Concept of Beam Position Monitor with Frequency Multiplexing*

Abstract

Two most widely used beam position monitor (BPM) systems (manufactured by Bergoz [1] and Instrumentation Technologies [2]) implement switching technique to eliminate errors associated with drifts in the channel gains. High stability is achieved by an alternative routing of signals from all pick-up electrodes (PUE) through the same chain. Such an approach creates problems with turnby-turn acquisition as well as measurement noise. In this paper, basing on the advances of digital signal processing that allow identical gains for the wide frequency ranges, we propose separating signals in the frequency domain. The experimental set-up and test results are presented. Practical realization of the beam position monitors is also discussed. INTRODUCTION Development of ultra-bright synchrotron radiation sources and high luminosity colliders requires unprecedented levels of beam stability. To achieve needed steadiness one can use a channel switching technique [1, 2] or utilize a pilot tone [3, 4]. The channel switching produces very low drifts but also manifests itself as narrow band measurement noise at the switching frequency. Another disadvantage is lack of the turn-byturn (TbT) capabilities. Pilot tone technique can provide both stable long-term beam position monitoring and TbT data: however it requires extra hardware to be placed inside an accelerator tunnel. In this paper we propose a new approach based on the separation of the signals from different pick-up electrodes (PUE) in the frequency domain combined with digital processing. Processing in the digital chain is a key element, because it provides stable and equal gain in the wide frequency range. EXPERIMENTAL SETUP The proposed signal processing scheme is shown in Fig. 1. For simplicity the scheme with two pick-up electrodes, A and B, is used (four-PUE implementation will be discussed later). Signal from each pick-up is divided into the two channels with equal amplitudes by splitters S1 and S2. Two local oscillators Osc 1 and Osc 2 set the intermediate frequencies (IF). Signals from each PUE are down-converted with mixers M1-M4 and cross combined with combiners C1-C2. The first analog-todigital converter (ADC) processes signal A at the first intermediate frequency and signal B with the second IF; the second ADC processes signal A at the second IF and signal B with the first IF. Due to symmetry of the processing chain the firstorder errors from the splitters and combiners (such as inequality of division and summation) and the amplitude variations of the local oscillators are cancelled; only the second-order terms remain. For the analysis purpose the variation of mixers insertion losses are included into the corresponding errors in splitters and/or combiners. Components characterization The tests were performed utilizing a LeCroy WavePro 7300A digital oscilloscope, ZFSC-2-4-S+ splitters/combiners, ZX05-10-S+ mixers by MiniCircuits, two N5181A RF signal generators by Agilent were used as local oscillators, and an RF and Clock Generator by Instrumentation Technologies for the signal source. The carrier frequency was suppressed by the lowpass filters with cut-off frequency depending on the chosen IF. The carrier frequency was set to 481.57 MHz to minimize the effect of reflected wave from the mixers. First, the splitters and ADC were verified with two measurements in which signal after splitter goes directly and then is crossed to the two channels of the oscilloscope. The measured waveforms were fit with sine, and obtained amplitudes were used to calculate unevenness of the splitters and the gain inequalities of the ADCs. The channel gains were different by 6×10 and the splitter was found to have 1.4×10 unbalance. Beam offset was simulated by insertion a 1 dB attenuator (theoretical transmission of 0.8913) into position A or position B. The measured transmission was 0.885. Tests with Low IF The initial measurements were performed with low IF, where the reduced sampling rate allows ADCs to have less noise and more effective bits. The local oscillators were set to 478.93 MHz (IF1=2.64 MHz) and 478.01 MHz (IF2=3.56 MHz). The SLP-5+ low-pass filters by MiniCircuits have cut-off frequency of 5 MHz. The sequence of 10 μs duration was recorded with 1 Gs/sec rate. The recorded signals ware fitted with a two-tone sine waveform:

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Cite this paper

@inproceedings{Pinayev2010ConceptOB, title={Concept of Beam Position Monitor with Frequency Multiplexing*}, author={Igor Pinayev}, year={2010} }