MODIFYING THE HAMTRONICS EM-5 FOR 9600 BPS PACKET OPERATION STEVE OOODE, WNG 140 W. WOOD APT. 314 PALATINEJLL

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1 MODFYNG THE HAMTRONCS EM-5 FOR 9600 BPS PACKET OPERATON STEVE OOODE, WNG 140 W. WOOD APT. 314 PALATNEJLL Absrac Wihin he las year considerable aenion has been given o level hree linking of local area packe groups across he counry. Fxperimens using virual and daagram circuis are under discussion. This paper presens an inerface board and he necessary modificaions o he Hamronics FM MHz ransceiver allowing operaion a 9600 bis per second (bps) o provide a high speed link beween he local area neworks. Radio Selecion The purpose of his projec was o provide a high speed radio modem as a esbed for he level hree linking experimens. A minimum daa rae of 9600 bps wih minimum modificaions o an exising radio were he main objecives. n order o send 9600 bps daa wih minimal overhead a radio wih fas ransmi o receive and receive o ransmi urnaround ime was desirable. This implied a crysal conrolled ransmier wih pin diode anenna swiching. Since he ARRL band plan for 220 MHz conains allocaions for high speed packe links a 220 MHz ransceiver was chosen for he iniial experimens. A radio ha was sill in producion and available o all ineresed groups was also a prime consideraion in selecing he Hamronics FM MHz ransceiver. This uni conains a crysal conrolled receiver and ransmier providing 7 was of oupu power and pin diode swiching of he anenna. Receiver Modificaions The EM-5 is a EM ransceiver so i conains a discriminaor deecor in he receiver. Frequency shif keying can be deeced using a frequency discriminaor so i was chosen as he modulaion echnique. The maximum daa rae ha can be sen hru he FM-5 receiver using FSK modulaion was esed by frequency shif keying an RF generaor wih random daa a differen daa raes and observing he received daa on an oscilloscope. The paern generaed when he oscilloscope is riggered on he ransmied daa clock is called an eye paern. Figure 1 shows he eye paern of he ransmied daa on he op race and he received eye paern in he boom race for a 9600 bps daa rae. One bi period is approximaely wo divisions in lengh. As can be seen in he picure, he received eye is open wih minimal jier a he bi crossings indicaing ha 9600 bps daa can be sen hru he EM-5. Figure 2 shows he eye opening of 12 kbps daa. The eye a 12 kbps is beginning o close wih increased jier a he bi crossings bu he FM-5 could be used o send 12 kbps daa. Figure 3 shows he eye paern of 16 kbps daa. This eye is closed wih an unaccepable amoun of jier a he bi crossing. This daa limi is due o he bandwidh of he F filers in he FM-5 and could only be exended by increasing he bandwidh of he F filers. Since minimum modificaions o he radio were desired he maximum sandard daa rae was se a 9600 bps. The modificaion o he receiver is simply o connec a shielded wire o pin 9 of Ul (he discriminaor oupu) which is run o pos deecion filers on he inerface board. The performance of he receiver a 9600 bps can now be esed by measuring he he bi error rae (BER) of he receiver. The BER of a daa sysem is he probabiliy of no receiving he ransmied bi correcly. This is normally expressed in percen or decimal form. For example, a sysem wih a BER of 1 x 10-3 or 0.1% has he probabiliy of receiving he ransmied bi incorrecly once in every 1000 bis. Bi error rae is measured by canparing he ransmied daa bis wih he received daa bis and couning he number of errors. The BER for packe radio is dependen on he inpu signal srengh o he receiver, so a graph of BER versus inpu power o he receiver is he normal measure of daa sysem performan=. Figure 4 shows he BER performance of he FM-5 receiver a 9600 bps. Figure 4 shows ha a he 20 db q-uieinq level of previously on he TAPR 1200 bps audio modem iich was repored in QEX (1). Figure 5 is he BER performance of he TAPR 1200 bps modem. This figure shows ha a he 20 db quieing level of he Moorola Synor radio he BER was measured o be 1.7 x This shows ha he 9600 bps radio has a sysem sensiiviy ha is approximaely 1.5 db beer han he 1200 bps audio modem. The direc modulaion of he RF carrier is a more efficien modulaion echnique han he audio subcarrier modulaion of he 1200 bps modem providing a sligh improvemen in sysem sensiiviy raher han a degradaion wfiich migh be expeced in going o a higher daa rae. The QEX aricle shows ha he AX.25 proocol begins o receive packes a he 10-3 BER and i is expeced ha any linking proocols will also begin recepion a he 10-3 BER. We can hen say ha a EM-5 modified for 9600 bps operaion will have a receiver sensiiviy abou 1 db beer han i's 20 db quieing sensiiviy. 4.45

2 3 KHz Deviai Figure bps Eye Paerns i Figure 2 12 Kbps Eye Paerns Figure 5 TAPR 1200 bps SYNTOR SHG Figure 3 16 Kbps Eye Paerns -12s

Transmier Modificaions e The Hamronics EM-5 uses a phase modulaor in he ransmier secion.

The resisor and capacior formed a flying connecion o which a shielded lead was aached and run o he inerface board, The EM lineariy of his modulaor was esed by measuring he frequency shif a he oupu of

A linear modulaor was used in place of a frequency shif modulaor for wo reasons.

This means ha some mehod of conrolling he ransmied specrum mus be provided in he ransmier. Figure 8 shows he resuling ransmi specrum Ghen daa is fed direcly ino a FSK modulaor.

3 Transmier Modificaions e The Hamronics EM-5 uses a phase modulaor in he ransmier secion. The ransmier was modified o provide direc frequency modulaion by ranoving C2S and replacing i wih he circui shown in Figure 6. The varacor and shun capacior were mouned on he bocxn of he board. The resisor and capacior formed a flying connecion o which a shielded lead was aached and run o he inerface board, The EM lineariy of his modulaor was esed by measuring he frequency shif a he oupu of he ransmier wih various DC volages ino he modulaor. Figure 7 shows he EN lineariy of his modulaor. As can be seen, he modulaor is linear over a 3 KHz range. A linear modulaor was used in place of a frequency shif modulaor for wo reasons. Firs, since no modificaions were made o he receiver F bandwidh he resuling daa sysem should fi ino he presen 40 KHz channel spacing 220 MHz band plan. This means ha some mehod of conrolling he ransmied specrum mus be provided in he ransmier. Figure 8 shows he resuling ransmi specrum Ghen daa is fed direcly ino a FSK modulaor. A 40 KHz spacing here is sill considerable energy which can inerfere wih he users on his adjacen channel. By filering he daa in a premodulaion filer and hen feeding i o a linear EW modulaor he specrum shown in Figure 9 is produced. This specrum has grealy reduced energy in he adjacen channel. The second reason for no feeding daa direcly ino a EN modulaor wihou premodulaion filering is ha he high frequency componens of he daa can excie crysal spurs resuling in he ransmier operaing a many frequencies, some of which may be ou of he Amaeur band.. Figure bps Transmi Specrum Unfilered cc 4C 8 b $5 0 Figure 6 FM-5 Transmier Oscillaor Modificaion M i Figure 7 i, i... ; FM Lineariy /!, SHG The opimum deviaion for he 9600 bps daa sysem was seleced by measuring he RF power required for a 10-3 BER a various deviaions. Figure 10 shows he RF power required for a 10-3 BER a 1.5 o 5 KHz deviaion. Deviaion was defined as he peak deviaion ien sending a 1-O paern. From his curve i is seen ha he deviaion for bes sensiiviy occurs a 3.25 KHz deviaion. Since he daa sysan should work wih normal frequency offses encounered wih V-F radios due o emperaure, aging and volage effecs, daa was also aken measuring he effec of deviaion on frequency offse. Figure 11 shows he RF power required for a 10-3 BER a 2, 3 and 4 KHz deviaion wih up o 4 KHz frequency offse. Taking ino accoun he frequency offse performance of he receiver and he FM lineariy of he ransmier he opimum deviaion for he sysem was se a 3 KHz. 4.47

4 Now ha we have defined he sysem deviaion he proecion given o adjacen channel users by he ransmi splaer filer can be measured. Commercial manufacurers of voice radios measure adjacen channel proecion in accordance o an Elecronics ndusries Associaion WA) specificaion (2) which requires a minimum of 70 db proecion o he adjacen channel. There are no specificaions a he presen ime for daa radios so we will have o make our own definiion. The EA voice spec essenially says o adjus he adjacen channel signal srengh o sensiiviy, hen raise his level by 3 db and degrade i back o sensiiviy by he inerfering adjacen channel ransmier. We can run adjacen channel ess by saying ha he daa sysem sensiiviy is 10-3 BER. The premodulaion filered FSK signal was adjused o give 10-3 BER and hen his level was raised 3 db. An inerfering signal was hen placed a various frequency offses and he proecion provided o his inerfering signal was measured. Figure 12 shows he proecion provided o a carrier, a premiulaion filered sysan (splaer filered) and an unfilered sysem (no splaer filer) a 9600 bps. As can be seen, 70 db proecion is provided o a carrier which should be similar o he EA es. 70 db proecion is also provided o he described 9600 bps daa sysem a 40 KHz channel spacing. Wihou splaer filering only 34 db proecion is provided which can cause inerference o operaions on he adjacen channel. 11 g SHG n An addiional facor which mus be considered in packe 'operaion is how he ransmier is keyed up and down. Since his is done more ofen in packe operaion han voice operaion he frequen key up can produce a specrum of is own ha can be higher in splaer han he daa modulaion specrum essenially negaing he effec of he premodulaion daa filer. Once again here are no commercial specs for his frequen ransmier key up bu a logical spec o se is ha his key up specrum should be below he daa modulaion specrum. The inerface board for he 9600 bps radio conains wo DC key up swiches which conrol his key up specrum. The 9.1 vol line o he oscillaor is keyed up 5 ms before he 12 vol line o he drivers and says up for 5 ms afer he drivers are urned off. This reduces he key up specrum o an accepable level. The modificaions o he ransmier require removing Cll, R3 and VRl. A separae 9.1 vol regulaor wih ransmi swich is on he inerface board and is brough ino he radio in place of he resnoved pars. A final modificaion o he ransmier requires removing R24. was found ha he charging of capaciors in he ransmier audio gave a spike a abou 30 ms afer key up o he phase modulaor. This inerfered wih he daa ha was being ransmied a ha ime. Removing he audio from he phase modulaor cured his problem. 4.48

5 Daa Randomizer Since VHF radios can have frequency offses as large as he chosen sysem deviaion for he 9600 bps daa sysem, fixed decision levels for he received daa canno be se. This problem is remdied in his sysem by assuming a randan daa inpu and deriving he daa decision level by averaging he received daa. Unforunaely he AX.25 proocol does no provide random daa so some mehod mus be found o randomize he ransmi daa and hen unrandamize he received daa back o he original ransmi daa. This is provided on he inerface board. A 17 sage shif regiser is configured o provide a self synchronizing daa randomizer. This randanizer essenially divides he ransmi daa by a polynanial a he ransmier and hen muliplies he receive daa by he same polynomial a he receiver. The polynanial for his randanizer was seleced o randomize AX.25 daa sufficienly for ransmission over he 9600 bps radios. Alhough his is no a sandard maximum lengh polynomial i does sufficienly randomize AX.25 daa wih a minimum of exra pars. The randamizer has one drawback in ha i requires 17 addiional bis o be received correcly for he packe o be received. Since his is 17 bis in addiion o he over 1000 bis in a packe his does no significanly change he BER required for he sysem o begin receiving. nerface Board Figure 13 is a schemaic of he inerface board required for he 9600 bps modificaions o he Hamronics!?%5. Ul is a quad op amp which provides he 5 pole 5 KHz Bessel filer used for he ransmi premodulaion filer and a 2 pole 5 KHz Buerworh filer used as he pos deecion receiver filer. The filers were designed o give 70 db adjacen channel proecion a 40 KHz in he ransmier and o provide opimum BER performance in he receiver. U2 and UlO provide he 12 v and 9.1 v key up conrols. U5 and U6 form he daa randanizer wih U7 providing he necessary swiching beween ransmi and receive. U4 and U3 form a clock recovery circui necessary for proper daa randomizer operaion. A clock lock oupu is also provide by U4 and U3 which is used as he daa carrier deec funcion. U8 provides a 1-O paern o he ransmier during receive o keep he oscillaor on cener frequency and ready o ransmi. U2 provides a ime ou imer for he ransmier and he daa slicer for he receiver. Channel frequency and deviaion of he radio ar e adjused by removing jumper 1 and shoring jumper wo. C26 in he FM-5 is hen adjused for cener frequency. Jumper 1 is hen replaced and he deviaion on he 1-O paern is se for 3 KHz. he building of a pin diode anenna swich. Field ess were hen conduced beween K9NG in Palaine, ll. and W9TD in Hoffman Esaes, ll. which is approximaely a five mile pah. K9NG used he 7 wa EM-5 and w9td used he 1 wa T-51/R-220 pair. Two TAPR ki boards were jumpered for wice clock operaion and conneced via he modem connecor bps packes were hen sen beween he wo saions. Boh saions used a l/4 wave ground plane anenna. These field ess confirmed he bench ess of he 9600 bps radios. The Txd of he TAPR boards were se o 1 resuling in a 20 ms delay for ransmier key up. 128 bye packes were sen in abou 150 ms. These packes jus open squelch in a normal EM receiver and sound like noise burss. 100 Kbye files were ransmier in approximaely one and a half minues. Modifying Oher Radios The inerface board presened here can be used o modify oher radios for 9600 bps packe operaion. As was menioned in he field es resuls, a Hamronics T-51 and R-220 excier/receiver pair were also modified wih similar resuls o he m-5. One area ha may cause a problem wih oher radios is he ransmier key up. Since oher radios may key up differenly han he EM-5 or T-51 he mehod of conrolling he key up used for hese radios may no work for ohers. f a all possible, he key up of he inended radio for modificaion should be observed on a specrum analyzer and verified o no creae a large ransmi specrum when keyed up a an 8 Hz rae. Cemens on Higher Daa Raes The filers on he inerface board can be direcly scaled for higher daa raes when radios wih larger bandwidh FS become available. should be realizd ha any increase in daa rae from he 12 kbps maximum allowable in he sandard 15 KHZ bandwidh of an EM receiver will degrade syse;n sensiiviy in a direc relaionship. For example if a bps sysem was desired he F bandwidh would have o increase by approximaely 10 imes and a 10 db degradaion in sysan sensiiviy would be experienced. This means ha link saion separaion would have o be decrease or ransmier power would have o be increased o make up for his degradaion in sysem sensiiviy. The apparen free lunch in sysem sensiiviy ha was obained by going o direc FSK will no be available for higher daa raes and he degradaion in sysem sensiiviy will be a radeoff in designing link saions o use hese higher daa raes. Field Tess An m-5 ransceiver was modified as described in his paper along wih a Hamronics T-51/R-220 excier receiver pair. The inerface board was mouned exernal o he FM-5 and all conrol leads. 3 were run hru a 9 pin D connecor mouned on he back of he radio. The excier/receiver pair used he same inerface board as he EM-5 bu required 4.49

6 Conclusions- The necessary inerface board and modificaions o allow packe operaion of he Hamronics EM-5 ransceiver a 9600 bps have been presened. Field ess have proven he design o be workable. Every aemp has been made o design his radio inerface o be a good neighbor in he 220 MHz band. Mjacen channel proecion raios were measured for he sysem and provide a minimum of 70 db proecion o he adjacen channel users. Since he EM-S provides a maximum of 7 was of oupu powerr linking groups may be ineresed in designing a power amplifier. This pow= amplifier would have o be designed o provide fas anenna swiching and proper key up. Fuure projecs could also include a radio designed specifically for 9600 bps packe operaion or higher daa raes. Aciowledganens would like o hank Paul Newlax& AD71 for providing he sae machine code used in U3 which implemens clock recovery and daa carrier deec. would also like o hank Gary Kaaz, W9TD for performing he 9600 bps field ess. Finally a large hank you goes o Pee Eaon, wf39elw for originally suggesing his projec and o Lyle Johnson, WA7GXD and he res of he TAPR gang for heir encouragemen hroughou he projec. References 1) Goode Seve, K9NG, Vhe Bi Error Rae Performance of he TAPR TNC Modem", QEX 18, Augus ) Elecronics ndusries Associaion, '*Minimum Sandards for Land Mobile Ccxrmunicaion FM or PM Receivers, MHZ", RS-204C, January

7 TO RADO FM L 1800 DSCRMNATOR RCV FLTER 1 1 -LLL 4.7uF 5v T3 5v A 4 DATA SLCER 9 RTS 13 ; RC 14 ; CLOCK 1 h 1 l%ool TME OUT TMER 1 15 ; GND Q5 2N3906 Q4 2N u3 RlOO 470 ; DCD CLOCK 32X CLK NOTE ALL CAPACTORS ARE N PCOFARADS UNLESS OTHERWSE SPECFED.

EXPERIMENT #9 FIBER OPTIC COMMUNICATIONS LINK

EXPERIMENT #9 FIBER OPTIC COMMUNICATIONS LINK INTRODUCTION: Much of daa communicaions is concerned wih sending digial informaion hrough sysems ha normally only pass analog signals. A elephone line is such