Frequency to Voltage (F-V) and voltage to frequency convertors (V-F)
· 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=kf fi 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 *TH *R*fi as TH =7.5 C /1X10-3
Ripple Voltage, Vr(max) =7.5 C /Ci
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 Vi to current Ii = Vi/R into summing junction.
When switch SW is open the current flows into capacitor Ci and charges it, and node voltage Vo1 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 TH.
The threshold of mono shot = 7.5 V and TH= 7.5 C/10-3
During TH , V01 ramps upward by amount ∆V01=(1mA-Ii) TH /Ci
Time duration TL for vo1 to return to 0 is TL = C∆V01/Ii
TL+TH = 1mA TH /Ii = T