Other IC oscillators are available as voltage-controlled oscillators (VCOs), with the output rate variable over some range according to an input control voltage. There are many uses for a VCO, and so there are many offerings from the chip manufacturers. Some of these have frequency ranges exceeding 1000:1. Examples
are the original NE566 and later designs like the ICL8038, MAX038, XR2206/7, and 74LS624–9 series.
The 74LS624 series, for example, generates digital logic-level outputs up to 20MHz and uses external RC’s to set the nominal frequency. Faster VCOs like the 1648 can
produce outputs to 200 MHz, and there are much higher frequency VCO techniques (like Gunn-diode oscillators and YIG oscillators) that operate in the many gigahertz range.
Where linearity is important, a precision voltage-to-frequency (V/F) converter like the AD537, LM331, or AD650 really does the job, with worst-case linearity of
0.15%, 0.01%, or 0.005%, respectively. Most VCOs use internal current sources to charge and discharge a capacitor, and many therefore provide triangle-wave outputs. The classic Exar XR2206 goes further – it includes a set of “soft” clamps to convert the triangle wave to a not-too great sine wave; they call this a sine shaper, and it produces a sine-looking wave with <1% distortion. Depending on external timing components, it goes from fractional hertz (at the low end) to 1MHz (at the high end),19 with a sweep range of 1000:1 and a temperature stability of frequency of 0.002%/◦C. You can also use it as a triangle-wave generator, in which mode it lets you adjust the duty cycle from 1% to 99%.
VCO chips sometimes have an awkward reference for the control voltage (e.g., the positive supply) and complicated symmetrizing schemes for sinewave output. It is our opinion that the ideal VCO has yet to be developed. Many of these chips can be used with an external quartz crystal, as we discuss shortly, for much higher accuracy and stability; in such cases the crystal simply replaces the capacitor.
You can make VCOs with techniques other than RC (or current-driven) relaxation oscillators.
|Engineer.Labs | Hardware Design Engineer (Freelancer)||
Articles, News, Tutorials