Voltage to Frequency & Frequency to Voltage Conversion. Multivibrators
Definition
A voltage-to-frequency converter is a circuit that produces an output pulse train whose frequency is directly proportional to the input voltage.
A frequency-to-voltage converter is a circuit that produces a dc output voltage proportional to the input pulse frequency.
A multivibrator is a regenerative switching circuit having two or more stable states, used to generate square waves, pulses, and timing signals.
Main Content
1. Voltage-to-Frequency Conversion
Basic principle
- The input voltage is converted into a train of pulses whose repetition rate depends on the magnitude of the voltage. A higher input voltage produces a higher output frequency.
Typical operation
- The circuit usually uses an integrator, comparator, and a reset mechanism. The input voltage charges a capacitor at a rate proportional to voltage; when the capacitor reaches a threshold, a pulse is generated and the capacitor is reset.
Mathematical relation
More specifically, for many practical circuits, where is the conversion constant determined by circuit components.
Why it is useful
- Frequency is easier to transmit over long distances and is less affected by noise, drift, and attenuation than analog voltage signals.
Example
- If a sensor output of 2 V corresponds to 1 kHz and 5 V corresponds to 2.5 kHz, then the frequency acts as an encoded representation of the original analog quantity.
2. Frequency-to-Voltage Conversion
Basic principle
- The input pulse frequency is converted into a dc voltage level that is proportional to the number of pulses per unit time.
Typical operation
- The circuit often uses a pulse shaper, charge pump, monostable multivibrator, low-pass filter, or frequency discriminator. The pulse train is averaged into a smooth voltage.
Mathematical relation
In practical cases,
Important requirement
- The input pulses should have a reasonably constant amplitude and duty cycle for accurate conversion; otherwise the output may be nonlinear or unstable.
Example
- In a tachometer system, the rotational speed of a motor may be represented by pulse frequency. An F-V converter changes it into a voltage that can drive a meter or controller.
3. Multivibrators
Astable multivibrator
- Has no stable state. It continuously switches between two states and generates a square wave or rectangular waveform. This is widely used as an oscillator and timing source.
Monostable multivibrator
- Has one stable state and one quasi-stable state. A trigger pulse causes it to switch temporarily before returning to its stable state. It is used for one-shot pulses, timing delays, and pulse shaping.
Bistable multivibrator
- Has two stable states. It remains in either state until externally triggered to change state. It functions as a flip-flop, memory element, or toggle circuit.
Role in V-F and F-V circuits
- Multivibrators are used to generate pulses, shape waveforms, set pulse widths, and control timing intervals in conversion systems.
Example of use
- A monostable multivibrator can generate a fixed-width pulse each time an input threshold is reached in a V-F converter.
Working / Process
1. Voltage-to-Frequency conversion process
- The analog input voltage is applied to an integrator.
- The integrator produces a ramp voltage whose slope depends on the input level.
- When the ramp reaches a reference threshold, a comparator triggers a pulse.
- The pulse resets the integrator, and the process repeats.
- Thus, a larger input voltage causes the threshold to be reached faster, increasing the output frequency.
2. Frequency-to-Voltage conversion process
- The incoming pulse train is first shaped and standardized if necessary.
- Each pulse is converted into a charge packet or timing event.
- The pulses are averaged using a capacitor and low-pass filter.
- The average output level becomes a dc voltage proportional to pulse frequency.
- Higher pulse frequency means more charge delivered per second, resulting in a higher dc output.
3. Multivibrator operation process
- In an astable multivibrator, capacitor charging and discharging repeatedly force the circuit to switch states.
- In a monostable multivibrator, a trigger causes a temporary state change and the timing capacitor determines the pulse width.
- In a bistable multivibrator, a trigger pulse toggles the output from one stable state to the other.
- The switching action depends on regenerative feedback and the RC time constants.
- These timing behaviors are used to generate pulses for conversion and control applications.
Advantages / Applications
Noise immunity in transmission
- Frequency-based signaling is less sensitive to noise than voltage-based signaling, making V-F conversion useful in remote sensing and telemetry.
Instrumentation and control
- F-V converters are used in tachometers, speed measurement, servo systems, and digital panel meters.
A/D and D/A interfacing
- V-F and F-V techniques provide a bridge between analog sensors and digital processing units.
Pulse generation and timing
- Multivibrators are widely used in clocks, pulse generators, timers, wave shapers, frequency dividers, and trigger circuits.
Industrial automation
- These circuits are used in process control, motor speed monitoring, alarm systems, and event counters.
Signal conditioning
- They help in converting noisy or slowly varying signals into more manageable frequency or voltage representations.
Data communication
- Frequency encoding can be used for robust signal transmission over long cables or harsh environments.
Summary
- Voltage-to-frequency conversion changes an analog voltage into a proportional pulse frequency.
- Frequency-to-voltage conversion changes pulse frequency into a proportional dc voltage.
- Multivibrators are switching circuits used to generate pulses, delays, and oscillations in such systems.
- Important terms to remember: V-F converter, F-V converter, astable multivibrator, monostable multivibrator, bistable multivibrator, pulse train, threshold, integrator, comparator