Why not use square wave but sine wave as LO signal?

a ideal switch operation can be get if a square LO is applied to the switching quad in a mixer. But conventionally we use a sine LO . Why? Is it difficult to generate a high frequency square wave?

No, it's not difficult, but the local oscillator output is also used to be multiplied by other signals, and if it's square instad of sinusoidal, it would have different harmonics at output which will be much difficult to filter out...

I hope that this helps,
Ahmad,

i think using square wave is the ideal case and i don't see any disadvantages in it except that it needs a Buffer from the VCO

The problem is generating the square wave. At microwave frequencies it takes more resources (chip area, current drain, S) to make a square wave.

Overdriving the LO port of the mixer is almost as good.

To get square wave, we use dividers to drive mixers. But in this case LO has to have 2X of frequency of operation.The main advantage of having square wave is to prevent 2nd harmonics and lower transient noise..

how about the power of the signal?

as also flatulent said, the LO stage is not intensionally switched on/off with a sine wave. it is just difficult to obtain a square wave at high frequencies. you may use a buffer at the input of your LO port to switch your LO as abrupt as possible, but depending on your operating frequency, you'll never get a square wave as you'll always have a low pass effect due to the parasitics etc.