Cavity BPMs for Happex and G0

John Musson

Triplet Configuration…X, Y, and I

• TM010 Mode for I

• TM110 Mode for X & Y– Slugs provide proper

excitation, reducing TM010 x-talk

– Nominal output: 54nV-uA/um (-132 dBm)…per MAFIA simulations

Cavity Response

___________________Courtesy Jürgen Schreiber, ECFA/DESY LC workshop, Amsterdam, April 1-4, 2003

I & Q Demodulation

I Q DE- MOD

I @ 28 Msps

ADC

70 MHz LO

90 Degree I & Q

56 Msps ADC

0

23

1

+

1

COUNTER

REG

ADC70 MHz LO

56 Msps System Clock 14-Bit 2’s

Complement

0

23

1

+

1

REG

Q @ 28 Msps

=

+I +Q -I -Q +I +Q -I -Q +I +Q -I -Q +I +Q -I -Q +I +Q -I -Q +I +Q

Receiver

Functional Description

Receiver Parameters

• Noise Floor: -91 dBm (200 uV)– 3690 uA-um per shot (per cavity simulation)

• Bandwidth = 100 kHz• Processing Gain = 6MSPS/100 kHz = 18 dB • Best-case Resolution at 50 uA ~ 9 um (at full

BW)• Additional Integration Gain (16ms): 32 dB• DAC Output BW ~ 75 kHz (200 ksps)• Calculated resolution for Eff = 50 %: 0.7 um

– Hoping for 1 um

EPICS Interface

Happex Run

BCM Crosstalk

Helicity-correlated position differences, vs stripline,1nm

ResolutionBCM DD

Glitches

BCM Linearity

Helicity-correlated position difference. xtalk

Same plot, better data!

Happex Run

Bad News…..• Limited Dynamic Range

– Required external amps and filters• Crosstalk…~45 dB of C-C isolation

– BCM signal would corrupt X & Y• Software Problems

– Register overflow resulted in glitching and “Bedposts”• Synchronous Detection => Dedicated MO

– Phase noise and distribution issues (“LOL”)

Data COURTESY l. Kaufman, K. Paschke, R. Michaels

In Addition

• Setup is a learned behavior!– We devised a procedure, which proved to be

more difficult than expected with actual beam.

• Hall personnel eagerly participated…..– More eyes– Technical understanding of benefits and

limitations– Fantastic model for future systems

G0 Improvements

• Hardware– Crosstalk path identified. IF “traps” installed on Local Oscillator

lines => > 60 dB– Amplifier removed from BCM (I) channel– Additional bench testing to understand

• Software– Register rollover identified, corrected, and tested.

• MO– Try asynchronous operation, due to large Phase Noise in Halls

• Hall personnel also system-savvy!– Data courtesy R. Suleiman

Non-Linear Behavior

“Double Bounce” at Zero X-ing

Glitching

Known Improvements• 50 nA Sensitivity

– Currently have 74 mm resolution! Need additional 37 dB to achieve 1 mm. LNA?

– I-cavity is not a problem…plenty of signal• Shore up all hardware fixes (ie. LO-IF traps)• Firmware and Additional tests. • Setup Procedure (EPICS) and All-Save• Return to Synchronous MO

– Must improve MO Distribution to Hall(s)• Cavity Investigation on Downstream X&Y

– How can we duplicate beam+cavity behavior in the lab?

• Thank you to all for data, feedback, and especially patience!

CORDIC Algorithm

• COordinate Rotation DIgital Computer– Jack E. Volder, The CORDIC Trigonometric Computing Technique, IRE Transactions on

Electronic Computers, September 1959 – Ray Andraka, A Survey of CORDIC Algorithms for FPGA Based Computers, FPGA '98.

Proceedings of the 1998 ACM/SIGDA sixth international symposium on Field programmable gate arrays, Feb. 22-24, 1998, Monterey, CA. pp191-200.

• Iterative method for determining magnitude and phase angle– Avoids multiplication and division

• Nbits+1 clock cycles per sample• Can also be used for vectoring and linear

functions (eg. y = mx + b)

Concept

• Exploits the similarity between 45o, 22.5o, 11.125o, etc. and Arctan of 0.5, 0.25, 0.125, etc.

• Multiplies are reduced to shift-and-add operations

Angle Tan ( ) Nearest 2-N

Atan ( )

45 1.0 1 45

22.5 0.414 0.5 26.6

11.25 0.199 0.25 14.04

5.625 0.095 0.125 7.13

2.8125 0.049 0.0625 3.58

1.406125 0.0246 0.03125 1.79

0.703125 0.0123 0.01563 0.90

cossin

sincos,',' yxyx

iiiiii

iiiiii

dxyKy

dyxKx

2

2

1

1

Y

X

Binary search, linked to sgn(Y)

Successively add angles to produce unique angle vector

Resultant lies on X (real) axis

0,1

0,1

i

ii yif

yifd

)2arctan(1i

iii dzz

i

iid )2arctan(

Functionally.....

with a residual gain of 1.6