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Print 11Chap10.Tif (13 Pages) 10.1 mTKwmIoR L nseauring wyetm with tel*mwtry in gsnwral oonaiwts of transducers, signal ccnditicning circuit*, an airbcrne mltiplexnsr and a radio frequsncy tmnamitter in ths aircraft, and on ths ground an rf rwosivwr, a dwmltiplwx*r and a d&a wtorags sywtem (usually * tape racordsr); in moat a*** *data (pre)pmo*seing *y?Jtem with displaya ia WddSd *or on-line data an¶lywiW. The te1ematry part Of the systm will be disoussed I* this chapter. It oonwiata of the mltiplwxwr with the aasooiafed data modulatora, the rf link, the damltiplsxer, the data dwmodulators and the grand meording equipat. The methods of on-lina data pmcsssing till also ba briafly ccneidsred. In ccmscticn with on-line d&s pmcassing, tslamatry has bsccme a powerful msans of incressing the capability and sfficisncy of flight tssting. Telwmetry of flight teat data haa a numbar of advmtagas c"*r ths n** of on-board moording: a tslsmwby sywtsm has less weight and volume, it i* I*** ssnsitivs to *tireme *mirom*ntal condition* than the m-board ~ccrdsr and it has bettsr quick-lock and on-lin* data pmosasing capabilitias. In *cm* type* of flight teat it wculd be almost impoa*ibl* to ccllsot a tificisnt quantity of data without telemetry. The u** of a second telemetry link from the ground-station to the aircraft (tele- oonmmnd link) can provide further ~mprcvement of the fli&t teat efficiency. On the other hand there am a few drawback*: ths rang* ia limited by tbs physical characteristic* of wave pmpagation, and there are problsma with tha mcmti,,g of on-beard a,,t*n,,s* and with dmponts of data mcapticn dus to fading inths r&c fmqusncy channel by multiple pmpagaticn. Detailad ocmparisms srw given in Ref. 1 and 81 wet) alao Ohaphr 8. The tslsmstry chin is shown in Figure 10.1. Figure 10.1 Blook diagrsm of a t*lem*try *yat*m The airborne aystsm consists of the mltiplex*r, tha rf tnmmittar and tha on-boar.3 antanna. Tbs radiated *l*ctromgn*tio WV* inducss an rf voltage in ths mesiting antanna. Tbia vcltage i* ampli- fied snd filtered by the mcaiver in crdsr to alimimts mwmt*d signal* and noise. The detitiplsrsr d*ccmpa**s the recsiv*r output signal and mccv*re the original data. Wmlly ths data pmcsseing equipment conv*rk* the racsived data into the proper form for th* UBOI. x) Some modulation mathods m&e urn* of the well-known trads-off between signal-to-noise ratio and bandwidth. They obtain better signal-to-noise ratios at the output of the dsmadulator by manipulating bandwidth ad shape of the spectrum. As s rule it will be advantageous to use a modulation n&hod which oocupies 811 the available ohanne bandwidth. 10.3 The usual mcdulaticn methods *=a shown in SPECTRAL CARRIER Figures 10.3 end 10.4. We have to distln- DENIITYt IPECTRUMOFTHE goirish betuesn the continuous modulation SPECTRN the frequency domain by its spectrum (awe DENSITY t Fig. 10.3). For the pulss modulation methods, the parea*t*,ra of a pulse oarrier (amplitude, duration) are ccntmlled by the signal fm rl,-l,-AF '< Gdf,+f,+aF voltage (awe Fig. 10.4). In this cam the FREWZG modulated carrier can bs displayed more P FREQUENCYYOcuL*TION IFM) clea*1y in *ha *ml* domain by Its time *lJnctmn. The time function of the modulated carrier for amplitude modulation (AMlis: } . CC8( WC . t) (10.1) with the carrier wa”B UC. CO8( w,.t). The ncrmalizsd signal time fun&ion *(t) is bounded by + 1. The range of the modulation factor m usually is m&l. The spectrum cf the modulated wave consists of one line at the csn-isr frequency oca"d an upper and lower sideband. The upper sideband is ob- AMLI- tained by shifting the signal spsctnun by luCE w. along ths w-axis. The low** sideband is the image of the upper sideband, symmatri- aalto "c. Therefcre, the bandwidth of the AM spedmm is twice the bandwidth of the origin.41 wiglIe. epctrm (see Ref. 1, 2, 4 ad 5). The main drawback of AM i8 that most of the power cf the modulated way* i* n- &red for the carrier. In ths dsmcdulatcr the oarrisr is ussd only for stitching the Aid wave in order to r*cov*r ths demcdulatsd signal. It contains no signal infczmaticn. Since ii is passable to obtain the witch- ing signal for the demodulation pmcess in Figure 10.4 IUse modulation msthcds other waya, - e.g. by mmipulating the sidebands -, it i* mere efficient to suppress the carrier before transmission (double-sideband suppraee- ad-carrier modulation (DSB)). Howevsr, ths hardvars of DSB ia more ccmplex than that of the simple AM. In the cas* cf AM and DSB all the signal infcxmation ia contained in each of the two sidebands. Tnerefcre, one sideband may bs suppressed by filtere and the sir&*-sid*be.nd suDDz.ass*d-carrier modulation (SSB) i* obtained. The bandwidth of SSB is equal to that of the signal,and half ths bandwidth of AM and DS3. 'I%* signal-to-noise ratio is equal to that of DSB but, b*cau** of ths smaller bandwidth, an* can handle twice as many data channels in a given frequency range as lnth AM and DSB. The main drawbacks cf SSB ar* ths high degree of hardware complexity and ths lack of M: responss. Th* spectra of DSB end $23 may easily be derived from ths Ay ape&rum dieplayed in Figure 10.). 10.4 For a deeper understanding ci modulation it may be wcrth mentioning that a close connection erista with the sapling theorem, wiuch is discussed in Chapter 6. lie aemme a signal speotmm with an upper frequency limit f.. When the" the oartier frequency fC IS less *hII 2 I.' thsrs will be flaqusncisa at which ths unmcdnlatad and the modulated signals overlap. This will cause aliaaing error* (or ermm of aommisicn as they are called In Chapter 6) when the signal is demodulated. The AM pmce88 may be regarded as the sampling of a signal s(t) by the ains wave. Frepusncy modulation (EWE (Fwxe 10.3b) IS a wideband modulation method which me&es use of an ex- tended bandwdth zn order to ~mprcve the signal-to-noise r&u,. Because of the relatively simple hardware FM IS of grsat importance for flight testmg. In FM the frequency of the carrier wave is modulated in the following way fm * fc + AF.S(t) (10.2) whsre fc is the frequency of the modulated carrier, AF is the frequency deviation and B(t) is the ncnnal- lzed signal (see above). It can be shown (see Ref. 1, 2, 3, 4 and 5) that the increase of the sqm,l-tc- nciss ratio is pmperticnal to the ratio M-F (10.3) I M ie the 80 called mcdulatlon lnder and f. tile highest frequency ccmponsnt of the Signal spectrum (8.0 Fig. 10.3b). Unfortunately,the bandtidth cf the FM-spectrum also increases aa e linear function of AF, thus limiting the obtainable gain because of the general restnotions cn available bandwidth. For FM subcarrxers mcdulat~cn lndxces of 5 are used in practice for fllgbt testing. A special case of FM is the phase modulation (PM). T%is ia accomplished by letting the signal a(t) control the carrier phase instead of the carrier frequency. Its special feature is a preemphasia which incrsasss the amplitude of the signal spectrum linearly with frequency. In pulse amDlituda modulation (PAM) (Fig. 10.4a) the signal a(t) ~8 sampled at discrete points 1" time. This pmcass is described in detail in Chapter 6. All considerations of that chapter apply directly tc PAM. Fulse duration modulation (FDML ('Pig. 10.4h) E4K?L is obtained by ccnvetiing the amplitude of the PAM samples into a pulse duration. Thus a train t 2 n of pulses with variable width is generated, and the d.yne.mic range of the signal is transformed fmm the amplitude domain to the time domain. The minimum value of the signal s(t) corraspcnds to the shortest pulse dureticn) the maximum value oarresponds to the longest pulse duraticn. The amplitude of the PDM pulse train rsmains constant. PDM, 88 FM, is a wideband mcdulaticn method, It is only used for simple ayeterns (sectmn 10.5). For the sake of completeness, pulse Doei- ticn modulation, or pulse phase modulation (PFM , is also displayed in Fig. 10.40, though it is *YPLINDE/ not a standard modulation method in telemetry. In this 0~80 the relative psitmn of * pulse is controlled by the signal. A time reference El” I/ is required for demodulation. u ” Due to the great technclc,$cal progress in integrated cirouits, the use of pulse code mcdu- letion (PCM~haa become important during the last few yeam.In this method the PAM samples are 10.5 ooded in l %r.wd~ of R pubes, wing only the levslm 0 and 1. Being a digital method, any mwimd aocuracy mn be obtained by pmpsr ohoioe of the word length II. Besides, PCMWea axcellent "ae of the law of exobangeability between signal-to-noise ratio and bandwidth. FCMis widely used in time domain multiplexing my.tams. Serial fomat XX oar, be derived from a PAM signal by a" analog-to-digital oon- verter with a aerial output. The olook fmquenoy for the A,D cometier rmpt be A-times the PAM sempling fnpuw,oy. This ob"iowly #ho"8 the inamase in b-width.
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