# Using ICL8038 as a VCO

I wanted to use this function generator IC as a voltage controlled frequency output especially to obtain low frequency sine waves. But unlike many implementations, I wanted to control the frequency not by a using potentiometer but instead by applying a voltage directly to its inputs. For powering the chip, I use a ±15 V dual power supply with 133 mA rating.

From the 8038 datasheet there is a section about sweep shown below: Above the sweep voltage is shown between the V+ and pin 8.

But instead, I tried the following scheme where I apply the DC voltage signal between pin 8 and GND. Here is how I use it at the moment: As you see above Vin is coupled directly to pin 8, and in my case the component values are Ra = Rb = 47 kΩ; C is selectable 4.7 μF or 100 nF; RL = 15 kΩ and pin 12 is directly connected to V-.

And here is a photo from the board: Since I couldn't find a similar implementation I have couple of questions regarding my implementation:

1. In my case I apply the input voltage(call it Vin) directly between pin 8 and the GND, and to get sine wave output the Vin must be such that 6.5V < Vin < 15V.

I can observe on a scope that the output frequency can be varied by varying Vin.

But since I couldn't find an exact example for the input I'm not sure what I'm doing is correct for the input.

Is it proper to just apply DC voltage Vin to pin 8 as in my case?(What confuses me is that the sweep voltage is shown between the V+ and pin 8). Okay in my case it is inversely proportional i.e. the output freq. is increasing with Vin decreasing which is not a problem for me but I'm wondering whether it is fine to use it this way.

2. In almost all examples there is a resistor between pin 12 and V-. But in my case I get the least distortion when I connect pin 12 to V- directly. Indeed whenever I introduce any resistor between 0 to 100k sine wave gets distorted.

Again since it is counter to examples I'm wondering is it proper to use this pin without resistor and directly connecting to V-?

3. How can it be known how much current would pin 8 sink at 15 V DV? It is because of I supply lets say 15 V to the pin 8 through an opamp, the opamp might not source enough current if "pin 8 input impedance" is low,

Note: I have also came up with this document but couldn't find any answer to my doubts.

• The ICL8038 is obsolete and has not been made for many years. Intersil was purchased by Renesas a few years ago. – Audioguru Jul 8 '20 at 20:55