USB 6341 counter output

Hi,

I'm trying to use the output counter  of USB 6341 for generating a square wave with a frequency between100 KHz to 1Mhz.

I used 100Mhz and 20 Mhz time bases and the 10Mhz ref clock as the source clock.

I checked the output signal in the oscilloscope.The performance was good for frequencies close to 100KHz.However, the output signal was distorted for frequencies higher than 300 Khz  

Is there a way of achieving a non distorted waveform for this frequency range?

Thanks,

Leonid