  Electron spin resonance with a scanning tunneling microscope (ESR-STM)
combines the high energy resolution of spin resonance spectroscopy with the
atomic scale control and spatial resolution of STM. Here we describe the
upgrade of a helium-3 STM with a 2D vector-field magnet ($B_z$ = 8.0 T, $B_x$ =
0.8 T) to an ESR-STM. The system is capable of delivering RF power to the
tunnel junction at frequencies up to 30 GHz. We demonstrate magnetic field
sweep ESR for the model system TiH/MgO/Ag(100) and find a magnetic moment of
$(1.004 \pm 0.001)$ $\mu_B$. Our upgrade enables to toggle between a DC mode,
where the STM is operated with the regular control electronics, and an
ultrafast-pulsed mode that uses an arbitrary waveform generator for pump-probe
spectroscopy or reading of spin-states. Both modes allow for simultaneous
radiofrequency excitation, which we add via a resistive pick-off tee to the
bias voltage path. The RF cabling from room temperature to the 350 mK stage has
an average attenuation of 18 dB between 5 and 25 GHz. The cable segment between
the 350 mK stage and the STM tip presently attenuates an additional
$34_{-3}^{+5}$ dB from 10 to 26 GHz and $38_{-2}^{+3}$ dB between 20 and 30
GHz. We discuss our transmission losses and indicate ways to reduce this
attenuation. We finally demonstrate how to synchronize the arrival times of RF
and DC pulses coming from different paths to the STM junction, a prerequisite
for future pulsed ESR experiments.
