Spirent simulators are fitted with highly accurate internal oscillators, an external reference should only be used on a standalone simulator if the frequency standards stability is equal to or better than the simulators internal oscillator which has a Signal Stability of 10.23MHz ± 5 x 10-10 per day (after warm up).
An external frequency standard provides a stable reference signal which is beneficial when you want to lock all units in a multi-box test system to the same clock signal. A recommended external frequency standard would be a HP5065A Rubidium standard accurate to <±1x10-9 Hz..
The external reference signal should have a level of -10 to + 10 dBm.