Erratic counter trigger from battery powered ttl source

Question

I am using a new Monarch optical tachometer with TTL output to drive e-series counters. Using a function generator or other TTL conditioner it works fine. However, using the Monarch, either battery or plugged into line power, I get very erratic frequency readings and frequently a -200140 error. I am on a pcmcia 6036e and pxi-6052. Using LV7.1 daqmx measure digital frequency 1-counter. The analog observation of the TTL pulses shows slightly different min-max levels and pulse width, but no smoking gun. Any ideas?

Answer 1

Jim,
What gate frequency are you using in your example?
Depending on the frequency you are using, you could have a synchronization error. This refers to the measurement variation due to the gate and source edge not being synchronized. In the example you mentioned, you should be using a 20MHz timebase, where each source cycle lasts 50ns. If the gate frequency you are using is of 50KHz, the number of 50ns periods that occur during one cycle of a 50KHz signal is 400 +/-1, which yields a frequency measurement of between 49.88 and 50.13KHz. On the other hand, if the gate frequency you are using is of 5MHz, then the number of 50ns periods that occur during one cycle of a 50MHz signal is 4+/-1, which yields a frequency measurement between 4 and 6.67MH
z.
Again, the reason why you might be getting very erratic readings could be because of the frequency that you are outputting from the optical tachometer into the gate of your board.
Please let me know if this helped. Thanks!
LA