Jump to content

As of July 17, 2015, the LabJack forums here at forums.labjack.com are shut down. New registrations, topics, and replies are disabled. All forums are in a read-only state for archive purposes.

Please visit our current forums at labjack.com/forums to view and make new posts. To post on the current forums, use your labjack.com login account. Your old LabJack forums login credentials have been retired. There are no longer separate logins for labjack.com and LabJack forums.


Photo

how to measure frequency?


  • Please log in to reply
7 replies to this topic

#1 sylvester

sylvester
  • Members
  • 32 posts

Posted 30 November 2013 - 08:54 AM

Hi,

I have attached a frequency signal to the counter of a Labjack U12. However, I can not read the frequency neither using Ecount nor the provided example LJcounter. No error message, but no counts, the timer works. I can read a frequency of 6.06 kHertz with a multimeter between CNT and GND. The signal comes from a Seabird conductivity cell, that gets 24V DC current and generates a frequency signal (signal ground is on power ground). I have connected the ground of the 24V power supply to Labjack GND. The voltage I measure between CNT and GND is 0.4V AC on the DVM ( 0.28V in DC mode). I checked with a second U12 to exclude a hardware problem, same result. I wonder if it is not possible to measure frequency with this setup with a U12 at all or did I connect something wrongly?

Any help appreciated

Sylvester



#2 LabJack Support

LabJack Support
  • Admin
  • 8677 posts

Posted 01 December 2013 - 08:59 PM

The voltage on CNT must go above 3V to be logic high and below 1V to be logic low.  Does not sound like your signal is doing that.  One possible solution is to use a comparator.  For more info try searching with the term "comparator" at labjack.com:

 

http://labjack.com/search



#3 sylvester

sylvester
  • Members
  • 32 posts

Posted 02 December 2013 - 09:41 AM

Ok, I need to get it working asap. I have a LJTick InAmp here; can I use that to amplify the signal for this purpose?

(I know it does not fit directly into the U12, I need to use some cables for connection)



#4 LabJack Support

LabJack Support
  • Admin
  • 8677 posts

Posted 02 December 2013 - 09:55 AM

You said you currently have a 0.4 VAC signal.  Is that RMS and centered around 0.0?  That would mean you have a AC signal that goes from about -0.56 to +0.56 volts.

 

Is the amplitude the same regardless of frequency?



#5 sylvester

sylvester
  • Members
  • 32 posts

Posted 02 December 2013 - 10:50 AM

Not sure, if my multimeter measures the voltage correctly at about 6kHertz frequency. I measured 0.4 V with AC setting and about 0.28 V with the multimeter set to DC, therefore I assume the signal is positive.

The output of the conductivity cell is a frequency signal proportional to the conductivity measured. Therefore the amplitude should not change with frequency.



#6 LabJack Support

LabJack Support
  • Admin
  • 8677 posts

Posted 02 December 2013 - 11:04 AM

Can you provide a link to tech specs for the sensor?  In particular, we need to know the max and min of the output voltage signal.



#7 sylvester

sylvester
  • Members
  • 32 posts

Posted 02 December 2013 - 12:59 PM

This is the data sheet;

seems the signal is 1V square wave

http://www.seabird.c...heets/4data.htm



#8 LabJack Support

LabJack Support
  • Admin
  • 8677 posts

Posted 02 December 2013 - 04:58 PM

It also says it is capacitively coupled, so I suspect it is a +/-0.5V square wave.

 

So what would happen if you connect the positive to INA+ on your LJTick-InAmp, connect the negative to INA- on the LJTick-InAmp, and also jumper INA- to GND?

 

You should not be exceeding the -10V to +15V "No Damage" spec of the LJTIA, so that seems OK:

 

http://labjack.com/s...inamp/datasheet

 

I would use offset=0.4 and gain=11 on the LJTIA.  That would give you an output of -5.1 to +5.9, which the LJTIA can't do, but you might just get the output limits of 0 and 3.5 volts which would work as an input to CNT.




0 user(s) are reading this topic

0 members, 0 guests, 0 anonymous users