I design the attached circuit:
75 MHz Sine wave that connected to R and C in parallel:
In the output, the amplitude of the sine wave is changing according to the C, more capacitance smaller amplitude of the sine wave.
When I tried that in the lab I got 2 results:
1. When I connected the function generator as the source I got the same results – higher C lower amplitude (of the sine wave in the output)
2. When I connected it to oscillator (http://www.farnell.com/datasheets/75480.pdf) I'm getting different results: the output is a sine wave, but if I make C higher the amplitude becomes lower until the sine wave (in the output) is "flipping" (kind of changing phase) and, if I keep raising the C higher, the sine wave in the output starts getting higher.
What are the differences between the two?
How can I make the oscillator work like the function generator in this case?
Best Answer
I'm going to assume that you used something like a breadboard to mount your components, and standard breadboard jumpers several inches long. And there lies your problem.
75 MHz is just too high a frequency to get reliable results on a standard breadboard unless you use great care. The biggest problem is the ground connection of your scope, which needs to be as short as possible, and connected to the circuit as close as possible to the ground pin of your oscillator. Just as an experiment, try looking at your oscillator output on the scope and try moving the probe around (changing the angle of your hand and probe).
Another way to look at this is to consider that your physical setup is almost certainly not exactly what your schematic indicates. Long wires will have parasitic inductances which become important at 75 MHz.
Second, especially with the oscillator, is making sure you have properly decoupled it. Use a 0.1 uF ceramic cap connected as close to the power and ground pins as is humanly possible, with the cap leads as short as possible.
Comparing your oscillator and an external function generator is fraught with possible difficulties. As a start, the FG is probably putting out fairly good sine wave. Your oscillator, on the other hand, is doing its best to put out a square wave - notice the 3 nsec rise and fall times. This will produce large signal components at multiples of 75 MHz. Granted, the large load capacitances you use will tend to filter these harmonics out, but with varying effects.
And finally (although perhaps I should have listed it as first), as asdex has pointed out, your oscillator is completely unsuited for the circuit you are investigating.