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voltage to frequency conversion


A function that is often useful is conversion from a voltage to a frequency (which is a wide-range voltage-controlled oscillator) or from a frequency to a voltage. This may be useful in telemetry, where a frequency can get through a link, such as a telephone line, while a direct current cannot. Another application is tachometry, where a voltage proportional to speed is desired, and an alternating voltage can easily be generated proportional to speed. It is also fun, and a good illustration of some of the circuits in other units of this series.

Since these functions are so useful, it is no surprise that integrated circuits have been created where everything needed, except for some resistors and capacitors to set the parameters and act as filters, is in one package. We will use such packages here. For voltage to frequency conversion, the LM331 is available in an 8-pin DIP, looking much like an op-amp. Going the other way, we will use the LM2907, which comes in a 14-pin DIP. Incidentally, the LM prefix refers specifically to National Semiconductor, but there are other sources, and these will use their own prefixes. The prefixes are so that government orders using them will also choose the manufacturer at the same time, under the supposition that a different prefix is a different part. Also, there are often similar parts with slightly different specifications, usually in the operating temperature range. The 331 is specified from 0°C to 70°C, the “commercial” range, while the 231 and 131 have extended temperature ranges (which are of no benefit to most users).

Let’s start by building a voltage to frequency converter. The insides of the 331 from an operational viewpoint are shown in the figure below. The pins numbers are marked beside the circles representing them. All the external components are shown that are necessary to make a working circuit. The diagram is a little complicated, so it will be explained piece by piece. All the stuff around the box marked “single shot” is for the purpose of holding the voltage at pin 1 (on the right) the same as the input voltage Vin (on the left). The 100k resistor and 0.1 μF capacitor are a low-pass filter to eliminate rubbish in the input voltage, and can be neglected in what follows. The input voltage and the voltage at pin 1 are fed to the inputs of a comparator, the triangle. When the voltage at pin 1 drops below the input voltage (comparing the voltages at pins 6 and 7), this comparator triggers the single shot, which then fires, saturating the output transistor and pulling pin 3 to ground.

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