Generate PWM Signal in LTSpice
A 100kHz signal must change every 1/T seconds, or 0.00001s, If you want a square wave with a 50% duty cycle then the Tperiod sets the time that the signal will be on and Ton sets the time during that period or 'window' that the signal is at Von.

Calculate your period: 1/f=Tperiod
Calculate how long the signal is on for 50% of the time (symmetrical square wave) would be t*Duty cycle = Ton.  

If your looking to generate a PWM signal then use a PULSE Voltage source with
a signal changing at 100kHz Tperiod is set to 0.00001s. The Ton will be half that for a 50% duty cycle, or 0.000005s. The rise and fall times may be left at zero. If you wanted a 20% duty cycle the Ton time would be 0.000002