Voltage to Frequency Converter Circuit

A Voltage to Frequency Converter (VFC) is an electronic device or circuit that converts an input voltage signal into a proportional output frequency. The output frequency is typically a square wave or pulse train, where the frequency varies in direct proportion to the magnitude of the input voltage. VFCs are useful for applications where a frequency signal is preferred over an analog voltage signal, often for reasons related to noise immunity, ease of transmission, or the ability to interface with digital systems.

How it Works:

1. Input Voltage: The input analog voltage is fed into the VFC, typically within a specific voltage range.
2. Frequency Generation: The circuit internally converts the analog voltage into a proportional frequency signal. This is done by comparing the voltage to a reference signal and generating pulses at a rate determined by the input voltage.
3. Output Frequency: The output is a frequency signal, typically a pulse train, where the frequency is directly proportional to the input voltage.

Voltage to Frequency Converter Circuit

Now here we will show the how to design a voltage to frequency converter circuit and explain how the circuit works. Here two LM358N op-amps are used, the first input op-amp is designed to operate as a comparator while the second op-amp is designed to operate as an integrator. The circuit diagram of the Voltage to Frequency Converter(VFC) circuit diagram is shown below.

If Vin is the analog input signal, if the output of first comparator op-amp is Vo1, if the output of the op-amp integrator is Vo2 then the frequency f of the output signal Vo2 is,,

\[f =k V_{in}\]

where k(Hz/V) is the sensitivity of the voltage to frequency converter(VFC).

Voltage to Frequency Converter(VFC) Circuit Working Principle

Here the voltage to frequency converter(VFC) circuit works. The output of the comparator is either positive or negative. Say the comparator output is negative(-Vo1), the diode 1N4184 is forward biased and therefore the capacitor(C) of the op-amp integrator starts charging up. The charging current is,

\[I_c=-\frac{V_{o2}}{R_d}\]

Since typically, Rd<<R, the capacitor charges rapidly.

During this charging process, the output Vo2 is ramps up upto the saturation upper threshold voltage of the operational amplifier. When the output voltage Vo2 reaches the threshold voltage, the comparator is triggered and the output voltage of the comparator goes positive +Vo1. The time duration of this charging process is say t1. When the comparator output goes positive, the diode is reversed biased and there feedback path is opened. The input voltage Vin provides the charging current for the op-amp LM358 integrator. The output Vo2 decreases or ramps down at the rate determined by the input voltage. This discharging continues until the Vo2 decreases and reaches the lower threshold voltage of the op-amp comparator. Let this period of discharging be t2. This charging and discharging of the capacitor of the voltage to frequency converter repeats continuously.

The frequency f of the output signal Vo2 is determined by the input voltage Vin. Typically, the charging time t1 is much less than the discharging time period t2 and so we can write approximately,

\[f=\frac{1}{t_2}=\frac{R_1}{2R_2RCV_{sat}}V_{in}\]

or, \[f=kV_{in}\]

where,

\[k=\frac{R_1}{2R_2RCV_{sat}}\]

where k(Hz/V) is the sensitivity of the voltage to frequency converter(VFC)

Key Features:

• Linearity: A well-designed VFC provides a linear relationship between input voltage and output frequency.
• Noise Immunity: Frequency signals are less prone to noise interference compared to analog voltage signals.
• Digital Compatibility: The output can easily be counted or processed by digital circuits like microcontrollers or counters.
• Transmission: Frequency signals can be transmitted over long distances with less degradation than analog signals.

Typical Applications:

1. Analog-to-Digital Conversion: VFCs are often used in data acquisition systems where an analog voltage needs to be converted into a digital format. By converting the voltage to a frequency first, a digital counter can easily read and process the signal.

2. Sensors and Transducers: Many types of sensors (e.g., temperature sensor, pressure, or flow sensors) produce an output voltage. Using a VFC, this voltage can be converted to a frequency that can be processed by digital systems.

3. Telemetry Systems: In long-distance signal transmission, VFCs can convert a sensor's voltage signal to a frequency for better transmission quality over noisy environments (e.g., industrial telemetry applications). The frequency is less affected by environmental factors compared to analog voltage.

4. Phase-Locked Loops (PLL): VFCs are often integral parts of PLL circuits, which are used for clock generation, synchronization in communication systems, and frequency modulation/demodulation.

5. Motor Speed Control: In systems where motor speed needs to be controlled or monitored, the motor's speed is often converted to a voltage using a tachometer. A VFC then converts this voltage to a frequency that can be used for digital speed control.

6. Frequency Synthesis: VFCs are used in applications like frequency synthesizers, where a variable frequency output is needed based on a control voltage. This can be useful in communication systems like radio transmitters and receivers.

7. Measurement Systems: In precision measurement systems, such as those used for measuring small changes in resistance, capacitance, or inductance, the varying voltage can be converted to a frequency using a VFC, which can then be easily read with digital equipment.

Examples of VFC ICs:

• LM331: A popular VFC IC with a linear voltage-to-frequency conversion over a wide range of input voltages.
• AD654: Another common VFC with high accuracy and good temperature stability.

In summary, VFCs are vital in applications where converting an analog voltage to a frequency signal simplifies further processing, improves noise immunity, or facilitates better signal transmission and integration with digital systems.