Change the sampling rate for digital signal generation

I have a question about changing the sampling clock rate when using the digital output
lines to generate signals. 

I'm using DAQmxCfgSampClkTiming (TaskHandle taskHandle, const char source[],
float64 rate, int32 activeEdge, int32 sampleMode, uInt64
sampsPerChanToAcquire); to set the sampling clock source for digital signal
generation.

But I found out the parameter float64 rate doesn't really change the sampling
frequency. For example, if I use "100kHzTimebase" for the const char
source[] clock source parameter, no matter what number I specify for  float64
rate, the sampling rate is always 100kHz.

I don't understand where I did wrong, and I was wondering what's the easiest
way to change the sampling frequency to an arbitrary number I want.