Connecting low-pass filtered output of one comparator into a second comparator

Question

I have the following situation: I have a signal with periodic, short (~2ns), ~1.5V pulses spaced ~10-20ns apart. I would like to detect when the pulses disappear, i.e. when there is no pulse for ~1us or so.

I am currently planning on using a first comparator to detect the pulses, then a low-pass filter to followed by a second comparator to keep the output on as long as the pulses arrive frequently enough. Something like the following:

Here, V3 is the pulse source, V1 is the threshold for the "pulse-detection" comparator, and also for the output of the low-pass filter. (I understand that the threshold for U2 might need to be different from the one for U1, let's just assume that the thresholds work out)

I tried to simulate this in KiCADs spice simulator, but ran into errors regarding singularities in the system, which makes me a bit worried.

As I understand it, the first comparator should just charge the capacitor C6 through R5, and drain it again in between the pulses (so it should work, even if there is no actual load attached to the RC filter). Will this circuit work as designed?

Answer 1

The integrated voltage across C6 in your schematic is not related to the input voltage level. So you need another, probably lower reference voltage for U2A.

May I suggest a simple BJT solution?

^{simulate this circuit – Schematic created using CircuitLab}

If you use an RF transistor with FT >= 1 GHz as Q1 you can integrate the pulses in C1.

Q2 can be a slower standard type. It is not a proper comparator, but provides the required signal.
The simulator needs some time to work it out.