Voltage Controlled Oscillator with LM358

Thread Starter

Yusuf Selim Karataş

Joined Jan 14, 2018
14
Hi everyone.

I am trying to do a voltage Controlled Oscillator by using LM358. In datasheet of the Lm358 the schmatic is given for a vco.

I apply V+ as 12 V and apllying to Vc as triangular singal at 10 Hz . By changing the input voltage the output frequency should change. But while output voltage changes frequency stays stable at 10 Hz. what May be wrong ? How can i solve it? Thanks in advance.
 

dendad

Joined Feb 20, 2016
3,009
Are you varying the +VC input?
That is not the OpAmp supply voltage but the frequency control voltage. Apply your 12V to the OpAmp supply pins and the 6V to the V+/2 pin and then a variable voltage to +VC pin.
It is confusing as the supply is normally labeled as +VCC.
 

AnalogKid

Joined Aug 1, 2013
8,107
I apply V+ as 12 Volt and by doing a voltage divider i divide it by 2. So it becomes 6 V
Not good enough. And, you didn't answer the question - How are you deriving V+2. What resistor values are you using? V+/2 must be a low impedance voltage source, like the output of a regulator or at least a zener diode. You can approximate this with a voltage divider with relatively low resistance values, such as two 1 K resistors.

ak
 

Thread Starter

Yusuf Selim Karataş

Joined Jan 14, 2018
14
Are you varying the +VC input?
That is not the OpAmp supply voltage but the frequency control voltage. Apply your 12V to the OpAmp supply pins and the 6V to the V+/2 pin and then a variable voltage to +VC pin.
It is confusing as the supply is normally labeled as +VCC.
Yes i am doing just the thing you say. Vc is a triangular signal which is at 10 Hz . We change Vc's amplitude and wait changes in output frequency. But it stayes at 10 Hz.
 

Thread Starter

Yusuf Selim Karataş

Joined Jan 14, 2018
14
Not good enough. And, you didn't answer the question - How are you deriving V+2. What resistor values are you using? V+/2 must be a low impedance voltage source, like the output of a regulator or at least a zener diode. You can approximate this with a voltage divider with relatively low resistance values, such as two 1 K resistors.

ak
I am deriving from DC power supply 12 Volt and using 100k to divide it as i have seen in some schematics.
 

Thread Starter

Yusuf Selim Karataş

Joined Jan 14, 2018
14
What are the positive and negative peak voltage values of the triangle wave you are applying to Vc. Be specific.

ak
Its at 10 Hz. Negative voltage is Zero. We change the positive voltage and expect the output signals frequency change. But when we change the positive voltage of the input triangular signal 10mV to 20 Volt output frequency does not change
 

sghioto

Joined Dec 31, 2017
1,039
I breadboarded the above circuit using a TL082 because I didn't have a LM358 on hand, but would only work using a dual power supply.
The frequency would only adjust to about 100Hz with 12vdc at Vc.

Steve G
 
Last edited:

Thread Starter

Yusuf Selim Karataş

Joined Jan 14, 2018
14
I breadboarded the above circuit using a TL082 because I didn't have a LM358 on hand, but would only work using a dual power supply.
The frequency would only adjust to about 100Hz with 12vdc at Vc.

Steve G
Thanks a lot for sparing time for this. But i could not understand what you mean by saying dual power supply
 

RichardO

Joined May 4, 2013
2,273
@yusuf-selim-karatas What op-amp are you using? When operating from a single voltage power supply, this circuit needs an op-amp that has an output that goes near ground. The LM358 does this but an uA741 (or TL082) will _not_ go close enough to ground.


This is what I get using LTspice. The frequency varies by about 2:1. I am not sure why it is not more...

upload_2018-1-14_12-19-0.png
 

Thread Starter

Yusuf Selim Karataş

Joined Jan 14, 2018
14
@yusuf-selim-karatas What op-amp are you using? When operating from a single voltage power supply, this circuit needs an op-amp that has an output that goes near ground. The LM358 does this but an uA741 (or TL082) will _not_ go close enough to ground.


This is what I get using LTspice. The frequency varies by about 2:1. I am not sure why it is not more...

View attachment 143680
Thanks very much. I am using LM358. I also did on LT Spice and i got the expected result. But in the lab i could not. On your circuit capacitor should be connected between negative input and the output of the first op amp
 

crutschow

Joined Mar 14, 2008
23,358
@yusuf-selim-karatas What op-amp are you using? When operating from a single voltage power supply, this circuit needs an op-amp that has an output that goes near ground. The LM358 does this but an uA741 (or TL082) will _not_ go close enough to ground.


This is what I get using LTspice. The frequency varies by about 2:1. I am not sure why it is not more...

View attachment 143680
Could you post your .asc file?
Thanks.
 
Top