In 10/2016, PSI started the beam commissioning of Aramis, the hard X-ray undulator line of the SwissFEL free electron laser. The injector, linac and transfer line BPMs have 3.3 GHz low-Q cavity BPMs to support 2-bunch operation with 28ns bunch spacing. In contrast, Aramis as well as the future 2nd soft X-ray undulator line called "Athos" are equipped with 4.9 GHz high-Q cavity BPMs, since the undulator will only be operated with single bunches by means of a fast beam distribution kicker system. The undulator BPM system uses 4.9GHz double-resonator pickups having a nominal loaded-Q of 1000. The associated front-end electronics applies single-stage analog down conversion to a 135 MHz IF frequency, subsampling ADC and digital quadrature down conversion. A detailed description of the BPM pickup and frontend design and signal processing will be given. Laboratory test and calibration methods and results of pickups and electronics will be compared with first beam measurements.