There are number of ICs available which convert frequency to voltage, but that cart also be done with discrete components as shown in this circuit. Such a simple design has of course, its limitations: the input signal must be a square wave, must have constant amplitude, and be provided by a low impedance source (≤50 Ω).
The circuit is called a transistor pump and is related to the charge pump that is used, for instance, in a voltage doubler or voltage inverter. Here, the added transistor arranges for C1 to be charged very rapidly (U1=low) when the left-hand side of that capacitor is pulled toward zero because of u1. The right-hand side of C1 clamped to a potential of Uc2-UBE. This means that the circuit is not dependent on the pulse width of the input signal. During the leading edge U1 the charge on C1 is transferred to C2, which is discharged through R1.
The transfer function of the circuit is derived from the balance in which the increase of Uo during a leading edge must be equal to the voltage reduction caused by R1 in each period. This yields the following formula
With component values as shown in the diagram, the output voltage increases by about 4.3 mV Hz-1 if the level of the input signal is 5 V. The speed with which the output voltage reacts depends on the tie constant of the circuit.
Since the value of both R1 and C1 is fairly large, the reaction is not very fast.There are number of applications where that does not matter however.
The ripple on th output voltage can be calculated by
With values as before the ripple 400 mV. The supply voltage for the circuit must be at least 2V higher than the output voltage at the highest input frequency to be measured. The circuit draws a current of not more than few mA
Filter R2-C3-C4 smooths current peaks during the charging of C1 and thus limits the RF interference the circuit produces.
The output must be terminated into a high impedance (digital voltmeter or buffer). If that is not possible a high value terminating resistor (≥10R1) should be used.