We will start with a description of the output section of the project. I know that oscilloscopes don't have outputs but sound cards do, and a number of software vendors have used this to their advantage. We will cover the details of that subject later. In the meantime, let's look at the output circuit.
Tip
48 KHz is only a theoretical maximum. The actual practical maximum frequency you are able to measure might be much lower. For information on the Nyquist–Shannon sampling theorem, refer to http://en.wikipedia.org/wiki/Nyquist%E2%80%93Shannon_sampling_theorem.
The following circuit diagram shows the output buffer circuit. This circuit is used to isolate the soundcard from the circuit being tested.
Output buffer circuit
The circuit may look odd to some of you, but there is method in our madness. Many operational amplifiers have extremely high bandwidth at unity gain. This causes them to oscillate. So the easiest way to avoid this is to add some negative feedback.
