Creating a cosine wave of 140 MHz out of 80 MHz clock
It's not entirely clear what you're asking, but you probably want to consider the Nyquist-Shannon sampling theorem. 

As explained by Wikipedia, this says, "a bandlimited function can be perfectly reconstructed from a countable sequence of samples if the bandlimit, B, is no greater than ½ the sampling rate (samples per second)".

More explicitly, for a single-frequency signal like a cosine, if the only analog filtering you do is low-pass filtering, you can in theory perfectly reconstruct the signal from samples only if the sample rate is at least 2x the cosine frequency.

To get very specific, if you want an output frequency of 140 MHz, you want to generate your samples at at more than 280 MSa/s. Higher oversampling would make the analog filtering problem easier.

There are alternative approaches, but you'd need to add a well-chosen bandpass filter to your circuit.