Generate an high frequency sinusoid

[Lando]

Hi,

I hve to generate high frequency sinusoid (90-100 kHz). I have a NI USB-6259(BNC) board. When I try to generate a sinusoid with  frequency of 100 kHz, since the maximum sampling frequency is about 1.25 MHz, I have only 12 points per period. Thus the resolution of the output signal is very low and the sinusoid, seen with an oscilloscope, is evidently quantized. But if I generate the same signal with a program realized in Visual Basic and using a board with a lower maximum sample frequency, the signal seen on the oscilloscope is very smoother.

I tried the Labview software I made with this second board, in order to check if it was a board problem, but the signal was still bad.

The code I used for the software in labview is a mere copy of the code present in the DAQ assistant express VI. Unfortunately I do not hve the Visual basic program source code, so I can't say how it works.

Now, my question is: There's a way to generate smooth signal at high frequency even with a small amount of point per period?

[rpursley8]

Add and RC filter to the output.  Resistor can be about 1 kohm, Capacitor about 1 nF.  Resistor in series, followed by a capacitor to ground.

[Lando]

I can't use a filter because I need to measure the phase very precisely and a filter would introduce an undesired phaseshift. There's a way to do it using LabVIEW?

[rpursley8]

What is this second board with a lower maximum sample frequency?

DAC boards put out voltages at a specific sampling rate.  If the other board has a lower sampling rate but looks smoother (if you do not see quantization), then it has to have a lowpass filter (or some kind of band-limited chip) in the output path.  There is no magic in the code.

Message Edited by rpursley8 on 04-26-2010 12:21 PM