HIGH RANGE VCO
The VCO are an essential piece of the project. They will produce the shape and contour of the images. They'll 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 synced 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). The VCO is built using a 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. The XR2206 is easy to use, offers a large range of frequency and HAS an amplitude modulation input which makes it even more interesting.
NEGATIVE TICK AT THE RESET AND CLAMP
However, to get stable image we need to lock the vco to internal 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. The circuit shows the Clamp circuit (the buffer opamp) used to removed the negative tick and constrain the waveform into its normal range. The other two opamps reduces gain and offset the signal to 1VDC.