keeping real time frequency in point by point signal generation

[rols]

hi.

i need to develope a system where I generate a multitone signal for an energy meter calibration procedure. i'm using labview 7.1 and a daq board pci 6024 E. the problem is that i need to perform a control loop over the generation, so the point by point generation comes to mind. keeping the real frequency of the signal to be generated is fundamental. with buffered generation keeping the frequency is not an issue, but when i perform point by point generation the frequency of the generated signal is not as precise as i wish it to be. my question is: is it possible to keep real time frequency in point by point signal generation? how could that be done? i´ve been trying to find examples but haven´t been able to find one!!

[GerardoG]

Hello rols,

This is a classic case where you most likely need a deterministic, real-time system. Your post didn't specify, but I assume you are using a desktop Windows PC to run your signal generation. The Windows OS (and other general purpose operatings systems) are not deterministic. This means that they cannot consistently close your control loop at any meaningful rate. Background tasks such as OS services, hard driver reads/writes, interrupt response, etc. will steal CPU cycles in an unpredictable fashion that kills the determinism of the system.

There are several options that you have for real-time performance with LabVIEW. You can have a dedicated real-time system, or you can use an FPGA card. With a PC real-time system, you can use the same code you are using on Windows box and the same hardware. FPGAs offer more performance but you're calculations need to be done in a fixed-point datatype, so you don't have the floating-point support.

Here's a nice tutorial on choosing a real-time system,

Selecting your LabVIEW Real-Time Deployment Platform

and here's some more information on FPGAs:

FPGA Starting Page

I hope this helps.

Gerardo