13.4 Summary
Oscillators and frequency generators find use in a wide variety of applications. They may be realized from simple single op amp topologies or use more elaborate special purpose integrated circuits. Basic op amp oscillators are usually constrained to the frequency range below 1 MHz. Two sine wave oscillators that are based on op amps are the Wien bridge and phase shift types. Both oscillators rely on positive feedback in order to create their outputs. To maintain oscillation, the amplifier/feedback loop must conform to the Barkhausen criterion. This states that in order to maintain oscillation, the loop phase must be 0 ∘ , or an integer multiple of 360 ∘ . Also, the product of the positive feedback loss and the forward gain must be greater than unity to start oscillations and revert to unity to maintain oscillation. In order to make the gain fall back to unity, some form of gain limiting device, such as a diode or lamp, is included in the amplifier’s negative feedback loop. The oscillation frequency is usually set by a simple resistor/capacitor network. As such, the circuits are relatively easy to tune. Their ultimate accuracy will depend on the tolerance of the tuning components and, to a lesser degree, on the characteristics of the op amp used.
Besides sine waves, other shapes such as triangles and squares may be produced. A simultaneous square/triangle generator may be formed from a ramp generator/comparator combination. Only two op amps are required to realize this design.
The voltage-controlled oscillator, or VCO, produces an output frequency that is dependent on an external control voltage. The free-running, or center, frequency is normally set via a resistor/capacitor combination. The control voltage may then be used to increase or decrease the frequency about the center point. The VCO is an integral part of the phase-locked loop, or PLL. The PLL has the ability to lock onto an incoming frequency. That is, its internal VCO frequency will match the incoming frequency. Should the incoming frequency change, the internal VCO frequency will change along with it. Two important parameters of the PLL are the capture range and the lock range. Capture range is the range of frequencies over which the PLL can acquire lock. Once lock is achieved, the PLL can maintain lock over a somewhat wider range of frequencies called the lock range. The PLL is in wide use in the electronics industry and is found in such applications as FM demodulation, FSK based communication systems, and frequency synthesis.
Timers can be used to generate rectangular waves of various duty cycles as well as single-shot pulses.
