VCO with op amp and NE555

Thread Starter

alpheratz

Joined Nov 10, 2019
17
Hello everyone.
I'm working on a VCO project inside a LTSpice and I have a little problem with the quality of my wave.
It is the classical VCO solution with a NE555 configured as schmitt trigger comparator. I don't use a BJT but CD4066 because I want a low offset voltage.
Please help me to understand the reason why.
Range of frequency: 0.2V-20V of Vin for 200Hz - 20kHz.
+Vcc = 15V so I have a triangle wave from 5 to 10V, between the timer's thresholds.
As opamp I have OPA277, powered to +15V and -15V.
The reason why I put C2 is because I want to filter spikes present especially at 20kHz (20V of Vin) - see the image.
Where do these spikes come from? I also noticed that if I increase R1 and R2 (10x times) (and therefore decrease C1 to have same time constant) I don't get any spike but it also tends to be slower and have the peaks of the triangle a kinda curved. This is why I want to keep low resistors.
But I don't know where spikes come from and is it proper to filter them like that?
 

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Tako

Joined Oct 21, 2014
65
For spikes, you would need to find their source, that is, track the patch. If adding C2 helps, for me it's fine. Depending what are professor's requirements and how deep would you understand the problem, you may "dig". During my studies, I met many different approaches of professors.

If adding C2 does not worse required parameters of your circuit, for me, it's fine.
 

Thread Starter

alpheratz

Joined Nov 10, 2019
17
I think the cause of the spikes is the low slew rate. YES it is bigger than what I need for my ramp but it's not enough to change the ramp during the peak. In fact using an op amp with bigger slew rate helps with that. Or maybe it's just abut op amps bandwith (1MHz vs 20kHz of triangle wave) that somehow filters higher harmonics of the triangle wave causing that low quality peaks (which are rich in harmonic components).
 

Tako

Joined Oct 21, 2014
65
Exactly. You need to dig and hence, check one thing, then another and so on. That's why, depending on the project, sometimes such solutions that "I assume that and used that and it help" are acceptable, but for others it is important to understand everything in details.
 
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