"How to measure very low frequency?"

I'm using Labview 5.1 and a data acquisition card PC-LPM-16PNP. How can I measure a low frequency signal like the human pulse of 72 pulses/min or 1.2 Hz. Thanks.

Virgo,
When measuring very low frequencies, I recommend measuring the period between pulses and converting to frequency.
To measure the period, there are a few methods. For something like a heartbeat, I would probably use a peak detector to locate the peaks and measure the distance between the peaks. For other signals, it may be easier to locate zero crossings. Sometimes filtering or smoothing is required to remove noise and improve the results.
Bruce
Bruce Ammons
Ammons Engineering

Similar Messages

Very low frequency caused by sample frequency in FFT analog input?

I'm measuring a very low frequency on my analog input, this frequency is in connection with the sample frequency of the Analog Input. At a sample frequency of 1000Hz I see a frequency of 0,05Hz in my FFT, at a sample frequency of 500Hz I see a frequency of 0,02Hz.
Attached is a screenshot of an example how I see this very low frequency.
My hardware: NI USB 6008 --> measuring on AI-0 (in this example the input is unwired). But in my real measurement I see the same FFT + signals I want to see (about 2 Hz).
In my real measurement I windowed the FFT (1-3Hz) so I see only the FFT I want to see. But I suspect that my complete signal moves along with this very low frequency of 0,05Hz. I saw this in my measerement.
What did I do wrong?
Attachments:
screenshot.JPG ‏66 KB

First, do you live in Europe? If so, that 50Hz could be power-line pick up.
Antialias filtering must be done in hardware before the DAQ. Because of the way aliasing works if you have sampled the signal it's already too late, you're hosed and no amount of digital filtering can remove the aliased signal. In terms of filter specifications, the filter cutoff needs to be at twice the highest frequency you are interested in seeing. For example, if you are looking for signals in the 2- to 5-Hz range, your antialiasing filter should cutoff at around 10Hz.
Obviously good signal management is also needed: shielding, appropriate signal termination, proper lead dress and spacing from known noise sources, etc...
Mike...
PS: There were no attachments to your last post.
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Why am I getting very high values for the very low frequency region of a random signal?

I am tyring to produce a power spectrum graph for a Tachogram data, related to Heart Rate Variability Analysis. This data can be thought to be as a random signal, but has a frequency spectrum range of 0 - 1HZ.
The problem that I am facing is, I am getting very high values for very low frequency region, closer to DC value. Even the DC value is really high, in the range 10^8. It is suppose to be a low number. Any suggestions would be appreciated.
Thanks.

Here is what my work is all about. I am trying to develop a software for Heart Rate Variability analysis. I am not sure if you are aware of heart beat waveforms, they are bunch spikes, occuring at irregular intervals. We have to do analysis on this waveform. How ? First we have to create a plot called Tachogram. This is done by, for example, let's name the first spike R1, the second one R2, and the third one R3, and so on. This is how the co-ordinate points are created. (R1, R2-R1), (R2, R3-R2), and so on...
Here R1 is the time instant at which the spike R1 occurs, and R2-R1 is the time difference between these peak occurences. So, if we plot these values, then it gives us one like what you saw on your time domain plot. What we have to do with this signal is, we have to apply FFT technique to produce its power spectrum for analysis. Generally the frequency range of the spectrum goes from 0 - 0.5 Hz. They part this range into VLF, LF, and HF, and analyze how much power is distributed in these regions. The distribution would tell you if you were stressed out,or have worked out, things like that. Normally, you can view spikes around LF, HF region. If you had viewed the power spectrum for the data that I sent, then that's what it pretty much looks like. I think I have said a lot about it now. Hope you get it.
We were talking about the "other component", I can't really see where it lies, must be in the 60-80 sec. range in the graph. If you look at the spectrum, I am getting a huge value around 0.0033 Hz, how do I minimize or remove this ?
Also, I am using Unevenly Spaced Signal Spectrum.vi, I am not sure what excactly is the unit for the Power Spectrum output. Technical Papers denote, Periodogram algorithm used by this VI, gives Power Spectral Density ? If that's right, then the unit is V^2/HZ.
Please advise.
Again, I should thank you for your time and effort.
I have to meet a deadline this week in finishing this project, and your help is immense to me at this point.
Thanks & Best Regards,

Very Low Frequency Converter

Hello
There are numerous devices that transmit audio via very low frequency (VLF). I quote:
"Carrier current devices are a combination of technologies. They are a cross between
wired microphones and subcarrier transmitters. The only difference is that the signal
is not transmitted via radio waves, but rather through a wire pair. A person cannot
accidentally intercept or detect a carrier current signal by simply tapping into a wire
like with wired microphones. A carrier current device works by picking up room audio
through a microphone. The signal from the microphone is then modulated by a low
frequency circuit, which produces a carrier current signal at approximately 100–200
kHz. A common example of carrier current devices are the newer wireless telephones,
intercoms, or baby monitor type devices that plug into the electric socket and use the
pre-existing wiring rather than having wiring run all over the house. A special circuit,
which can demodulate the low frequency signal, is used as the receiver.
Only a sophisticated receiver with a low frequency probe can detect this sort of device."
Can Audition modulate the signal back into an audible range?

At 100kHz, yes it's true - you'd need an audio interface running faster than twice the modulating frequency to demodulate all of the sidebands correctly. Not it's not absolutely impossible to do this, but it's a very expensive way of doing what hardware does very cheaply! To do this in Audition, you'd need initially to get the modulated carrier in (so if it was 100kHz, that would require an interface that runs at 200kHz to satisfy the Nyquist requirement) and all of the currently available crop appear to stop at 192k), and then run the signal through an envelope detector to capture the sideband signals. Problem is that Audition doesn't have the envelope detector, so you're stuffed anyway, even if you used 100Hz. 100Hz would give you all sorts of problems on top of this, because you'd have to use FM and then you'd have to deal with all of the reflected sidebands.
So to state this baldly, Audition is an audio editing program, not a radio receiver!
If you want to read more about this (and I should warn you that this gets complicated) then here are a few links:
Demodulation - Wikipedia, the free encyclopedia
audio-rate frequency modulation
http://www.secretmango.com/jimb/Whitepapers/radio/radio.html

How to measure and log frequency with fieldpoint CTR

Hi,
I am developing a data acquistion and control system for an engine dynamometer using the fieldpoint modules and Labview. One of the most important signals is the engine speed, measured in RPM. The RPM signal is a 0-12V pulse where one pulse equals one revolution of the engine. As well as being an important piece of data for later analysis, engine RPM will also be in the input into a PID controller, so the signal must be both accurate and have a high measurement frequency.
Currently I am using the FP-CTR500 modules to measure the frequency of the signal. I am already aware of the included frequency measurement VI example, as well as the one posted before for low frequency measurements, and I have gotten both to work with my setup. I would be using the low frequency VI becuase the max frequency measurement would be in the 200Hz range.
The first problem I am having is with the structure of the VI and how the data is output. The case structure in the VI activates when the counter is read and resets the counter, then switches to the next case. I would like the RPM number to output out of the case structure into a write_to_file VI and PID controller input. The problem is that when the case switches, the counter is reset to 0, which will be recorded in the written file.
This is some example output data (RPM):
1232
0
2321
0
2400
0
2521
0
The data is being written correctly, but of course I can't have 0 readings when the case structure changes. This would be especially problematic when input into a controller VI.
The next problem I am having is with sampling rate. If I were to use the low frequency measurement VI, the sampling rate of RPM would be variable based upon the the speed of the signal. Or, the original frequency measurement VI has an adjustable sampling rate. Of course, in my system there are a number of other signals that need to be recorded at the same time. I have found that as I am collecting data, the "write to file frequency" is entirely dependent on the read frequency of the frequency measurement. Therefore, if I had set the read frequency VI to read at 1 Hz, data will be recorded only every 2Hz. Ideally I would like an overall measurement frequency of all channels (mix of analog and digital) to read between 20-50Hz, but if I am limited by the frequency measurement.
Any ideas on how to solve this problem, either through Fieldpoint or Labview?
I can post my VI if this help.
Thanks,
Huang

Thanks for your reply.
I should probably describe my current setup before I go into anymore details with the problem. As for my specific setup, I am using an FP-1000 connected with an AIO600, AI110, CTR500, and TC120 all running through the RS232 line to a desktop running labview 8. The actual counter module is reading a tachometer signal output from a separate engine controller. THe output is a 0-12V ON 50% duty cycle signal. As for data logging, i am simply using a "write to measurement file" Express VI. I have a while structure which holds all of the express VIs which access the fieldpoint IO, and these are all routed to the "write to" VI.
As for the specifics of the data logging problem, as I said, when I set the count frequency of the "Fieldpoint Frequency Measurement" VI to 1Hz, (which means the VI calls the case structure at 2Hz), the "write to measurment file" VI is called at the same rate (2Hz). Which means that the overall logging rate of the VI is only 2Hz. Is there someway to decouple this?
I was able to solve the problem of calling the frequency variable from the case structure by using a local variable which is called outside the case structure.
And now I have been having a lot of problems with reading the actual frequency from my engine controller. THe actual signal will only range from 0 to around 200Hz. I noticed, by comparing the actual signal to what was being read in my labview program, that after around 80Hz the signal increases by around 1.5times more than the actual signal. ie. Actual signal = 100hz, Read Signal = 150Hz. After trying to figure out what was the problem, I decided to change the Noise Filter settings to 200Hz. It actually worked for all the frequency ranges up to around 150Hz but after that the filter attenuates the signal to the point where the actual signal is 160Hz, but is being read at 100Hz. My question is if there is a way to change the actual filter setting outside of the 2 given setpoints (200hz and 40khz) or if you have any other suggestions on how to fix this problem? I was thinking of creating a noise filter input in MAX so that i could play with the values in Labview, but am i only limited to those two filters?
Thanks again for your help,
Huang

Can you tell me how to measure audio signal frequency in measurement studio with visual basic 6.0?

I'd like to measure two differential input signal(I.e. A+ and A-),below is two test projects,
1.connect A+/A- to PCI 6052E card's ACH0/ACH8 ,and refer to GND for testing
2.connect respectively A+/A- to PCI 6052E card's ACH0/ACH8 for testing
Could you tell me the details of the second project?and give me a illustration in measurement studio with visual basic 6. 0 about the second?
In addition,I would like to measure the audio signal frequency,could you give me a illustration in measurement studio with visual basic 6.0?
Wait for your reply!

Dear kelven,
Thank you for contacting National Instruments.
To address your question, there are Analog Input shipping examples that demonstrate how to measure an analog input differentially, as you specify in (2).
As for measuring the audio signal frequency, you may want to check out the following KnowledgeBase. The link is:
http://digital.ni.com/public.nsf/websearch/4b08380530d72d3b86256340004c3005
Let me know if you have any further questions or if this does not resolve your issue.
Thanks again and have a great day!
Chad AE
Applications Engineer - National Instruments

Low-Frequency measurements using counter/timer

I am trying to measure speed, and am using the FP-502 counter timer module to count the pulses from my sensor. The field point example that uses a fixed width gate pulse will not work for my appliation because I need a gate pulse so big that it updates way to slow.
Does anybody have any sugestions on how to measure a low frequency signal using a counter/timer?
Thanks
Dan

Hello guys
Thanks for all your sugestions I have the final results. I got it to work but I would like to add some comments for anybody who is trying to attempt this.
Triggeing on the gate is a must for low frequency measurements. I used the divide by sugestion however using a factor of 3 only gets you about 60 Hz after that you can't sample fast enough. My application required me to measure from 0 to 150 Hz. I added a "gear shifter" routine where the terminal count was set to 3 on frequencies lower then 45 Hz and 15 for frequiencies greater than 45 Hz. Using a terminal count of 15 for all measurements made the udate rate on really low frequencies way to slow i.e. .5 seconds for 20Hz (check the math on this).
I a
lso needed to cascade the counters. At low frequencies the count went higher than 6550. Using the previos channel function of the fieldpoint module enable me to get 32 bit worth of terminal count data.
Finally I needed to add a timeout condition for 0 Hz. If there are no pulses coming from the senser the program displays the last value read. That won't be 0. So I added a time out routine that if I didn't see a pule in a certian amout of time, output 0 and reset the gate count.
Thank you for you help you relly saved me. I hope my comments made sense.
Dan

27" iMac screen low frequency humming + moving too freely on the base

Got a new 27 incher on tuesday after fedex lost the first one That's a different story.
The problems
1. The huge iMac moves a tad bit too freely on its base and when i try to tilt it to an angle, it doesn't stay at that angle it trys to go tilt back down ... maybe its gravity.. i don't know...
2. Very very low frequency humming noise from the iMac top left corner... its out when it goes to sleep. not the normal HDD/fan noise. I have a fan app, its not the fan... but its really annoying when working on it ...
i've seen a couple of posts about this. i'm taking this 27 incher to the store tomorrow... lets see how the geniuses deal with it. i hope they acknowledge this problem. does anyone else with the 27" have this problem?

Took it to the apple store, some genuises looked at it, confirmed it and sent me another one

Low frequency measurement from Parallel Port

Hi there...
I need to calculate the "on" and "off" time and duty cycle in pulse form from a parallel port. By making the circuit in 5 or 0 V, I just simply put it in my parallel port. The problem comes when I need to measure a very very low frequency. In this case, I want to measure the duty cycle from my operated refrigerator. I need to know when the thermostat goes "on" and when it comes to "off". In my experience, the thermostat will be "on" in about 5-10 minutes and "off" in about 20-30 minutes. So, the pulse might be take for a long periode each.
I've tried with Timing and Transition Measurement wizard or even by using Pulse Measurement.vi which is included in Waveform Measurement category. It only works for 2 Hz and . If I try to set it with 1 Hz or below, it comes the message :
"Error -20308 occurred at Timing and Transition Measurements -> Untitled 1
:4" (waveform index 0 of 1)
Possible reason(s):
Analysis: The waveform did not cross the mid reference level enough times to perform this measurement. Check the signal length, reference levels, and ref level units."
Could someone help me please ?
Regards,
Ricki

here is a quick shot to give you an idea
Greetings from Germany
Henrik
LV since v3.1
“ground” is a convenient fantasy
'˙˙˙˙uıɐƃɐ lɐıp puɐ °06 ǝuoɥd ɹnoʎ uɹnʇ ǝsɐǝld 'ʎɹɐuıƃɐɯı sı pǝlɐıp ǝʌɐɥ noʎ ɹǝqɯnu ǝɥʇ'
Attachments:
port logger.vi ‏22 KB

Measurment through a vibration sensor interfaced with Ni-PXI is very low & less responsive. I wana know any frequency limitations is there for PXi or the system uses any clipping ckts inside ???

Measurment through a vibration sensor interfaced with Ni-PXI is very low & less responsive. I wana know any frequency limitations is there for PXi or the system uses any clipping ckts inside ???

It would be helpfull to know what PXI hardware and what sensor you are using.
Up to now I only found mechanical limitations ....
(Creating a 100kHz sinus exitation with 160nm amplitude need more than 63km/s² (>6400g) acceleration )
Greetings from Germany
Henrik
LV since v3.1
“ground” is a convenient fantasy
'˙˙˙˙uıɐƃɐ lɐıp puɐ °06 ǝuoɥd ɹnoʎ uɹnʇ ǝsɐǝld 'ʎɹɐuıƃɐɯı sı pǝlɐıp ǝʌɐɥ noʎ ɹǝqɯnu ǝɥʇ'

Low frequency measurement (1 to 5 Hz) issues - Reg

Hello All,
i am facing a problem in measuring Low-Frequencies in the range of 1-5 Hz. I am using VIs used for pulse-width measurement and the card is PCI-6031E. i am generating frequency using PCI-6013 card.
Actually i am trying to develop an application for measuring the speed (RPM) of the motor which runs very slowly. so if i miss one pulse per second also, when i multiply that with 60 it will be big error.
So if any can give me a solution that will be big help to my issues.
Thanks.

Hello sathiskumar,
I'm not sure what you mean by missing a pulse but I think what is
happening is that your counter is rolling over and starting to count
from zero again. Every time the counter rolls over, it will
output a pulse. You can connect this signal to another counter
and count how many times your first counter has rolled over. That
way, if you don't receive a pulse on your first counter before your
counter rolls over, the second counter will have a record of how many
times this has happened.
I hope this helps!
Eric
DE For Life!

How to tune the threshold to a very low value

Hi,
how to set the thershold to a very low value for database 9i. on which critieria we will tune this thershold value of DB

user606944 wrote:
I/o thershold value?There is no threshold value... unless there is some base line to measure against.
Also, I/O threshold values in what respect? Number of reads/sec? Writes/sec? Latency per I/O call? Number of bytes per second?
A solution is only as good as the problem definition. You have not defined any problem.. thus it is quite difficult to suggest any type of solution to you.

How to measure the frequency of the data plotted on the waveform chart

i have made a virtual instrument in labview that acquire data (say sine wave) into a spreadsheet file and also on the chart from the function generator. I measured the frequency of the data from the chart but it is coming out to be very less. How can i calculate the frequency of the plotted sine wave and also from the spreadsheet file.

If you want to get an idea of the base-frequency of an periodic signal, you can measure the number of samples in between two maxima or in between two minima of the periodic signal.
Multiply the number of samples by the sample-interval (1/Fsample in [sec]) gives you a rough estimate.
FFT, I mentioned is one of the more advanced calculation-methods.
The method calculates the spectrum from a waveform.
Hope this helps,
Patrick

Low Frequency Measurements using counter timers

I am trying to measure multiple speeds using FP-CTR-502 modules. I am having a resolution problem. Looking at the post at the following URL (http://forums.ni.com/ni/board/message?board.id=110&message.id=3601&query.id=42252#M3601) looks like exactly what I need to do, but I'm not sure how to program this. Does anyone have any example code to get me headed in the right direction?

Hi Tcrower,
Here is the example that Carl L. was describing in his forum post along with his instructions. Also, there are thorough instructions for using the program in the front panel as well as in File >> VI Properties >> Documentation.
You can measure frequency in two ways using a counter.
1) Measure the time it takes for a set number of pulses to pass or,
2) Count the number of pulses that pass in a set amount of time.
For low-frequency signals it is best to measure the time duration of a set number of pulses. If we tried to calculate the number of pulses that passed in a set amount of time, we would need a long gate period to get accurate results.
The drawback of measuring the amount of time it takes for a set number of pulses to pass is that the sample period varies with the frequency of the signal.
To accurately measure low frequencies with a CTR module, select two counter channels, one output channel, and one gate input channel. Count Input 0 counts the pulses of the signal that we are measuring. The output of the counter is set to high for x pulses and low for x pulses, which is used as the gate of Count Input 1. The source of Count Input 1 is the internal clock reference (the example can be modified to use an external clock reference). During the high phase of Count Input 0, Count Input 1 counts the pulses of the clock reference.
During the low phase, we read channel 1 and reset the counter.
For example, if the output of Count Input 0 is set to high for 4 pulses and the signal has a frequency of 20 Hz, the output of Count Input 0 is high for 200 ms. During the high phase, we will count 200 counts if we are using the 1 kHz internal clock reference. So to calculate the frequency, we have F = #pulses/duration = timebase * terminal count/counts = 1000 Hz * 4 terminal counts/200 counts = 20 Hz.
Regards,
Hal L.
Attachments:
Low_Frequency_Measurement_(CTR).zip ‏77 KB

Low frequency Voltage measurement

Hi,
I am using PCI 6025E to control load bank and to measure voltage, current and frequency of a generator.
While current and frequency measurement is not a problem, voltage measurement is toublesome.
Since on test bench already there are few PCI boards and huge number of sensors, I do not want to add NI 9205 or any other additional boards and clutter the area (other people are also working there).
The problem with voltage measurement is that out of three generators two run from 400 RPM to 1600 RPM i.e.. somewhat 15-55 Hz.
I had two solutions for this:
1, Putting a step down transformer (230 Vac / 5 Vac) and then measure the voltage. But when testing at somthing like 15 Hz I have to deal with magnetic saturation with commercially available transformer. I do not want to take pain to go on built my own transformer.
2, Using high precision voltage Divider. The problem is that what should be the galvanic isolation? Isolation transformer, but then again low frequency problem.
Hence could someone suggest me since there is a bit confusion in my head.
Thanks.
Solved!
Go to Solution.

If you have a sine generator (15Hz migth be hard for the soundcard ) you can do a calibration of your transformer.
(If only the voltage amplitude is of interest , use a isolated DMM as reference and run your generators)
Or you buy a isolation voltage sense amplifier. (weidmueller, phoenix, wago, ...)
Greetings from Germany
Henrik
LV since v3.1
“ground” is a convenient fantasy
'˙˙˙˙uıɐƃɐ lɐıp puɐ °06 ǝuoɥd ɹnoʎ uɹnʇ ǝsɐǝld 'ʎɹɐuıƃɐɯı sı pǝlɐıp ǝʌɐɥ noʎ ɹǝqɯnu ǝɥʇ'

"How to measure very low frequency?​"

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"How to measure very low frequency?"