High frequency measurement

[RRRRSSSS]

I need to measure a frequency around 3 Mhz the signal is in bursts.

I have the example 2 counter.vi working with a siggen with a continuous output but can't get a reading on the burst.

Below is the vi and scope shots of the signal I need to measure.

[Rohama_K]

Hi,

It appears you are measuring an analog signal.  What is your hardware set-up? I'm curious because according to this, if you are using an X Series DAQ device, the maximum sampling rate is 2 MHz for analog signals. The differences between NI DAQ devices can be found here. 

Rohama K.

[RRRRSSSS]

I was using a USB-6221 for my initial testing..I have a USB-6341 on order for the project.

[Rohama_K]

NI counters will not register this signal because this signal does not follow TTL Logic and that is what the counter is expecting. To learn more about TTL Logic, read this KB. Also, to sample an analog signal you must sample the signal at or above the Niquist rate to avoid aliasing. Both devices you have cannot sample a 3MHz analog signal at that Niquist rate. 

[RRRRSSSS]

now your done busting my balls on why it won't work, which was obvious on my first post.

my questions was

"I need to measure a frequency around 3 Mhz the signal is in bursts"

how bout throwing me a bone here and suggest something that will work.

[johnsold]

I suspect that the best way for you to measure the freqeucny of the bursts is to use an analog acquisition with a sampling rate of at least 10 MHz (higher is probably better).  Then do an FFT on each burst. The images you posted earlier show about 30 cycles in a burst.  That is enough to get a reasonable response from an FFT.  You could also detect zero crossings and calculate the frequency from that.   FFT may be easier.

Does the frequency vary within a burst?  Does it vary from one burst to the next? What is the range of frequencies in the bursts? What frequency resolution do you need? How fast do you need to analyze the data? Between bursts or seconds later?

Lynn

[RRRRSSSS]

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@johnsold wrote:

I suspect that the best way for you to measure the freqeucny of the bursts is to use an analog acquisition with a sampling rate of at least 10 MHz (higher is probably better).  Then do an FFT on each burst. The images you posted earlier show about 30 cycles in a burst.  That is enough to get a reasonable response from an FFT.  You could also detect zero crossings and calculate the frequency from that.   FFT may be easier.

Does the frequency vary within a burst?  Does it vary from one burst to the next? What is the range of frequencies in the bursts? What frequency resolution do you need? How fast do you need to analyze the data? Between bursts or seconds later?

Lynn

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The frequency is constant in each burst and is the the same from one burst to the next.

Resolution does does not need to be real acurate .5 mhz would work, but the more acurate the better..

I have lots of time to analyze the data at least 30 seconds to capture and analyze.

[johnsold]

Here is an example showing what I had in mind.  FFT seems to work well.

The frequencies are scaled down by three orders of magnitude (kHz rather than MHz). I saved it to version 8.0.  I got a strange warning about the multiply output so let me know if it does not work.  Also let me know what version of LV you are using.

Lynn

[RRRRSSSS]

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@johnsold wrote:

I suspect that the best way for you to measure the freqeucny of the bursts is to use an analog acquisition with a sampling rate of at least 10 MHz (higher is probably better).  Then do an FFT on each burst. The images you posted earlier show about 30 cycles in a burst.  That is enough to get a reasonable response from an FFT.  You could also detect zero crossings and calculate the frequency from that.   FFT may be easier.

Does the frequency vary within a burst?  Does it vary from one burst to the next? What is the range of frequencies in the bursts? What frequency resolution do you need? How fast do you need to analyze the data? Between bursts or seconds later?

Lynn

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What NI hardware samples at 10 MHz analog??

example provided works,I'm using LV2011

[johnsold]

A quick search at the Products and Services page of NI's web site shows the PCI-6115 and PXI-6115 have 10 MS/s rates.

All of the devices listed as Digitzers/Oscilloscopes have sample rates of 15 MS/s or more.

Lynn