A simple voltage-to-frequency converter (VFC) with modulated duty cycles is proposed. The proposed VFC is composed of an operational amplifier, a voltage-to-current circuit, a charge and discharge circuit, a comparator, and a Schmitt trigger. The 50% duty cycle signal is always obtained at the output of the Schmitt trigger and multiple duty cycles can be obtained at the output of the comparator by simply adjusting an external voltage level. Due to its simplicity, the proposed VFC circuit can be easily combined with other circuitry to extend its practical application. The errors caused by the circuit non-idealities are investigated to quantify the trade-offs between performance and cost. The proposed VFC, which features modulated duty cycles, is realized in a standard 0.18-m 1P6M complementary metal-oxide-semiconductor (CMOS) technology, and an active area of 210mx190m is occupied. Experimental results show that the proposed converter is useful for output frequency ranges from 315kHz to 2.53MHz with an input voltage range of 0.2-1.6V and a power consumption of 0.8mW.