s. ~M:/~a3J/C~! JECCOE~S

I

5

AMI/HDB3/CMI DECODERS1=

5. 1 OBJECTIVES• Describe and examine the functional aspects of the decoders

A.'iI /UDB3 / C!iI .• carry out the error rate measurement on the connections consisting of:

coders ~~I/HDB3/C!iI; transmitter; line+noise; line receiver; decoders.-\"'iI/HDB3 /C!iI.

5.2 DESCRIPTION5.2.1 ~'iI/HDB3 Decoder

The circuit receives the binary unipolar signals "+" and "-" and thenegated RCK clock ·(with frequency equal to the data speed), supplied bythe Data Recovery and Clock Recovery circuits (fig.S.l).

A decoder operates on a 4-bit string, which are read in correspondenceof the rising edge of the clock. The NRZ decoded binary data aresupplied across the output in correspondence to the rising edge of theclock and a delay of 4 clock periods (see example of fig.5.2). Thedecoder carries out also a control on the binary signals .~'iI/HDB3 "+"and "_" , and in presence of violation of the code it supplies an errorsignal (high level for the duration of a clock period) across the Errordetector output. The error is signalled in the following cases:• HDB3 mode: when two consecutive violations are detected having the

same polarity• AMI mode: when a violation is detected• AMI/HDB3 mode: when the binary signals AMI/HDB3 "+" and "_" applied to .

the input of the decoder are simultaneously at high level.

C.LOCKRECOVERY

2''N' moc. AWI/HD83 DECODER LINE2<

'e •• our IN- EOUAlIZER21 & ALBO Ifl IN

A•• [CJ "DU33 3C

22 IICi <J I\,OUT.["01D[I[CrDR 30

OUl·l! DATARECOVERY

fig.5.1

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s. AMI/~OB3/C~I ~£COOERS

~ECEIVED SIGNAL HOBJ

3~Ot<EN U?51eNAL •.•.• e '-'

CLOCK

RECEIVED DATA

-- -....1 11DELAY I I~ ~r!~ ~

fig.5.2

5.2.2 CMI DecoderThe circuit receives the CMI amplified signal and the RCK clock (withfrequency double the data speed), supplied by the Data Recovery andClock Recovery circuits (fig.5.3). Across the output of the decoder, youobtain the data signal with the related clock.

CMI~CK CLOCKPCM OUT 28CIoII RECOVERY

DC. DECODER RCK C1l CMI LINEEOUALlZER

At ALBO RI IN3) ,.

<J T\'ell DATA12 RECOVERY

fig.5.3

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...

5.3 EXERCISESRequired material• modules T20E/T20F• power supply +/-12 Vdc• osci lloscope.5.3.1 Exercise 1: Decoding of AMI/HDB3 signals1. At the beginning set up the system in fu~Imode (fig.5.4):

module T20E• FAST/SLOW deviator: FAST• STOP/READ deviator: READ• PATTERN: 4xO/4x1• CLOCK: 320kHz• NOISE: minimum• LINE: 160 KHz; attenuation to the minimummodule T20F

• &~I/HDB3 deviators: AMI• TP20 with TP 25

TP21 with TP26• AMI/HDB3-CMI deviator: AMI/HDB3

• insert jumpers J8 and J10interconnections

flat cable between T20E and T20FTP5 of T20E with TP16 of T20FTP27 of T20F with TP10 of T20Eground of TX OUT with ground of LINE INTP11 of T20E with TP34 of T20Fground of LINE OUT with ground of TX INTP 23 of T20F with TP8 of T20ETP 24 of T20F with TP9 of T20E

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2. examine the data signal across the input of the coder (TP16) andacross the output of the decoder (TP24). The same data should beobtained (i.e. the same wave-form) delayed of about 9 clock periods

3. examine the data signal and the corresponding clock signal together(TP 23 and TP 24)

4. set up the system in HDB3 mode and carry out the last measurements.again.

5.3.2 Exercise 2: Measurement of the error rate

PC.r--c0cU"~~ =-~~ I~ ~ I

I,---,.",..---, I

ICltllrulr\aI~'1 :

-- IIIII

..If( Kf'tJi"C( o o

TH"

no!

5. Select the Sequence data signal (pseudocasualsequence of 64 bit)on T20E. and press the RESET pushbutton

6. the value read on the "BIT COUNTER" display, multiplied by 103,corresponds to the number of received bit. The "ERROR COUNTER"display shows the number of the bit received wrong

7. if the noise is not introduced into the line there should not bewrong bit. In case there should be, this could'be caused by a badclock extraction; regulate the variable capacity (on the back of themodule) until the error bit counting stops

IIIIIIIIIIIIIII

0'" IL.--=.;-.:..,::-..J~ii~il[lr::-.c;f~ :Lr-;;.r-ln7 :

-c n l"":- I_______~_~ ~ J_~_tlnllCllt~1C o 0

U III •• " ••

.on

... ~1Ct,_ •• _rJIC:I. •••••••• _",,, .__ ..•"" •• , __ 11

= :==~.-

TlIIINC C[NUATOI

fig.5.4

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s. AMr/~Da3/CMr OECOOE~S

8. to simulate real conditions, insert the noise in line and carry outthe error rate measurement operating as follows:

• set the deviator on READ and reset the counters pressing RESETwhen th~ received bit number reaches a certain value (e.g.

5000x10 bit) set the deviator on "STOP" and read the error bit• the ratio between the wrong and transmitted bit constitutes the

error rate on the connection9. vary the line attenuation and the noise amplitude and carry out

other error rate measurements

5.3.3 Exercise 3: Measurements on the CMI connection

10. Set the. sy'stem in CMI mode, changing the· last set-up in thefollowing way:

module T20FTP18 with TP25

• TP 19 wit h TP 26• AMI/HDB3-CMI deviator: CM!

tI,i

• insert jumpers J7 and J9interconnections• TPS of T20E with TP1S of T20F

11. repeat the last measurements for the systems AMI and HDB3.

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