BPM Signal Processing with Diode Detectors
Marek Gasior
Beam Instrumentation Group, CERN
Outline:
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Why using diode detectors for processing BPM signals ?
Diode detectors in systems for which the signal amplitude is not (that) important (two examples)
A compensated diode detector
Diode ORbit measurement system (DOR)
DOR lab measurements (resolution, stability, linearity, calibration)
DOR measurements with the collimator BPM and the SPS beam
DOR measurements with the LHC beam
M.Gasior, CERN-BE-BI
8th DITANET Topical Workshop on Beam Position Monitors, 16-18 January 2012, CERN
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Why diode detectors ?
 Diode detectors can be used to convert fast beam pulses from a BPM into
slowly varying signals, much easier to digitise with high resolution.
In this way amplitudes of ns pulses can be measured with a lab voltmeter.
 As the diode forward voltage Vd depends on the diode current and
temperature, the output voltage of a simple diode detector also depends on
these factors.
 The detector output voltage can be proportional to the peak amplitude or an
amplitude average of the input pulses.
Input (Vi) and output (Vo) voltages
of a peak detector with
an ideal diode
Input (Vi) and output (Vo) voltages
of a peak detector with
a real diode
Input (Vi) and output (Vo) voltages
of an average-value detector
M.Gasior, CERN-BE-BI
BPM Signal Processing with Diode Detectors
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Simple diode detectors
Charge balance equation for the following assumptions:
- a simple diode model with a constant forward voltage Vd
and a constant series resistance r
- constant charging and discharging current, i.e. output
voltage changes are small w.r.t. the input voltage
Vo
Vi  Vo  Vd
T

R
r
Vo
1

Vi  Vd 1  r  T
R 
A numerical example: LHC, one bunch.
For LHC τ ≈ 1 ns and T ≈ 89 μs, so for Vo ≈ Vi
one requires R/r > T/τ.
Therefore, for r ≈ 100 Ω, R > 8.9 MΩ.
• Vi Vd
• n bunches
- For large T to τ rations peak detectors require large R values
and a high input impedance amplifier, typically a JFET-input
operational amplifier.
- The slowest capacitor discharge is limited by the reverse
leakage current of the diode (in the order of 100 nA for RF
Schottky diodes)
M.Gasior, CERN-BE-BI
BPM Signal Processing with Diode Detectors
Vo
1

Vi 1  r  T
R n
3
Simple diode detectors for BPM signals
ideal : p12  c12
real : p12  c12
Vi1  Vi 2
Vi1  Vi 2
Vo1  Vo 2
Vi1  Vi 2
 c12
Vo1  Vo 2
Vi1  Vi 2  2Vd
 One diode detector for each BPM electrode.
 Subtracting signals before the detectors (e.g. by a 180° hybrid) is no good, as the resulting signals would be:
• smaller (→ larger nonlinearities);
• changing signs when crossing the BPM centre.
 The diode forward voltage Vd introduces a significant position error.
 Vd depends on the diode current and temperature.
 Simple diode detectors are good for applications when the signal amplitude is not that important.
 Two examples:
• Tune measurement systems
• An LHC safety system: Beam Presence Flag
M.Gasior, CERN-BE-BI
BPM Signal Processing with Diode Detectors
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Diode detectors in tune measurement systems
 In tune measurement systems one measures frequency of beam oscillations.
 For proton machines often such a system must be optimised for sensitivity, e.g. for the LHC the system has to see μm turn by
turn beam oscillations (emittance conservation, tolerances of the collimation system)
 Often the BPM signal amplitude is much too large to be processed by the front-end electronics, so it must be reduced (i.e.
signal thrown away). For example, the LHC tune stripline pick-ups give up to some 200 V peak voltages.
 Diode detectors can work directly with such large voltages (one diode is good for some 70 V of signal).
 The large DC voltages corresponding to the beam offset are removed by series capacitors.
Otherwise these voltages would waste the dynamic range of the following acquisition chain.
 The resulting small signals corresponding to beam oscillations are amplified and filtered, prior to be digitised with high
resolution audio ADCs.
M.Gasior, CERN-BE-BI
BPM Signal Processing with Diode Detectors
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LHC tune measurement system based on diode detectors
BPLX
D6R4.B1
M.Gasior, CERN-BE-BI
BPLX
B6R4.B2
BPM Signal Processing with Diode Detectors
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LHC beam spectrum, one pilot bunch
FFT1 continuous system at βH βV 400 m,
B2, 1 bunch of 5×109, fill #909, 13:35 11/12/09,
8K FFT @ 1 Hz, 20 FFT average
 Absolute amplitude calibration was done by exciting the beam to the amplitudes seen by regular LHC BPMs.
M.Gasior, CERN-BE-BI
BPM Signal Processing with Diode Detectors
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Beam spectrum observation on an electron machine (PSI)
 Work done in the framework of feasibility studies for stabilisation of the magnets in the CLIC final focusing system.
 Very many thanks to our PSI colleagues for their great hospitality.
 The measurement was done with the LHC tune system harware, only sightly modified:
• detectors for electron beams (diodes upside-down w.r.t. protons !)
• bandwidth of the analoque front-end shifted from 0.5 – 5 kHz to 1 – 1000 Hz.
M.Gasior, CERN-BE-BI
BPM Signal Processing with Diode Detectors
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SLS beam spectrum measured with the diode detector system
SLS
100
-40
-60
spectrum #2 (red) shifted by 2 Hz (upper freq. axis)
10
loudspeaker on @ 111 Hz ( )
average of 32 8K FFTs (all spectra)
-80
1
-100
Magnitude [dBFS]
One tone amplitude [nmrms]
orbit FB off, no excitation
orbit FB on, excitation on @ 80 Hz ( )
no beam signal
0.1
-120
0.01
-140
0.001
0
100
200
300
400
500
600
700
Frequency [Hz]
 Absolute amplitude calibration was done by exciting the beam to the amplitudes seen by regular SLS BPMs.
M.Gasior, CERN-BE-BI
BPM Signal Processing with Diode Detectors
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LHC Beam Presence Flag system based on diode detectors
 BPF is an LHC safety system, optimised for
reliability and robustness.
 Very simple, the only inputs are beam signals
and 230 V (in particular no computer interface,
no timing signals).
 In addition no programmable devices, no
adjustable components, no ADCs.
 4 independent system channels for each beam
use signals from 4 BPM electrodes, resulting in
quadruple redundancy for the final “high beam
intensity” enable.
M.Gasior, CERN-BE-BI
BPM Signal Processing with Diode Detectors
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Compensated diode detector
 Compensated diode detector consists of two diode peak detectors, one with single, second – with two diodes.
All three diodes are in one package, for good thermal coupling and symmetry of the forward voltages Vd.
 Two operational amplifiers are used to derive 2 Vd voltage and to add it to the output of the two-diode detector. This way
the resulting output voltage is equal to the input peak voltage.
 This is the simpler and most promising scheme, found in a very popular text book on electronics.
 To get an “ultimate peak mode operation”, the discharge resistors can be omitted. In this case the discharge is done by
the reverse leakage current of the diodes.
 The asymmetry in the charging conditions becomes less important for larger input voltages.
M.Gasior, CERN-BE-BI
BPM Signal Processing with Diode Detectors
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Diode ORbit Front-End
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Signal of each pick-up electrode is processed separately.
The multiplexer is foreseen for calibration with beam signals.
The low pass filters decrease the signal amplitude.
The conversion of the fast beam pulses into slowly varying signals is done by compensated diode detectors.
These slow signals can be digitised with high resolution, averaged and transmitted at slow rates.
All further processing and calculations are done in the digital domain.
Simple and robust hardware, high resolution, no beam synchronous timing required
M.Gasior, CERN-BE-BI
BPM Signal Processing with Diode Detectors
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Diode ORbit front-end connected to two LHC BPMs
M.Gasior, CERN-BE-BI
BPM Signal Processing with Diode Detectors
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DOR FE prototypes are still in the “state of the art”
M.Gasior, CERN-BE-BI
BPM Signal Processing with Diode Detectors
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DOR prototype: lab measurements
 Inputs of all 4 FE channels parallel, simulating beam in the
PU centre.
 Input signal: 10 MHz sine wave, FE gain 25 dB (max).
 Raw results, without any offset and gain calibration.
 24-bit 8-channel ADC sampling at 11.7 kHz, samples
averaged in the microcontroller to 50 Hz equivalent
sampling, then to 1 Hz for the plots.
 Front-end channels have 10 Hz LP filters before the ADCs.
 Amplitude changes due to temperature sensitivity of the
signal generator.
 The amplitude faster jumps may be due to some internal
calibration of the generator or somebody (i.e. me) touching
the input cables, as during the night signals were much
smother.
M.Gasior, CERN-BE-BI
BPM Signal Processing with Diode Detectors
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DOR prototype: lab measurements
The plot repeated from the previous slide
 Beam position calculated as for an LHC arc pick-up
with 49 mm electrode distance.
 ”Natural” long term stability shown, no calibration.
M.Gasior, CERN-BE-BI
BPM Signal Processing with Diode Detectors
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DOR prototype: lab measurements
 All 4 inputs in parallel, simulating beam in the PU centre.
 Input: 10 MHZ sine wave with slow triangular modulation to
simulate intensity changes, FE gain 25 dB (max).
 Raw results, without any offset and gain calibration.
 Beam position calculated as for an LHC arc pick-up with 49
mm electrode distance.
 ADC sampling at 11.7 kHz, samples averaged to 50 Hz
equivalent sampling.
 Front-end channels have 10 Hz LP filters before the ADCs.
M.Gasior, CERN-BE-BI
BPM Signal Processing with Diode Detectors
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DOR prototype: lab measurements
 Now channels 2, 3, 4 are correlated with channel 1,
resulting in the coefficients (offset, gain), in the ADC full
scale (FS) units:
ch2: -0.001720, 1.002262
ch3: 0.000325, 0.998747
ch4: -0.001247, 1.000359
 Channel difference improved from 103 to 105 level, by
some 2 orders of magnitude.
 Position error improved also by some 2 orders of
magnitude.
 For amplitudes larger than 20 % of the full scale the noise is
not larger than some 50 nm peak-peak, i.e. some 10 nm
rms. This is with the 25 dB gain of the high-frequency input
amplifiers, 50 Hz ADC equivalent sampling and 10 Hz
analogue bandwidth for the orbit changes.
 The final diode front-end will be equipped with a calibration
circuitry capable of connecting the same signal of
programmable amplitude to all inputs.
 The input circuitry will allow the same with beam signals,
i.e. the same beam signal can be connected to both frontend channels processing one pick-up plane.
M.Gasior, CERN-BE-BI
BPM Signal Processing with Diode Detectors
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100 µm
DOR prototype: lab measurements
 Input signal made different with fixed attenuators, simulating
an offset beam.
 Simulated beam offset is the same for both pick-up planes
(both planes connected in parallel).
 Input signals, gain and sampling as before for the simulated
centred beam.
 Errors some 2 orders of magnitude larger than for the centred
beam.
 Correlating channels does not help significantly.
 Position measurement dependency on signal intensity for large
beam offsets will be addressed in the future development.
M.Gasior, CERN-BE-BI
BPM Signal Processing with Diode Detectors
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DOR prototype: lab measurements with a calibration mux
M.Gasior, CERN-BE-BI
BPM Signal Processing with Diode Detectors
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DOR prototype: lab measurements with a calibration mux
corr. for 1 s every 1 h
correlation for 60 s
corr. for the whole meas.
as above, projected to a 49 mm aperture
whole
M.Gasior, CERN-BE-BI
BPM Signal Processing with Diode Detectors
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DOR prototype: lab measurements with a calibration mux
 Measurement with 4 equal signals and calibration mux
(not switching), results scaled to 49 mm aperture.
 Correlation coefficients used to calibrate the channels,
the same coefficients for the whole measurement.
M.Gasior, CERN-BE-BI
BPM Signal Processing with Diode Detectors
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DOR on collimator BPM: Measurements with the SPS beam
 Main role of the embedded BPM system is to
indicate when the beam is in the middle
between the jaws, i.e. when the signals from
the opposing electrodes are equal.
 Geometrical factor, a constant for “static”
BPMs, changes with the jaw distance,
fortunately not dramatically, less than a factor
of 2.
 Signal amplitude changes with the jaw
distance by a factor of 3.
 Very interesting case from academic point of
view (lab-type measurement possible with real
beam, online calibration by BPM precise
movement).
LU
LD
RU
RD
Drawing from A.Nosych
 Measurements with one LHC nominal bunch
normalised U (D) position 
normalised L (R) tilt 
M.Gasior, CERN-BE-BI
RU ( D)  LU ( D)
RU ( D)  LU ( D)
L( R)U  L( R) D
L( R)U  L( R) D
right upstream button
left upstream button
right downstream button
left downstream button
BPM Signal Processing with Diode Detectors
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DOR on collimator BPM: beam tests on the SPS
M.Gasior, CERN-BE-BI
BPM Signal Processing with Diode Detectors
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LHC: standard vs. DOR processing comparison
 BPM electrode signals are split and send to both systems
 Beam position calculation using the same polynomial
 Aperture 61 mm
M.Gasior, CERN-BE-BI
BPM Signal Processing with Diode Detectors
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LHC: standard vs. DOR processing comparison
M.Gasior, CERN-BE-BI
BPM Signal Processing with Diode Detectors
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LHC: standard vs. DOR processing comparison
M.Gasior, CERN-BE-BI
BPM Signal Processing with Diode Detectors
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Conclusions
Diode detector strengths:
Diode detector weaknesses:
 Simple and robust.
 “Perfect sampling” even for very short bunches
without any precise timing.
 Slowly varying signals at the output, easy for signal
processing and high resolution digitization.
 Low rates of the digital data.
 Natural noise gating (i.e. very low gain for noise
between beam pulses).
 No limit for beam offsets, as signals from each
electrode are processed separately.
 Excellent resolution.
 Surprisingly good long term stability.
 Limited dependence of the output voltages on the
number of circulating bunches.
 Limited dynamic range of the linear operation, variable
gain amplifiers required to compensate important
bunch intensity variations.
 Only turn by turn, bunch by bunch not easy
(some ”reset techniques” required).
 Large bunches favoured
(good for masking undesirable reflections).
 Important dependence of the measured position on
signal intensity for large beam offsets.
It will be addressed in the future development.
M.Gasior, CERN-BE-BI
BPM Signal Processing with Diode Detectors
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