  We discuss graphene nanoribbon-based charge sensors and focus on their
functionality in the presence of external magnetic fields and high frequency
pulses applied to a nearby gate electrode. The charge detectors work well with
in-plane magnetic fields of up to 7 T and pulse frequencies of up to 20 MHz. By
analyzing the step height in the charge detector's current at individual
charging events in a nearby quantum dot, we determine the ideal operation
conditions with respect to the applied charge detector bias. Average charge
sensitivities of 1.3*10^-3 e/sqrt{Hz} can be achieved. Additionally, we
investigate the back action of the charge detector current on the quantum
transport through a nearby quantum dot. By setting the charge detector bias
from 0 to 4.5 mV, we can increase the Coulomb peak currents measured at the
quantum dot by a factor of around 400. Furthermore, we can completely lift the
Coulomb blockade in the quantum dot.
