Google Groups no longer supports new Usenet posts or subscriptions. Historical content remains viewable.
Dismiss
Re: How do I read a pulsed input on an SCB-68 counter channel?
9 views
Skip to first unread message
Steve_B
Jul 29, 2008, 7:10:10 PM7/29/08
to
Hi Shyam,
Based on your description of the edge counting it makes sense why you are seeing an increased frequency measurement. In order to perform a frequency measurement, the counter is actually counting the number of clock ticks of a known time base between rising edges of your signal. The number of edges that occur can be turned into the period and frequency of your signal. If the edge count is counting more edges than it should, the higher frequency is expected. Can you try to measure another known signal, perhaps from a function generator or even from the board itself. You can route a pulse generation into a frequency measurement to check its behavior. If your signal is noisy the noise could be interpreted as edges of your signal which would cause a higher edge count/frequency measurement. You should not need to tristate the count direction pin for this measurement.
Based on your description of the edge counting it makes sense why you are seeing an increased frequency measurement. In order to perform a frequency measurement, the counter is actually counting the number of clock ticks of a known time base between rising edges of your signal. The number of edges that occur can be turned into the period and frequency of your signal. If the edge count is counting more edges than it should, the higher frequency is expected. Can you try to measure another known signal, perhaps from a function generator or even from the board itself. You can route a pulse generation into a frequency measurement to check its behavior. If your signal is noisy the noise could be interpreted as edges of your signal which would cause a higher edge count/frequency measurement. You should not need to tristate the count direction pin for this measurement.
Steve_B
Jul 31, 2008, 4:10:09 PM7/31/08
to
Hi Shyam,
The major factor that would come into play with the signal having some noise is that the counter is expecting a TTL signal and the oscilloscope is measuring an analog input signal. If your signal floats above the TTL thresholds or does not rise/fall within the specifications, the behavior is not predictable and it can be measured as a high or low by the counter. An oscilloscope will measure the analog signal and you may be able to measure the frequency of the noise on your scope by analyzing a smaller timeframe of the signal.
What Is the Definition of a TTL (Transistor-Transistor Logic) Compatible Signal?<a href="http://digital.ni.com/public.nsf/3efedde4322fef19862567740067f3cc/acb4bd7550c4374c86256bfb0067a4bd" target="_blank">http://digital.ni.com/public.nsf/3efedde4322fef19862567740067f3cc/acb4bd7550c4374c86256bfb0067a4bd</a>
The major factor that would come into play with the signal having some noise is that the counter is expecting a TTL signal and the oscilloscope is measuring an analog input signal. If your signal floats above the TTL thresholds or does not rise/fall within the specifications, the behavior is not predictable and it can be measured as a high or low by the counter. An oscilloscope will measure the analog signal and you may be able to measure the frequency of the noise on your scope by analyzing a smaller timeframe of the signal.
What Is the Definition of a TTL (Transistor-Transistor Logic) Compatible Signal?<a href="http://digital.ni.com/public.nsf/3efedde4322fef19862567740067f3cc/acb4bd7550c4374c86256bfb0067a4bd" target="_blank">http://digital.ni.com/public.nsf/3efedde4322fef19862567740067f3cc/acb4bd7550c4374c86256bfb0067a4bd</a>
0 new messages