I simulated the 1MHz PWM LTSpice circuit using the lt1715 high-speed comparator from Analog.com. The AC input signal feeds through into the triangle wave at the input to U2. Thinking it might be a simulation artifact, I reduced the maximum time step, increased the supply voltage and changed the frequency of the triangle wave, but the "birdie" remains. Naturally, it modulates the pwm output.
Is this feedthrough "real", or is it an artifact of the simulation model?