL.
The stability of a synthesizer can be enhanced by using an amplified signal from the shortwave time-and-frequency broadcast station WWV at 2.5, 5, 10, or 15 MHz, directly as the reference oscillator. These signals are frequency-exact to a minuscule fraction of 1 Hz, because they are controlled by atomic clocks. Most people don’t need precision of this caliber, so you won’t see consumer devices like ham radios and shortwave receivers with primary-standard PLL frequency synthesis. But it is employed by some corporations and government agencies.