Hi Laura,
With 16 bit samples, the default, a 1 Gigabit Ethernet link can only carry 25 MS/s. Thankfully, that's complex IQ samples, so while Nyquist applies the bandwidth is 1x the highest frequency.
http://files.ettus.com/manual/page_usrp2.htmlThe N200 has a fixed master clock rate (ADC/DAC rate) of 100 MHz. Only integer factors of that will be possible, if you look at your console output there should be a line saying that 40 MS/s is not a possible value and it is being set to a different one, either 25 MS/s or 50 MS/s. Either of those isn't what you are expecting so your flowgraph will not produce the correct values. 25 MS/s would be the best rate for you to use as it fits your 20 MHz wide signal with some margin on either side to accommodate filter roll off. It is also an even divisor of the master clock rate so you'll have better filter performance than using a rate that is an odd divisor.
Can you please test with the benchmark_rate that comes with UHD to see if your system is setup to transport samples at that rate?
You may also find that it is harder to write 25 MS/s (200 MB/s) to a harddrive than it is to run the waterfall plotting.