5Mhz is easily within the range of LC filters, that is, you don't need huge values to acomplish the design. A simple LC filter should do it, if you need extreme purity of the signal, a series resonant followed by a parallel should do it.
to beenthere : I need as pure as possible. I'll use it to generate the transducer of ultrasound
to bertus : Based on the page, the clapp and hartley oscillator is just working up to 1.67 MHz. May I know how to increase the workspace of oscillator up to 5 MHz? thanks.
to Bill_Marsden : I am still confuse what you are saying. I am still beginner on it. May I know the detail? or may be, where I can find the article of it in web?
to liitlefan : I think the crystal oscillator is square wave (am I false?). May I know How to change it to sinewave?
There are formulas on the page to calculate the frequency.
Smaller coils and capacitors will give a higher frequency.
These oscillators can be used upto 100 Mhz.
It will convert almost any waveform at 5Mhz into a sine wave. Finding the right values is the hard part, and the two coil and two caps don't have to be the same values, as long as they match their counterpart (an LC circuit).
There are a lot of oscillators, including crystal oscillators, that produce less than perfect wave forms. This circuit will usually clean them up.
At this point you will need to decide on the power you intend to produce. This should handle a square wave pretty well.