10
Jan
10

Electronics for Novices (and Experts!) – Voltage to Frequency – Part 1

This post is written to be understandable for electronics novices and interested laymen. Supplementary material will also be included in – Part 2 – for knowledgeable Engineers.

The voltage to frequency converter (VFC) is an important type of analog to digital converter (ADC). All such converters change a real-world, continuous-value signal, such as voltage, pressure, speed, or weight, into a form that is easily interpreted as a number. When designing industrial and commercial scales, I chose to use a VFC to measure weight. A scale base, containing transducers – devices that change the force from a weight into an electrical signal – provided that signal to the input of a VFC. The VFC converted the signal into a different signal, an output signal with oscillations that could be counted. The number from the count represented the weight.

The spring scale that many of us have in our bathrooms is an ADC. Our brains are the final component of that ADC, providing a decision for which dial reading is closest to the indicator line.

The VFC is a convenient type of ADC because it changes a voltage into something we can digitize very easily: frequency – a time measurement. Humans have been, for thousands of years, steadily  improving their ability to measure time.

The VFC is also usually capable of being continuously-integrating. This means that the input value (the ‘signal’) is measured without gaps. Unfortunately, most VFC’s that we can purchase, in ready-to-run form, have a distinct limitation. They are affected noticeably by temperature (when it is NOT the signal !). This error is mostly is two forms: an addition (offset) to the signal, or a change in conversion factor (a multiplier, or gain).

The following describes an utterly unique VFC, having ZERO first-order gain temperature coefficient. That is, it had almost no change in measurement with changes in temperature. I developed it in 1981 for use in precision weighing instruments. To this date, there is no other such VFC.

This is a charge-balancing VFC. Imagine having a glass measuring cup, with a tube providing a flow of water to be measured. A dipper, smaller than the measuring cup and having a precise volume, is used to remove water at intervals. The dipper is used when water reaches a certain height in the measuring cup. The dipper is, approximately, balancing the water to that height. Counting the number of dippers of water removed during a certain time period (events per time = frequency) is a measure of the rate that water is flowing into the measuring cup.

This Dipper VFC has a non-zero gain temperature coefficient. The dipper, generally, will change size with temperature. (The measuring cup size is irrelevant.) If the same dipper could also be used to put the water into the measuring cup, then counting dips:in and dips:out provides a measure with zero gain temperature coefficient.

Next: the real, electronic version of this Dipper VFC analogy.

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All original material Copyright James R. Stone 2010, except where specifically noted. Some images licensed under Creative Commons, or GNU Free Documentation License, or unlicensed and public domain.

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