Frequency to Voltage (F-V) and voltage to frequency convertors (V-F)

Frequency to Voltage (F-V) and voltage to frequency convertors (V-F)

Frequency to Voltage convertors (F-V)

·           F-V convertors applications: Tachometer in motor speed control Rotational speed measurement.

·           Two types of it: Pulse integrating Phase locked loop

·           F-V convertor produces an output voltage whose amplitude is a function of input signal frequency.

·           V₀=k_f f_i   kf is sensitivity of F-V convertor

·           It is basically a FM discriminator.

Input frequency is applied to comparator A.

Resistor R acts as feedback element.

Capacitor Ci enables charge-balancing,

High pass network conditions input signal

For negative spike of V 01, comparator COMP triggers one shot multivibrator with threshold 7.5V The output of multivibrator closes the switch SW, for a time TH, this causes voltage Vo to build up and inject thru R and this continues until current out of summing input of opamp is equal to that injected by Vo through R continuously.

Vo=10^-3 *T_H *R*fi           as T_H =7.5 C /1X10^-3

Ripple Voltage,  Vr(max) =7.5 C /Ci

Voltage to frequency convertor

Principle: Charge balancing technique-the process of charging and discharging results in frequency proportional to input signal F0= k Vi

Operation: Op-amp A converts input V_i to current I_i = V_i/R into summing junction.

When switch SW is open the current flows into capacitor Ci and charges it, and node voltage V_o1 produce ramp down.

When V₀₁ =0 CMP triggers and sends a triggering signal to one shot multivibrator that closes the switch SW and turns transistor Q ON for time T_H.

The threshold of mono shot = 7.5 V and T_H= 7.5 C/10^-3

During T_H  , V₀₁ ramps upward by amount ∆V₀₁=(1mA-I_i) T_H /C_i

Time duration TL for vo1 to return to 0 is TL = C∆V₀₁/I_i

T_L+T_H = 1mA T_H /I_i   = T

F₀=V_i/7.5 RC