  We describe instrumentation for a high-frequency electron paramagnetic
resonance (EPR) and pulsed electron-electron double resonance (PELDOR)
spectroscopy. The instrumentation is operated in the frequency range of
107$-$120 GHz and 215$-$240 GHz and in the magnetic field range of 0$-$12.1
Tesla. The spectrometer consisting of a high-frequency high-power solid-state
source, a quasioptical system, a phase-sensitive detection system, a
cryogenic-free superconducting magnet and a $^4$He cryostat enables
multi-frequency continuous-wave EPR spectroscopy as well as pulsed EPR
measurements with a few hundred nanosecond pulses. Here we discuss the details
of the design and the pulsed EPR sensitivity of the instrumentation. We also
present performance of the instrumentation in unique experiments including
PELDOR spectroscopy to probe correlations in an insulating electronic spin
system and application of dynamical decoupling techniques to extend spin
coherence of electron spins in an insulating solid-state system.
