10Mhz frequency acquisition

[David15]

Hi,

 I have a cRIO 9074 and Ni 9401 module, I need to acquire 6 frequencies around 10Mhz ,two problems:

-1)in scanengine the frequencies to acauire are to high, so I need to acquire it in FPGA mode.

how to implement the code in FPGA mode?

2) I need to appreciate the variation on sigle Hz of the acquired  frequencies.

someone can help me??

regards

david

[Stefano.Caiola]

Hi,

have you tried this in scan mode?

http://digital.ni.com/public.nsf/allkb/C9088DFDF803CD8B862575F3007C40FD

Detect a variation of 1Hz on a 10MHz signal is too high even in FPGA i think, it means detect a frequency variation of 0.0001% 

[crossrulz]

Your signals at going at 10MHz and the fastest you can sample at with that module is 10MHz.  That is not going to work well.  Even if you could go at the 40MHz for the FPGA base clock, you would not get the Hz accuracy you are looking for.  I would reacommend looking for a simple DAQ board that has counters on it.  Those counters would have a much better resolution for you.

[David15]

Yes I tried but after 1 Mhz the frequency value acquired is no longer correct.

any other idea?

[David15]

thanks for you answer,

If I go to 40MHz  or more(80Mhz)for the FPGA base clock,will have the same resolution problem?

thanks

[Hooovahh]

Nyquist

I know Wiki articles can be dry and hard to understand, so let me put it in easy to understand terms.  If you have a signal at 10Khz, and you want to characterize it by sampling it periodically, then to get the frequency you must sample at least 2 times the signal rate in this case 20Khz.  But when you use this sampling theory in the real world things aren't quite so perfect, and 2 times the rate just isn't enough, especially with edge cases like when you happen to sample at a zero crossing.  So realist generally say Nyquist should be 10 times.  If your signal is at 10Khz, then you should sample at 100Khz.  If it is 10Mhz, then you should sample at 100Mhz.

Now what you haven't said is what you intend on doing with these samples.  Lets say you measure a signal 100,000,000 times in one second...now what?  Are you going to graph all of that? log it?  Measure the frequency? Peak to peak?  Doing these things gets more difficult as the amount of data to process goes up.  So even if you got a 100MHz FPGA, and you were able to measure that signal that often, which you generally can't, then what do you intend on doing next?