bpm signal processing with diode detectors marek gasior beam instrumentation group, cern

M.Gasior, CERN-BE-BI 8th DITANET Topical Workshop on Beam Position Monitors, 16-18 January 2012, CERN 1

BPM Signal Processing with Diode Detectors

Marek Gasior

Beam Instrumentation Group, CERN

Outline:

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 BPM Signal Processing with Diode Detectors 2

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.

Why diode detectors ?

Input (Vi) and output (Vo) voltagesof a peak detector withan ideal diode

Input (Vi) and output (Vo) voltagesof a peak detector witha real diode

Input (Vi) and output (Vo) voltagesof an average-value detector


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

Simple diode detectors

rVVVT

RV doio

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.

- 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)

T

RrVV

V

di

o

1

1

nT

RrV

V

i

o

1

1

• Vi Vd• n bunches


21

211212 :ideal

ii

ii

VVVVcp

dii

ii

oo

oo

VVVVVc

VVVVcp

2 :real

21

2112

21

211212

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

Simple diode detectors for BPM signals


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.

Diode detectors in tune measurement systems


BPLXD6R4.B1

BPLXB6R4.B2

LHC tune measurement system based on diode detectors


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

LHC beam spectrum, one pilot bunch

Absolute amplitude calibration was done by exciting the beam to the amplitudes seen by regular LHC BPMs.


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.


0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0F req u e n cy [H z ]

0 .0 1

0 .1

1

1 0

1 0 0

One

tone

am

plitu

de [

nm

]

-1 4 0

-1 2 0

-1 0 0

-8 0

-6 0

-4 0

Mag

nitu

de [

dBFS

]

sp ec tru m # 2 (red ) sh if te d b y 2 H z (u p p e r fre q . ax is )

lo u d sp e ak er o n @ 1 1 1 H z ( )av e rag e o f 3 2 8 K F F T s (a ll sp ec tra )

rms

0 .0 0 1

SLSo rb it F B o ff, n o e x c ita tio no rb it F B o n , ex c ita tio n o n @ 8 0 H z ( )n o b eam s ig n a l

SLS beam spectrum measured with the diode detector system

Absolute amplitude calibration was done by exciting the beam to the amplitudes seen by regular SLS BPMs.


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.


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.

- input: 1 MHz sine wave- detector R = 10 M, C = 1 n + 10 n- generator 33220A, voltmeter 3478A

0 0.1 0.2 0.3 0.4 0.5

Input amplitude [Vp]

0

0.1

0.2

0.3

0.4

0.5

Out

put v

olta

ge [

V]

channel 1channel 2


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

Diode ORbit Front-End


Diode ORbit front-end connected to two LHC BPMs


DOR FE prototypes are still in the “state of the art”

HAND MADE

in Rapid Development Technology ®


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.

DOR prototype: lab measurements


Beam position calculated as for an LHC arc pick-upwith 49 mm electrode distance.

”Natural” long term stability shown, no calibration.


The plot repeated from the previous slide


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.



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, 1.002262ch3: 0.000325, 0.998747ch4: -0.001247, 1.000359

Channel difference improved from 103 to 105 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 front-end channels processing one pick-up plane.



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.

100

µm



DOR prototype: lab measurements with a calibration mux


whole

correlation for 60 s

corr. for the whole meas.

corr. for 1 s every 1 h


as above, projected to a 49 mm aperture



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.


right upstream buttonleft upstream buttonright downstream buttonleft downstream button

RU

LD

LU

RD Drawing from A.Nosych

DOR on collimator BPM: Measurements with the SPS beam

)()()()(position (D) UnormalisedDLUDRUDLUDRU

DRLURLDRLURL

)()()()( tilt (R) L normalised

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).

Measurements with one LHC nominal bunch


DOR on collimator BPM: beam tests on the SPS


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


Diode detector strengths:

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.

Conclusions

Diode detector weaknesses:

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.

bpm signal processing with diode detectors marek gasior beam instrumentation group, cern

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