creating internal sampling clock to take acquisition on analog input with PCI6251

[youngwater]

Hello,
 

I am using PCI 6251 and SCB-68pin E series connections and Labview 8.I have got 2 analog input channels (I'll call them AI-1 and AI-2)
 

AI-1 is a chirped (FM modulated) sinusoidal. the frequency is chirped around 200kHz.
on&nbsp; (AI-1), I realised Retriggerable Acquisition by using an example program (NI-DAQmx:Retriggerable AI Using Counter and an Analog Trigger, Filename: <a href="ftp://ftp.ni.com/pub/devzone/epd/retrigg_ai_analogtrigger.vi" target="_blank">retrigg_ai_analogtrigger.vi</a>).
for the moment, the program does nothing about AI-2.
&nbsp;
I would like to add the following functionality to this program:
&nbsp;
-a sw trigger (or sampling clock?) is generated each time the (chirped) sinusoidal on AI-1 makes a rising-edge-zero-crossing.
-this trigger (or sampling clock) is&nbsp;used for sampling&nbsp;second analog input channel (AI-2). That is to say,&nbsp;ONE sample is taken from AI-2 each time the signal on AI-1 makes a rising-edge-zero-crossing.
-&nbsp;as the AI-1 is a chirped sinusoidal,&nbsp;the step intervals between&nbsp;successive sampling points on AI-2 will be unequal.
- these unequal sampling intervals and sampled data values are written into a file for further signal processing.
&nbsp;
in order to clarify the application, I would like to express it as follows:
my signal on AI-2 is called U(t)
t(i): is the time at which the ith data point is taken
t(i)=t_0 + i * Delta
Delta: is the time interval between data points.&nbsp;
Delta is NOT equal for the samples.
I need two arrays written into a file:
U[t(i)]: data points
and
t(i)
&nbsp;
&nbsp;
in&nbsp;the application&nbsp;the data acqusition on AI-1 will be kept as a retriggerable&nbsp;controlled by an external trigger (trigger period is about 200ms)
I really can not see how the tasks defined above to be embedded into the program so that the acquisition on AI-2 will be contolled by the&nbsp;data of AI-1&nbsp;&nbsp;in real time between two external triggers.
&nbsp;
any help will be highly appreciated!!
&nbsp;
thanks in advance.
regards,

[demux]

Greetings Youngwater, What you are trying to reach is impossible with the type of board that you have. Both of them are possible, but just not at the same time.Your description demands a different timing+triggering for each AI channel. You cannot program this on a board that has one ADC and one triggering circuit.A workaround (besides buying another board) would be to measure AI2 at the same rate as AI1, check by array manipulation in software when AI1 goes through zero and discard the redundant data from AI2Downsides are that you would only be able to see zero-crossings of AI1 in case you were measuring it. The accuracy of timing would be set by the speed you decide to measure AI1. Kind Regards,WouterNational Instruments Belgium