Which is the best way to measure low frequencies?

Question

Hallo,I should measure the frequency of an input signal.Max frequency of signal is 100 KHz.I have made a simple system to detect rising edge of input signal using a flip flop.It samples the input signal every system clock cycle.To measure the input frequency I count the number of system clock periods between two rising edges of input signal.To test the system I' using a function generator.Reading the data acquired (using a fixed frequency) I have watched that every 5-10 datas there is an error.The number aquired is about 50% or 75% less than the others.Using a pull up resistor will be solved the error?Otherwise I need a system that verify glitches?-- Marco

Jan Panteltje · Accepted Answer

On a sunny day (Mon, 17 Apr 2006 16:59:07 +0200) it happened "Marco T." wrote in :

Hard to tell what your error source is. But in all cases you need to make a fast rise clean pulse. You need a Smitt-trigger. Basically an opamp with some positive feedback.

Peter Alfke · Answer

If you want to measure 100 kHz with 1% accuracy, you need to count the incoming pulses for at least 1 millisecond. Or you can measure the 10 microsecond period length, if you have a clock available that is faster than 10 MHz.

Measuring for a longer time reduces the impact of signal rise time and jitter, but requires a counter to establish the ms time base.

This is all covered in hundreds of text books... Peter Alfke

Jim Granville · Answer

For more precsiion, you can divide the Fu, before doing the time capture. Or, you can set up to capture a wide-timebase counter, and NOT reset the counter between captures - then you have the option to choose Cycles in SW, and the time is done by subtract, and you do not loose any time-ticks, so have best precision.

50% less is explained by false triggers on the wrong edge, 75% less is harder to explain, unless you are over more than one cycle ?

Hardware solution is to add a Schmitt trigger, and maybe also a RC filter.

SW patch is to read multiple captures, and reject ones that suddenly 'step' from previous readings.

-jg

Steve Knapp (Xilinx Spartan-3 Generation FPGAs) · Answer

[... snip ...]There is a frequency coutner reference design for Spartan-3E StarterKit board.I'm not sure how low in frequency that you want to go, but it doesmeasure from ~50 kHz to 100s of MHz.  It also uses the PicoBlaze 8-bitembedded controller macro.[ADV]:  Spartan-3E Starter Kit board[ADV]:  PicoBlaze Controller---------------------------------Steven K. KnappApplications Manager, Xilinx Inc.General Products DivisionSpartan-3/-3E FPGAs Spartan(tm)-3 Generation:  The World's Lowest-Cost FPGAs.

Marco T. · Answer

Many Thanks to Everyone!Marco

Which is the best way to measure low frequencies?

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