freq counting via Scmitt Trigger Circuits & ICs

hi,
am analysing an already existing circuit which is supposed to count the no. of pulses generated by a flow sensor and feed it to the counter
input of microcontroller to measure flow which is proportional to the flow... now what beats me is they pass the pulses thru a schmitt trigger circuit using opamps... then pass the signal thru a Low pass filter n then again pass it thru a schmitt trigger inverter... i mean y not directly pass it thru a schmitt trigger inverter... am totally clueless... thats question no 1.
question no 2 is... once i get neat square pulses they are passed thru toggle f/f to divide the freq by two n then its gated with a high freq signal of 3Mhz before feeding it to the microcontroller... now the question is my input signal (which is in form of pulses) has varing freq (range 0-150 hz)... so the no. of 3mhz pulses gated at low freq are high and at high freq are low.. so how do i count the pulses and its freq????.....
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hi again!..
i just figured out the ans of the 2nd question... somebody help me with the 1st one!
thanks aparna aparna wrote:

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Without seeing the filters, I can only guess, but it sounds like the 2nd filter is removing pulses that are too short, that sometimes sneak through the first schmitt trigger. It could also be setting a minumum width from the 2nd gate.
Kevin
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am not talking abt the 2nd filter ... am talking about the schmitt trigger using opamp itself... can i skip the opamp circuit and the following filter n directly feed the signal to the schmitt trigger inverter??!
Kevin Kilzer wrote:

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