Simultaneous frequency measuring

[Achim]

I have to measure for frequency signals simultaneously, so that I'm able to associate one measured value to the others. How do I synchronize this? Is this possible with the four counters on a 6601 board? Two of the signals have frequencies up to 15 KHz, two of them go up around 4kHz. I also have a 6014 board with an additional counter available, maybe I could use it somehow...I also have to synchronize the analog acquisition that I perform with the 6014 with the one of the frequency signals, namely a continuous TTL pulse.

Thanks in advance for any help!

[mhaag]

It sounds like there are two major parts of your application that you need to work on (1) synchronizing your counter operations and (2) synchronizing your analog input (AI).

Since you need to perform four frequency measurements, the easiest way to get the counters synchronized is to supply a common gate and trigger to all four counters so that they start together and all acquire their frequency measurements off of the same input clock/pulse-train. I have attached an application note that discusses a lot of the factors that you need to consider and how to perform multicounter synchronization. Also the following link also gives some more information on triggering multiple counters with a single pulse:

http://digital.ni.com/public.nsf/websearch/CE99A0B2A
9048A2A86256C630062F550?OpenDocument

Along with this, you mentioned wanting to synchronize your analog acquisition with one of the frequencies. I am assuming that this means you want to trigger your analog acquisition based on one of your four frequency signals. I am not completely clear on what your application needs to do here so I would look at what you have and at some of the information online and the shipping examples to see if you can find a good example to show you what you need to do in this case. If you need to set up a retriggerable AI, then remember that your hardware is not retriggerable; however, you can emulate this in software, which can be slow since it is not that efficient and is very system dependent, or you can use two counters to create a continuous pulse train.


Regards,
Michael
Application Engineer
National Instruments

[Achim]

Hi Michael,
thanks for you help! Your hints and document links were very useful, as well as another answer in another thread. In conclusion they all lead to an example I got from NI Germany Support. It was created for a 6602 but I modified to use it with my 6601. I attached the example llb to this document including my modification. It works quite good, but I wonder why counter 1 always counts one event more than counter 2 and 3. I used only three counters here, because for testing reasons (on my office pc) I used counter 0 to generate a contiuous pulse train as an input "simulation". All three gates used share the same signal to be measured, namely the output of counter 0. Could it be that counter 1 sees the trigger edge on PFI34 (as I configured it) a l
ittle bit earlier than the other two (delay for hw reasons)?

Kind regards
Achim

[mhaag]

One reason why you may be seeing slightly different counts and a delay is due to differet propogation delays to the various counters. If this is the problem this issue can often be resolved through pad synchronization. I believe the "Multicounter Synchronization App Note" attached to my previous response contains some information on this. For a general description you can refer to the following link:

What is Pad Synchronization?

My guess is that is the biggest reason for the effect your are currently seeing in your application. I would take a look at this and hopefully that will take care of it.

Regards,
Michael
Applications Engineer
N
ational Instruments