The VCO are an essential piece of the project. They will produce the shapes and contours of the images. The VCOs will be all the same. They will have a large range from 1 or 2 minutes to 1Mhz (despite the fact that the waveforms are less neat around this frequency).
1Mhz is actually a small limit for the VGA scanline. Meaning that once locked the vco will produce less vertical bar that it can produce horizontal bar (vertical sync). This is the result of a scanline twice faster that the one used in PAL or NTSC (about 15Khz). Although test will be run to run the clock generator lower.
The VCO is using the XR2206, obsolete but still available from China. Are they counterfeit? Clone? Rejected part at the factory? I don't really care as long as they work. Some external corrections are needed to constrain the waveforms into what it is expected for the project.
To get stable image we need to lock the vco to the monitor clocks (H sync or V sync) and the XR2206 hasn’t got a dedicated pin for it. I manage to get the VCO to reset but it creates a «tick» in the waveform that should be removed for processing. This is done with a clamp circuit that reduce the negative peak. The two other op-amps are here to make the waveform into the 1VDC signal range.
Approaching the MHZ range, the square output tends to become something else.
I needed to create a kind of « reshaper » circuit to get back a proper
square wave or at least something less ugly.