Instrumentation & Measurement Magazine 24-9 - 28

Fig. 7. Button BPM signal in the frequency-domain (upper row) and the time-domain (lower row) to a single Gaussian bunch for three different bunch lengths.
a bunch length σs = σtc0 >> 2r, the output signal waveform
vbutton
≤ 2r, the output signal waveform becomes like that of
(t) is a differentiation of the Gaussian bunch signal shape,
but as the bunch length gets close or smaller than the button dimension
σs
the Gaussian bunch signal shape. In practice however, parasitic
effects start to dominate at high frequencies, causing reflections
that result in a strong " ringing " effect of the output signal, and is
often observed in applications for short electron beam bunches.
BPM Signal Processing
The signals from the BPM electrodes need to be conditioned
and processed to extract the beam intensity independent beam
position information. This is realized by a set of electronics
hardware (analog, RF and digital), digital gateware and realtime
software that implements the discussed normalization
procedure, often following the Δ/Σ-normalization principle.
The exact concept, design, layout, and implementation of the
BPM signal processing system depends on a variety of factors,
e.g., type of accelerator, beam parameters and formatting, performance
requirements of the beam position measurement,
environmental conditions, etc., but also on less technical factors,
like budget and manpower resources, infrastructure, as
well as on laboratory standards, rules, and regulations.
As Fig. 8 illustrates, the BPM signal processing is a continuation
of the symmetry concept of the BPM pickup, used to
detect a small asymmetry, i.e., the beam displacement with a
set of highly symmetric signal conditioning and processing
electronics in presence of a high common-mode Σ-signal. The
more perfect the symmetry of the entire BPM system-pickup
and signal processing-the higher will be its performance potential,
e.g., in terms of BPM resolution and accuracy.
Read-out Electronics
Each BPM pickup is equipped with read-out, sometimes called
front-end electronics, to process the BPM electrode signals of
28
one or both (horizontal and vertical) planes. In most cases it is
preferred to locate the BPM electronics outside the accelerator
tunnel to avoid damage of the semiconductors and other elements
from the ionizing radiation during machine operation.
This requires rather long coaxial cables, TEM transmissionlines,
between the BPM pickup in the accelerator tunnel and
the read-out electronics located in a gallery or service building,
sometimes installed in temperature stabilized racks. However,
in some cases, e.g., to detect the position of a beam with very low
intensity, it cannot be avoided to locate a few signal conditioning
elements, e.g., pre-amplifiers, RF filters, hybrids, etc. in the
tunnel near the BPM pickup, or a special impedance-matching
network requires to be mount directly on the BPM feedthrough.
There exists a very large variety of BPM signal processing
concepts and post-processing schemas, impossible to cover all
of them in this introduction, Fig. 9 gives an overview of analog
signal processing options for BPMs [15]. Moreover, many of
the different BPM signal processing concepts reflect a present
state-of-the-art in electronics and telecommunication technologies
at a given time, but may be quickly outdated as these
techniques advance. " Modern " BPM front-end concepts utilize
digital based signal processing techniques and is often
based on I/Q demodulation or similar concepts known from
the telecom industry, in most cases the data acquisition is performed
with two or four identical channels, one for each BPM
electrode, see Fig. 10 [16]. For recent BPM read-out electronics,
the technology advances of analog-to-digital (ADC) converters
have the greatest influence on selecting a particular signal
processing schema [17]. Clearly, the trend goes to digital signal
processing techniques, which prevents imperfections and drift
effects often observed in analog and RF circuit sections due to
the aging of electronics components, their tolerances, and the
variation of their nominal values with the ambient temperature.
The digital hardware also enables more complex arithmetic on
the BPM signals [18], typically implemented as gateware in
IEEE Instrumentation & Measurement Magazine
December 2021

Instrumentation & Measurement Magazine 24-9

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