Hi Ramin,
The VCO center frequency is a function of R1, C1, and VDD as shown in this diagram:
Section 3.3 of this document gives a basic design procedure for selecting components to set the VCO frequency: https://www.ti.com/lit/an/scha002a/scha002a.pdf As an example, using Figure 9(a) the VCO center frequency is close to 400 Hz when R1 = 1M Ohm, VDD = 5V, and C1 = 2nF.
Let me know if you have any other questions.
Regards,
Connor
Hi Ramin,
I just used Figure 9(a) to find the intersection point between the R1 = 1M Ohm, Vdd = 5V curve and 400 Hz on the Y-axis. The x-axis shows C1 should be around 1 or 2 nF. You can follow a different curve depending on your supply voltage and value for R1. Table 4 in the document I linked in my last reply describes the design procedure and gives a few equations for calculating the min and max VCO frequencies as well.
Regards,
Connor
