For many situation, voltage to frequency converter is very useful. One of its application is transmitting measurement using standard voice radio transceiver. Although the following schematic only use two CA3130 op-amps, its voltage-frequency transfer linearity is better than 0.5% and the temperature coefficient less than 0.01% :

high precision voltage controlled oscillator circuit schematic diagram

high precision voltage controlled oscillator circuit schematic diagram

C1 will be charged with time constant (R9+R5)C1, flowing through R9, R5, D3, and C1 (Assuming that the output voltage of U1 is 15V). This will last until U1 (pin 2) voltage get higher than the non-inverting input voltage at pin 3. The output voltage of U1 will then drop to zero, and the non-inverting input pin 3 will drop to about 5V. Set by the R3 for hysteresis. C1 will now discharge its current through D4 and R6, at a rate determined by the voltage level of the U2 output. The U1 output will swing to 15V and the cycle will be restarted if the C1 voltage has decreased below 5 volt. The voltage level is the average of the pulse output of U1 at U1 positive input. The output of U1 is a positive pulse with constant width and the space is variable depends on the discharging rate of C1 which is depend on U2 output. If the voltage level at positive input is higher than the negative input, then the output of U2 will increase toward positive supply (15V) giving a reversed action to keep the level of negative and the positive input equal. If the voltage level of this positive input of U2 is smaller than the control voltage at the negative input then the output of U2 will decreased toward negative supply (ground), therefore the discharging rate of C1 will be increased and the pulse output of the U1 will have shorter space, thus increase the voltage level at C3 until the it’s equal to the control voltage level at negative input of U2.

D1, D2, and R3 are provided to introduce similar reference voltage variation of positive input as seen in the negative voltage variation caused by D3 and D4 at different temperatures because D3 and D4 are needed to separate the charging and discharging control of C1, and the diodes have different characteristics at different temperature. Potentiometer R10 can be used to correct the offset error of U2 and R9 to fine tune the voltage-to-frequency conversion rate to 1kHz/Volt. [RCA Application Notes]

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