Frequency-to-voltage converter

Frequency-to-voltage converters or frequency-to-voltage converters convert the frequency of a constant alternating voltage or a square wave into a frequency-dependent or even proportional direct voltage . The counterpart is a voltage-frequency converter .

realization

Frequency doubling of a square wave signal. The width of the needle pulses corresponds to the product RC. The mean value of the output voltage is proportional to the frequency.

The following options are available for frequency-voltage conversion:

1. Use of a frequency demodulator (usually not referred to as a frequency-voltage converter)
2. Series connection of a frequency triggered monoflop and a low pass
3. Generation of pulses of a defined length with an exclusive-OR gate after each rectangular edge and connected low-pass filter (mean value images)
4. Use of a frequency counter and connected digital-to-analog converter (by means of a microcontroller equipped for this purpose )
5. Operation of a voltage-controlled oscillator (VCO) in a phase-locked loop (PLL), which compares the control frequency with the frequency of the VCO: the control voltage of the VCO is the sought-after analog voltage

Variants 2 and 3 are used in particular for low frequencies, for engine control or in frequency and tachometers . The input voltage is first passed to a threshold switch ( Schmitt trigger ) in order to obtain a square-wave voltage with a steep edge from an oscillation with insufficiently steep switching edges .

In variant 2, one of the switching edges of the square-wave signal triggers a monoflop whose switch-on duration is below the shortest period of the input frequency.

In variant 3, an exclusive-or gate generates pulses of constant width with twice the input frequency, which speeds up the subsequent averaging.

The pulse train is directed to a low-pass filter, which forms the equivalent value from it. This is a signal proportional to the input frequency via the duty cycle . A disadvantage of both methods is the residual ripple ; if it is to be small, the inertia must be made large: the low-pass filter must have a significantly lower cut-off frequency than the input frequency. A higher-order low-pass filter can help.

application

• Tachometer , speedometer , analog issuing rate monitor
• Speed ​​and motor control (signal from an incremental encoder or from the motor itself)

literature

• Rainer Felderhoff: Electrical and electronic measurement technology. Basics - procedures - devices and systems. 8th edition. Carl Hanser Verlag, Munich 2007, ISBN 978-3-446-40571-4 .
• Herbert Bernstein: LF and HF measurement technology. Measuring with oscilloscopes, network analyzers and spectrum analyzers., Springer Vieweg Verlag, Wiesbaden 2015, ISBN 978-3-658-07377-0 .
• Kurt Bergmann: Electrical measurement technology . Electrical and electronic processes, installations and systems; 5th edition, Friedrich Vieweg & Sohn Verlag, Braunschweig 1993, ISBN 978-3-528-44080-0 .