Why does this comparator not output a square wave?
It will be one of two things &amp; more than likely both:


The probe you are using is not suitable, be it in frequency or in its compensation (the little screw on the side of the probe).


A 10 MHz scope is too slow for a 4.5 MHz signal


Here is the buildup of a squarewave up to the 100th harmonic (4.43&nbsp;MHz fund):



import numpy as np
from matplotlib import pylab
F= 4.43e6
t = np.arange(0, 2/F, 1e-12)
x = np.sin(2*np.pi*F*t) 
pylab.subplot(3,1,1)
pylab.title('Sinewave of increasing frequency: Fourier content of a squarewave')
pylab.plot(t,x)
pylab.grid(True)

for i in range(3,100,2):
    a = (1/i)*np.sin(2*np.pi*F*i*t)
    pylab.plot(t,a)
    x +=a

pylab.subplot(3,1,2)
pylab.title('Equivelent squarewave for summation of its harmonics')
pylab.plot(t,x)
pylab.grid(True)

y= np.zeros(len(t))

A= 10e6*2*np.pi*t[1]/(10e6*2*np.pi*t[1]+1)
for i in range(1,len(t)):
    y[i] = y[i-1] + A*(x[i] - y[i-1])
pylab.subplot(3,1,3)
pylab.plot(t,y,label='4.43MHz through 1 filter')
x = y
y= np.zeros(len(t))
A= 10e6*2*np.pi*t[1]/(10e6*2*np.pi*t[1]+1)
for i in range(1,len(t)):
    y[i] = y[i-1] + A*(x[i] - y[i-1])
pylab.plot(t,y)
pylab.plot(t,y,label='4.43MHz through 2 cascaded filters')

pylab.title('Result of passing a 4.43MHz squarewave through 1 &amp; two 10MHz 1st order filters')
pylab.legend()

pylab.grid(True)
pylab.show()


If the acquisition is only capable of 10&nbsp;MHz, the contributors will be attenuated and phase shifted producing a distorted waveform similar to the one you are seeing.

Cascading two 10MHz "filters" (one in the probe, one on the input of the scope) will further distort the waveform resulting in a signal closer to that seen on the scope. 

The mean of a 0-5V squarewave is 2.5V. if your scope as an "average input" it will also produce a similar waveform and tend towards 2.5V. I have been caught out a number of times looking at PWM only to see a very strange walking waveform ONLY to find someone messed with my scope and had enable "16sample averaging"