What would be output of frequency divider IC if there are strong harmonics in signal?

Assume we divide 5GHz by 2 using divider IC that works up to 6GHz, and there is strong 10GHz harmonic of 0dBm. Is it necessary to filter out this harmonic?

A divider will clock on any edge that meets it's logic level requirement. If the 10GHz signal is large enough to drive the divider it will do so. If there is a risk, use an LPF anywhere between 5GHz and 10GHz to supress the harmonic so it can't cause problems.

Brian.

nope. Since it is a digital gate, it is expecting a digital input, like a square wave. A square wave would have a strong 15 GHz component also, and you definitely would not want to filter it out.

If you are sending it a non-digital input, like a sine wave, and it only happens to have a 2nd harmonic content because you are overdriving an amplifier somewhere, then yes I would try to filter it out a little. You might have a non-square wave time waveform if there is too much 2nd harmonic, which my cause odd triggering