The Proton Engineering Frontier Project (PEFP) low-level radio frequency (LLRF) system for the 3-MeV radio frequency quadrupole accelerator (RFQ) and the 20-MeV drift tube linear accelerator (DTL) has been developed. A stability of 1 % in the amplitude and 1 degree in the phase is required. Therefore, the drift of the analog components should be low enough to satisfy these requirements. An analog chassis, as a prototype of the LLRF system, was con gured and tested. RF components, including an IQ modulator, an RF switch, a mixer, RF splitters, RF lters, a circuit for measuring tank phase and a trip circuit for high voltage standing wave ratio (VSWR), have been installed in this chassis. The analog chassis performs an error compensation of the RF amplitude and phase from the IQ signal, a down-conversion to the 10 MHz IF signal, an interlock for the arc and the high VSWR and a distribution for the RF and the clock signals. The amplitude and the phase stability of each component were measured to check the e ect on the whole system's performance. In the test with dummy cavities, a uctuation of 5 degrees was measured in the 340 MHz LO signal, but the relative phase between the two cavities was maintained within 0.2 degrees because of the e ect being the same for the LO signals of the two cavities.