VCO with inverse property.

[myfaithnka]

Hi all,

is it possible to make a vco with a property that frequency is inversely proportional to input voltage ?


thanks.

 

[chuckey]

By using an op-amp with diodes strung around it, it is possible to get more or less any law you want, just interpose it between the DC control voltage and the VCO.
Frank

 

[myfaithnka]

dear frank,

please tell more about diodes with op amp.

whats is your opinion about a 555 based vco, I dont need 100% modulation.

thanks.

 

[chuckey]

Think about a conventional op-amp with a negative feed back resistor, suppose the resistor is 100 K, giving a gain of 10. If you connect a diode(anode to output) via a 10K resistor across the 100K, then when the output reaches +.6V, the diode comes on and the negative feedback resistor is shunted by the 10 K, so the gain falls to about 9 X. If you now put another diode and a 1k resistor across the 10K resistor, when the output reaches +1.6V, the second diode comes on, and the negative feed back resistor becomes 100 and 10 and 1K in parallel, so the gain falls to 1. Conversely, if you set the bias so the output sits at 3V, all the diodes are on and as the output goes more negative, the diodes switch off INCREASING the gain .
Frank