# Which is the best way to measure low frequencies?

#### Do you have a question? Post it now! No Registration Necessary

Translate This Thread From English to

•  Subject
• Author
• Posted on
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

Re: Which is the best way to measure low frequencies?
On a sunny day (Mon, 17 Apr 2006 16:59:07 +0200) it happened "Marco T."

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.

Re: Which is the best way to measure low frequencies?
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

Re: Which is the best way to measure low frequencies?

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

-jg

Re: Which is the best way to measure low frequencies?

[... snip ...]

There is a frequency coutner reference design for Spartan-3E Starter
Kit board.
http://www.xilinx.com/products/boards/s3estarter/reference_designs.htm#frequency_counter

I'm not sure how low in frequency that you want to go, but it does
measure from ~50 kHz to 100s of MHz.  It also uses the PicoBlaze 8-bit
embedded controller macro.

http://www.xilinx.com/s3estarter

http://www.xilinx.com/picoblaze

---------------------------------
Steven K. Knapp
Applications Manager, Xilinx Inc.
General Products Division
Spartan-3/-3E FPGAs
http://www.xilinx.com/spartan3e
---------------------------------
The Spartan(tm)-3 Generation:  The World's Lowest-Cost FPGAs.

Re: Which is the best way to measure low frequencies?
Many Thanks to Everyone!

Marco