Hi Guys,

I'm struggling with a small project of mine. I need to measure very fine time measurements between two arriving TTL pulses and also trigger an event when the pulses happen at exactly the same time. I have a prototype board that has been printed but it doesn't seem to work when i have both time measurement and coincidence sections connected at the same time. Below is the front end of the board, The IC#start/stop are bnc connectors connected to single photon counting modules which produce TTL pulses. The resistor and capacitor at the input are there just in case i wanted a filter or voltage divider but at the moment the caps are not connected and the resistors are jumped. The signals travel up to a 5-3.3v level converter, and forward to a number of inverters and eventually an AND gate. The idea here was that it would act like a edge detect circuit and would only really trigger the AND gate when the input signal's from the START/STOP pulses arrive at exactly the same time. The output of the AND gate is connected directly a an arduino event trigger pin and the 3.3V output goes to a PIC24EP512mc202. When i test each part separately they work fine but together they seem to fail.

Any Ideas?

 

*Extra information: i am using the SN74F04 hex inverter and SN74F21 dual 4-input AND Gate

 

You are missing a crucial piece of information.
What is your definition of "exactly the same time"?
In other words, you need to specify a maximum time interval beyond which two events are considered not coincident.

 

Thanks for the reply

Well it doesn't have to be "exact", I allow for 10-15ns deviation. So if they arrive exactly together i should get a 10-15ns output pulse due to the delay in the hex inverter. If they arrive 7ns apart then i guess i will still see a very short pulse but anything beyond that should give me nothing. I have tested the circuit and it seems sufficient, i just can't figure out why it doesn't work when i have the level converter connected at the same time.

Also if there is a better solution i'm all ears ^_^, however i'm hoping i can get what i have working.

 

My point is there is no such thing as "exact".
You must specify a "window" that defines coincidence.

Once you have determined this "window", use a monostable multivibrator of pulse width equal to the "window" on each channel triggered by the event.
Then AND the outputs of the two monostable multivibrators to give you the "coincident" signal.