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
The following options are available for frequency-voltage conversion:
- Use of a frequency demodulator (usually not referred to as a frequency-voltage converter)
- Series connection of a frequency triggered monoflop and a low pass
- Generation of pulses of a defined length with an exclusive-OR gate after each rectangular edge and connected low-pass filter (mean value images)
- Use of a frequency counter and connected digital-to-analog converter (by means of a microcontroller equipped for this purpose )
- 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 .