LM339N Question

[system]

I understood that it was a zero crossing detector (hence my last comment). I did not realize (then) that it was an 8.25 mV window detector, which answers my question about the "threshold", except that it really isn't a "threshold" per se (technically it is of course) because that would be a point which separates one class of input signal (those below the point) from another (those above the point) but in the case of an AC signal , ALL signals CROSS the zero point so there is no distinction.
If , for example , the OP had a bandpass filter that blocks anything below a certain frequency (at which the amplitude falls below said "threshold" , say 33% below input amplitude) and everything above that frequency is above said "threshold", then one could correctly state that a "threshold" detector would separate frequencies below f_c from frequencies above. The zero crossing detector separates nothing. It will detect ANY AC signal. Thus, I would not call it a "threshold" . I would call it the "zero crossing band (or window)", if only to distinguish it from what I described.

That still doesn't answer my question about this comment:

and the 1.0V reference voltage holds off anything under 40 Hz

and whether some explanation is warranted.
(thanks for clarifying the voltage divider issue)

[Wawa]

The DC offset is small enough to call this circuit a zero crossing detector.
The slight offset is there to give the output a predefined state.
In this case, the -input is always at least 8.25mV higher than the +input.
So the output of the opamp is always low without an input signal.

A window detector uses a three-resistor voltage divider, and two opamps.
One opamp detects "higher than", and the other opamp detects "lower than".

As said, I think OP uses a generator that outputs a lower voltage at a lower speed.
You could detect a frequency threshold with that if you increase the DC offset.
But with an increased offset, it's not a zero crossing detector anymore.
Just a frequency detector.
Leo..