extending the TV scope

As explained last month, a normal TV set can be used as an oscilloscope. A simple converter for this purpose was described in detail: the `TV scope - basic version`. In an introductory article it was explained that this basic version could be extended, thereby eliminating its two major weaknesses: limited usefulness at high frequencies and lack of triggering facilities. Before discussing the details of the extension circuits, a fuller explanation of the underlying principles is in order.