Telemetrie-Messtechnik Schnorrenberg

Transcription

1 Funcion Descripion of Digial Telemery 1. Digial Telemery Sysems 1.1 Telemery Sysems wih PCM-Technology For he wireless ransmission of several informaion channels, several differen RF ransmission frequencies are necessary and as many selecive RF receivers, which would increase he cos/benefi raio disproporionaely. The soluion of his problem provides he digial ransmission echnique. The real ask of muli-channel elemery sysems is o inerlace and bundle he differen cannels imewise wih each oher in ha way, unill hey are available as a "2-wire line" o be ransferred wih one single RF ransmier. This ype of ransmission is realized by digiizing, muliplexing and PCM encoding of all signal channels. This kind of digial PCM ransmission (pulsecode modulaion) has been used for many years in he communicaions echnology, eg. worldwide ransmission of elephone-channels. The advanages of PCM ransmission speak for hemselves: - consan signal / noise raio by digiizaion of he signal - muliple use of one ransmission-channel by muliplexing - inerference free signal ransmission, low suscepibiliy o crossalk - direc acquisiion and processing of he received digial PCM signal o a PC Sender Encoder a() in TP f Gg D P 12 1 S Tak, S PM Empfänger biparallel worseriell PCM Decoder biseriell Überragungsweg a() ou f g TP D 12 P S 1 Figure 1: Block diagram of a 1-channel PCM ransmission

2 2. Generaion of a PCM signal The Pulsecodemodulaion (PCM) plays an ever increasing role in he collecion, ransmission and analysis of measured values. In he following he producion of a digial PCM signal and he process of sampling, quanizaion and coding are described in more deails. The analog signal a() is iniially amplified (condiioned), and in is bandwidh filered (Fig. 1). This is followed by an essenial sep, he discreizaion of he coninuous measuring signal. n elecronic swich (Sample & Hold) - conrolled by a clock generaor - akes individual samples from he signal, whereas he pulse ampliude in ime corresponds o he momenary value of he analog inpu volage. The oupu of he elecronic swich is a pulsampliude modulaed signal, he PM signal. The sampling heorem specifies he minimum frequency rae an analog signal has o be scanned, so ha he original signal can be exraced wihou loss of informaion from he re-sampling. The sampling frequency (fs) mus be greaer han wice he highes conaining frequency in he analog signal (fg): fs > 2 x fg In pracice her are 4 o 5 samples per bandwidh aken. The effec of pulseampliude-modulaion becomes more clearly by viewing he signals in he ime - and frequency domain (Fig. 2). The sampling process produces a sequence of pulses which conains according o he Fourier nalysis a DC componen and a sum of sinusoidal volages, which are ineger muliples of he fundamenal frequency. Zeibereich Fourier-nalyse Frequenzbereich f Signal f f g S S f fs = 1/s as-pulse (Dirac-Pulse) f S 2f S f f * S f * S PM f f g f s -f g f s f s +f g 2 f Figure 2: Scanning in ime- and frequency domain

3 In he frequency domain he pulse-samples creaes sysemaically specral-lines a inervalls of fs. he righ and lef sides of hese carriers arises modulaion sidebands (similar o ampliude modulaion) wih upper and lower sidebands a fs-fg, fs+fg, 2fs+fg, 2fs-fg, ec. The signal informaion is in each side band. For he signal ransmission is, however, only he red coloured baseband in Figure 2 is used. In he frequency range i s also eviden ha by a magnificaion of he signal-cuoff frequency he modulaion sidebands expand and would fall ino each oher. In his momen he so called "aliasing" appears, which only can be prevened by a higher sampling rae. In pracice his problem does no occur because he signalbandwidh is are already low pass filered (by ani-aliasing filer) a he fronend. The puls ampliude modulaed signal in Figure 1 is sill an analog frame of he inpu signal. Bu he samples can be much beer processed in digial form. In order o finalize he quanizaion and coding, he PM signal is fed o a 12-bi-/D-converer. The /D-converer convers (quanized) each PM pulse according o heir curren ampliude ino 12-bi words, a digial resoluion of 1024 seps. Therefore a PM pulse ampliude of 1 vol is digiised wih a resoluion of <1mV. The digiized PM signal is called a PCM signal. The 12-Bi-/D- follows a parallel/serial converer, which convers he 12-bi words ino a biserial daa sream and can be ransferred on a daa line or via a fiber opic RF line. To enable he receiver o synchronize on o he serial daa sream, sychron bis in fron of each daa word are ranfered as well. On he receiving side he same is done hing in reverse. fer he series / parallel conversion, he 12-bi words are convered wih he help of D/-converer in PM signals and hen low pass filered o smoohed ampliude coninuous signals. Each signal value is equal o he average of he corresponding quanizaion inerval. fer signal amplificaion according o he level adjusmen, he original measuring signal a() is availble again. 2.1 Muliplexing and Demuliplexing Muliplexing allows mulichannel, synchronous ransmission of several PCM channels. s shown in Figure 1 and Figure 2 he sample pulses ake only a very limied amoun of ime and beween he saples are relaively large ime gaps. ccording he ime division muliplexing, he 12-bi code words of several ransmied signals can be imed o sagger, so ha hey do no affec each oher, bu fill in he free ime gaps. This resuls in a PCM muliplex signal. Figure 3. illusraes he basic principle of he emporal inerlacing of several messages (code words) and he ransmission over a common line. The ime muliplexing process is done complee elecronic. Figure 3 shows four inpu signals, whih are cyclically scanned by a roaing swich. Synchronous wih he consequence of he incoming code words, he swich is driven on he nex inpu. he oupu of he swich he PCM ime muliplex signal is hen available. The period in which a code word is ransmied is called ime slo.

4 Sender 12-bi-Codewor Empfänger Überragungssrecke Pulsrahmen B Zeischliz einzelne PCM-Kanäle Muliplexen PCM-Zeimuliplex- Kanal Demuliplexen einzelne PCM- Kanäle Figure 3: Schemaic represenaion of ime muliplex and ime demuliplex bi sequence, which onains a codeword from each inpu signal, is called a pulsframe. The example lised in Figure 3 shows a puls frame consising of four code words from he inpu signals.... The required sampling frequency of he muliplexer for he complee ransfer of digial informaion is fs > 2 x fg x numbers of channels On he receiving side, he individual PCM signals are recovered from he ime division muliplex signal, i.e. he 12-bi code words are applied o he corresponding oupus. The roaing swich B disribues synchronous he code words o he four oupus. Like he ime muliplexing a he ransmier side, all operaions are execued complee elecronic. 3. Mulichannel elemery sysems Figure 4 and 5 show he basic consrucion of a muli-channel elemery sysem, as an example wih 4 ransmission channels. The PCM ransmission sysems consiss of wo unis, a PCM encoder for recording and coding of he signal values on he ransmi side and a PCM decoder o decode and oupu he measured values on he receiving side. Funcions of he encoder: - Signal processing of he analog inpu signal (sensor signal) - Bandwidh limiaion by Low Pass- - Simulaneous sampling by a Sample & Hold mplifier - /D conversion () of he sampled signal - Parallel-serial conversion of he 12-bi words - Inserion of synchronizaion bis (marks) - Converion ino a PCM code - FSK modulaion of RF ransmier

5 PCM-ENCODER 12 1 Muliplexer HF- Sender simulan sampling asrae Figure 4: Block diagram from a mulichannel PCM encoder Figure 4.1: Modular-MT32-Telemery-Sysem wih signal condiioning, analog o digial converers, encoder and RF-Transmier (433/866M) The decoder a he receiver side performs he following asks: - selecive amplificaion and demodulaion of he RF signal - Regeneraion of he incoming serial PCM signal - Generaion of an inpu signal o he synchronous clock - Deecion of synchronizaion bis and generae he corresponding signal addresses - Oupu daa in bi-parallel, word-serial formae o a PC inerface card (IF16) - D/ conversion of he daa and oupu as analog signals

6 PCM-DECODER PCM-ou 1 12 HF- Empfänger Demuliplexer PCM-Decoder asrae simulan Sampling Figure 5: Block diagram of a PCM muli-channel decoder Figure 5.1: Modular-MT32-Telemery-Decoder wih 8 analog oupus (+/-5V) 3.1 Synchronizaion of he encoder and decoder Thus he decoder is able o recognize he iming of he digiized measured values, a so-called synchronous word is insered. This sync word consiss of a fixed lengh of 4 bis and is insered a he beginning of each PCM Pulsframe. The decoder is synchronized o his sync word and is always in exac synchronism wih he corresponding ransmier. In addiion, he synchronous word provides abou is encoding oher useful informaion, such as he baery capaciy of he ransmier. Figure 6 shows he srucure of a single pulsframe, consising of sync word and 4 channels. The lengh of he serial PCM Pulsrahmens is 4 x s + 4 bis = 52 bis.

7 PCM-Pulsrahmen sync 4 bi 1. Daenwor 2. Daenwor 3. Daenwor 4. Daenwor 1. Synchronisaionswor 2. Synchronisaionswor Figure 6: PCM frame wih sync word, example: 4 channels The max. ransmied signal bandwidh of each channel is direcly relaed o he muliplexer scanning speed. Table 1 gives an overview of he achievable signal bandwidh () as a funcion of sampling rae () of he PCM sysem. The acual frame lengh (see Figure 6) is calculaed from he number of ransmied channels. Thus, he ransmission rae of a elemery sysem is he produc of sampling rae and frame lengh. ransmission rae (bi/s) = sample rae () x frame lengh (bi) Birae 40 Kbi/s 80 Kbi 160 Kbi 320 Kbi 640 Kbi 1280 Kbi Rahmen länge 16 Kanal bi 8 Kanal bi 4 Kanal bi 2 Kanal bi Table 1: Relaionship beween ransmission rae, numbers of channels and PCMframe lengh Transfer rae calculaions, e.g. for 8 channels: 8 x = 96 bi + 4 bi sync. = 100 bis Birae = 6400 x 100 bi = 640 kbi/s uhor: Werner Schnorrenberg, July 2009 copywrie