[Discuss-gnuradio] USRP RF output frequency is much faster than intended.

I build a flow graph like an image in the following link: http://i.imgur.com/PpXck09.jpg

After then, I connect the USRP and an oscilloscope and see output, which yields: http://i.imgur.com/JEKhnfx.jpg

As you see, both are square waves, but the frequency, the above is 1k and the below is 7.8 kHz.

If I set a frequency of signal source to 2k, the below says 16.6 kHz. (Of course, there are some deviation)

How could I understand this?
If the oscilloscope shows a frequency slower than the GNU radio,
I think that is acceptable since there are latency and processing time and something.

But the output is 7.8 times faster than what I inteded.
The only thing I can guess is... 7.8 is rounded up to 8, and 8 indicates '8' bits.
But it seems a bit unnatural and weird...

Could anyone give me a hint?

If you think it is more appropriate to be posted on USRP-users, I'll post it there later. :)

Regards,

Jeon.