  By operating the radio frequency single electron transistor (rf-SET) as a
mixer we present measurements in which the RC roll-off of the tunnel junctions
is observed at high frequencies. Our technique makes use of the non-linear
rf-SET transconductance to mix high frequency gate signals and produce
difference-frequency components that fall within the bandwidth of the rf-SET.
At gate frequencies >15GHz the induced charge on the rf-SET island is altered
on time-scales faster than the inverse tunnel rate, preventing mixer operation.
We suggest the possibility of utilizing this technique to sense high frequency
signals beyond the usual rf-SET bandwidth.
