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C A B L E F A B M C A T I O N P R O C E D U R E S F O R T H E B PM S Y S T E M *

W . B . S M IT H

Stanford Linear Accelerator Center

Stanford Univcnity, Stanford, California UfS S

The B eam Position Monitor (BPM) System is

t

critical component or beam steering

and control. It consists of the Beam Position Monitors (in the bcam linc), electronic

signal processors (U> ed rermitely) and the ca bles tha t interconnect the two.

Each BPM produces four signals. These four signals arc then compared electroni

cally with each other. Differences in amplitude among these signals is converted into

information about the position of the beam as it passed that particular BPM.

In orer to provide the nee ded accuracy Tor this system, all four signals from

each BPM must ar.ivc at the processing electronics simultaneously. This means the

interconnecting cables must be matched very precisely. Th e following procedures,

when followed precisely: will assure th e matching of BPM cables.

TOLER ANCE STANDARDS : We use RG223/U cosxial cable for the BPM System.

The m anufac turer specifies its propagation velocity is 1.54 nS/ft. For the BPM elec

tronics to operate accurately, each of the four signals from a single BPM must arrive

at the electronics within 0.5 uS. And since cable runs range from 100 to 300 feet, this

requires an accuracy of 3.0 inches or 0.3% (or better)

THE PROBLEM : Conventional cable measurement/cutt ing techniques produce typ

ical accuracies on the order of 1 , far too coarse for the BFM System. Hence, the

more sophisticated technique that follows.

THE TIME DOM,UN REFLECTOMETER (TDK): This

Ls

a marvclously sensitive

instrument. When properly calibrated, it is also extremely accurate. Phase I describes

'.he procedure to be followed to calib rate the instrum ent. All subsequent work is highly

dependent upon the "arc you give this calibration.

* Work supported by the Department of Energy, contract DE-ACOM6SFO0515.

PORT IONS OF THIS nEP OR T ARE ILLEGIBLE. I t

h i s b een r ep ro d u ced f r o m t h * .b t r t v i l r t f c

co p y t o p e rm i t th e b r o i d r t p * * i rb H W i l l -

abi l i ty .

P H A S E I , T D R C A L I B R A T I O N

s t e

P 1

:

Interconnect the various instrument* as shown on the attach ed Test System

Interconnection Diagram.

s t r

P 8= Turn on all electronic equipment. Allow at least Jhom for warm-up, longer

if at

all

possible.

Step 3 : Measure PRECISELY and cut one RGS23/U cable. Make its length equal

to or greater than the maximum length of the sets of cables to be made.

Actual cable length should be selected in orJer to facilitate fabrication a nd

operator preference. Terminate one end or the cable with SMA connector,

Leave the other end open.

S

"> P * Repeat Step 3, only this time make the cable length EXACTLY one-hail

the length of the Grst cable. PRECISION is the key to Strjs t, 3 and 4.

Any errors here will lead to greater inaccuracies in the BPM cable* to b e

fabricated. Th e cables made in Step 2 and Step 4, along with a ' lero-

length" (i.e. no cable) comprise a ibrce-point calibration system for t i e

TDR

Step 5 : Set the SPDT coaxial switch "A' tc T D R Mode". Set the TDR " V/D IV

dial to 0.05. Set the DP-IT uax iei sw itch *B* to position 1 . Set "Ri rc-

t ion" to "VAR" niode. Set "SCAN" to Normal". Set 'PERSIST ENCE "

to -NORMA L" mode. S "DISPLAY" to 'INTE RNA L". Set "MAGNI

FIER" to "XI" . And set 'MARKE R P0ST1ON (Div) to TIM ES 100"

and expand to 1 ft /div.

S tep 6 : F indtheleadingrdgeor tbereBectcdpi i l seoatheTDR'fCRTd> i t> lay . Tki t

wil l appear as in abrupt transition ia the trace from a hotiro ttaltu t to oe

tha t rises steeply. We will use the "knee" where this IrUM lk* takes place

aa the standard measuring point for all subsequent measurements with the

TDR . The waveform will M t l ike this:

M ' Leoaing edgt 5

/ (ejected pjite

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Step 7 : Having found the reflected pulse, set the "knee" on the X-AxU0 graticule,

using 4 combination of the marker dial tod the coarse and fine Horizontal

Position adjustments. When properly done, the "knee" will ait squarely on

the 0% graticule, the Marker Dial will read "000" and Switch "B" will be

in position "1".

Step 8 : Set the Marker Dial to the length of the longest cable (or to its nanosecond

equivalent, if in the time m ode). Set Switch

fc

B" to position "2". Use a

jeweler's screwdriver to adjust *VAR" so that the new "knee" falls exactly

on the0 graticuje.

Step 9 The adjustm ents you made in steps 7 and 8 affect each other. So repeat

steps 7 and 8 until you can step from one to the other without having to

adjust either one.

Step 1 0: Set Switch "B" to position "3" and adjust the Marker Dial for the length

of the shorter cable (or its nanosecond equivalent if in the time mode). The

new "knee

9

should be very close to the 0% graticule. If not, combine Steps

C and 10 and "rock" the calibration to the best compromise you can find,

^ou will find that the accuracy of your cable cutting will have a major

impact on the ease with which you are able to accomplish this calibration

procedure.

The calibration procedure for the TDR b now complete.

** WARNING ** WARN ING ** WARNING * WARN ING _**

IT IS VERY EASY TO BUMP OTHER ADJUSTMENTS WHILE MAKING AD

JUSTMENTS TO THE TDR OR WHILE WORKING WITH THE CABLES. EVEN

THE SLIGHTEST BUMP WILL DESTROY T HE ACCURACY. IT IS fMP_ERATJrVE

THAT YOU REPEAT THIS CALIBRATION PRODECURE ANYTIME YOU SUS-

PECT TH E TDR'S SETTINGS HAVE BEEN DISTURBED IN ANY WAY

WARNING " WARNING " WARNING " WARNING * "

THIS CALIBRATION PROCEDURE SHALL BE PERFORMED AT THE BEGIN

NING OF EACH DAYS WORK AND BEFORE BEGINNING WORK ON EACH

NEW SET OF FOUR BPM CABLES TO BE TRIMMED.

DISCLAIMER

mfMmmm

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PHASE n, CABLE TRIMMING

Step 1: Cut four cables. Make each two to four feet longer than the length pre-

stribed. Terminated one end of each cable with

AH

SMA connector (see

attached trim code charts for details).

Step 2

As in the calibration procedure (Phase I), Switch

"B"

should have no cable

attached to position "1", the shorter of the tw o calibration cables ( the

length of the longer calibration cable) to position "3". Attach one of the

cables to be trimmed to position "4".

Step 3 : Confirm TDR calibration by switching Switch "B" through the Grst three

positions.

Step

4'.

Set the vernier scale to the desired length (length mode) or delay (time

mode). Turn Switch "B" through the Grst three positions.

Step 5 : Carefully "prune" the unterminated end of cable # 4 until the reflected

pulse's "knee" falls within ONE ( ) MINOR DIVISION of the 0 graticule

of the TDR's display.

Step 6: Disconnect the two calibration cables. Connect the three remaining cnbles

of the set of four positions. * T , "2" and "3".

Step 7 j Repeat Step 5 for cables 1, 2 and 3.

Step 8 : Visually compare the four cables with the TDR by switching "B" through

the four positions. NO TW O CABLES SHOULD DIFFER BY MORE

THAN 1.5 MINOR DIVISIONS ON THE TDR GRATICULE. ALWAYS

TRTM CABLES # 1 , # 2 or # 3 as needed to make this match. Cable # i a

your standard now, and should NOT BE TRIMMED.

Step 0

;

Now that the cables have been carefully trimm ed, terminate the untcrmt*

natcd end with the appropriate connector (SMA or SHV) according to the

attached trim code charts.

NO TE. : If difficulty is experienced IT the termination procedure and a cable must be

ret rimmed in order to be properly terminated, repeat Step 8. If the cable

still mcetr the standard in Step 8, it is acceptable. If not, then a new cable

must be fabricated.

S

PHASE m, ATTENUATION MATCHING

Mismatches in attenuation ca n not be remedied. Thus any cable of a four-cable

aet that can not meet attenuation tolcraaces must be discarded from the set, and a

DCW cable fabricated to take its place. However, a longer cable discard might prove

satisfactory (once properly trimmed to the new length)

at>

a substitute for a mismatched

cable of a ahorter set. Hence it is recommended that the longest sets of cable* be t t* 4

for attenuation matching first.

ATTENUATION STANDA RD: Each cable of a four-cable set must be within

1

of

each of the other three.

Step 1: Set Switch "B" to position "I". Connect cable #1 to Kaput #1 . Now

connect the free end of cable #1 to ouput # 1 . Observe the TDK display.

The reflected pulse shouM disappear, confirming proper connection.

Step 2 : Set Switch "B" to position " 2 \ Connect cable #2 to Input # 2 and output

# 2 , in much the same fashion aa in Step 1. Repeat the procedure for cable*

# 3 and #4 . Be sure to use the TDR display to confirm i roper cooaectton

in each case.

Step 31 Set SPDT coaxial Switch *A" to *Rw" pofition.

Step 4: Using the oscilloscope, adjust the DELAY GATE GENERATOR (DGG)

that its output overlaps the leading aegatire pub* of li e PULSE

STRETCHER.

Step 5 : Maximize this overlap by maximizing the DIGITAL VOLTMETER (DVM,

reading.

Step 0: Step through the four positions of Switch

v

tl*. citing the DVM value* far

each position.

Step 7: Compute the deviation between the maxima and minima of the cable aet.

Reject any cable which eihibiti a deviation greater than A 0 535 of the

mean.

EX AM PLE : The DVM readings for a wt of cables arc: 610, 611, 600 u d M

(

respectively. The mean would be: (610 -- 611 4* 600 + 6 M) /4 or 3432/4 *

0.5%of 608 3.04.

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Hence allowable attenuation for this yet would have to be within the span from

604.06 to 611.04. O bviously, cables # 1 , #2 , and #3 meet this standard. But at a

value of 602, cable # 4 must be rejected.

Carrying this exam ple further, the replacement for cable # 4 produce s a DVM

reading of 614, A new m ean must be calculated (611) and the acceptable 0 .5 % span

calculated (3.055 ). Tbis new span is 607.045 to 014.055. Now all four cables arc

acceptable, although Cable #4 just barely makes it.

TRIM CODE CHART

S MA C O N N E C T O R :

S HV C O N N E C T O R :

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