Lipka, D. Lorbeer, B. JACoW Publishing Geneva, Switzerland 2673-5350 978-3-95450-204-2 10.18429/JACoW-IBIC2019-WEAO02 English 453-456 WEAO02 resonance cavity FEL electron electronics Contribution to a conference proceedings 2019 2019-11 https://doi.org/10.18429/JACoW-IBIC2019-WEAO02 http://jacow.org/ibic2019/papers/weao02.pdf The European XFEL facility is in operation with a maximum of 2700 bunches in one train. The highest bunch repetition rate is 4.5 MHz; this corresponds to a minimum time separation of 222 ns. The measurement of the beam properties for each bunch in a train is required. Therefore the beam position monitor (BPM) system needs to separate the signals from each bunch. All BPM types (button, re-entrant and cavity) fulfill this requirement except a few button BPMs installed inside of the cold accelerator module, where Pile-Up from the train can be observed. To identify the cause of this effect we measured the S-parameters during a shutdown of the accelerator, compared it with a similar BPM at the FLASH accelerator but located in a warm section and finally measured the spectrum of the button signal during beam operation. As a result, resonances were found at about 2.46 GHz with relatively high quality factor that remains within the frequency range accepted by the electronics.