lab sheet experiments

Download Lab sheet experiments

Post on 25-May-2015




5 download

Embed Size (px)


Lab sheet experiments


  • 1. INTRODUCTION TO TIMSTIMS is a modular system for modelling telecommunications block diagrams.Since block diagrams themselves represent telecommunications systems, or sub-systems,and each sub-system can probably be represented by a mathematical equation, then TIMScan also be said to be a telecommunications equation modeller.Most TIMS modules perform a single function. For example, there are multipliers, adders,filters, samplers. Other modules generate signals such as sinewaves, square waves, randomsequences.Complex systems are modelled by a collection of these simple modules. There are fewmodules that perform complex functions which otherwise could have been performed by acollection of simpler 1/2conventionsTIMS is almost self-explanatory, and a first-time user should have no trouble in patching upa basic system in a few minutes, without the need to refer to the extensive User Manuals.TIMS modules conform to the following conventions. inputs to each module are located on the left hand side of the front panel outputs from each module are located on the right hand side of the front panel modules become powered when plugged in, and pass signals via external patch leadsconnecting front panel sockets sockets involving analog signals are coloured yellow sockets involving digital signals are coloured red analog signals are user-adjusted to the TIMS ANALOG REFERENCE LEVEL, which is 4volt peak-to-peak digital signals are sent and received at TTL levels (0 volt and 5 volt) input impedances are high (>10 kohms) and output impedances low (> Figure 1: a DSBSC generatorHere the message source a(t) is shown as a single sinusoid. Its frequency () wouldtypically be much less than that of the carrier source ().A snap-shot of the waveform of a DSBSC is shown in Figure 2, together with the messagefrom which it was derived..message0E-Etime+ 1- 1DSBSCFigure 2: a DSBSC - seen in the time domain 14. Emona-TIMS DSBSC - generation L-03 rev 1.3experimentModel the block diagram of Figure 1 as shown in Figure 3. If an AUDIO OSCILLATOR isnot available, the 2 kHz MESSAGE from MASTER SIGNALS can be substituted. But thiswould be a special case, since this message is synchronous with the carrier frequency. Notealso the optional ADDER in Figure 3; this makes provision for a pilot carrier - see pilotcarrier below.messagesource( )carrier source fromMASTER SIGNALS( )DSBoptionalFigure 3: the TIMS model of Figure 1There should be no trouble in viewing the output of the above generator, and displaying it asshown in Figure 4. Ideally the oscilloscope should be synchronised to the messagewaveform.Figure 4: typical display of a DSBSC and the message.This is not the same as the snap-shot illustrated in Figure 2. An oscilloscope with the abilityto capture and display the signal over a few message periods could reproduce the display ofFigure 2.You can obtain the snap-shot-like display with a standard oscilloscope, provided thefrequency ratio of the message is a sub multiple of that of the carrier. This can be achievedwith difficulty by manual adjustment of the message frequency. A better solution is to usethe 2 kHz MESSAGE from MASTER SIGNALS. The frequency of this signal is exactly 1/48of the carrier.If an AUDIO OSCILLATOR is not available (the 2 kHz MESSAGE from MASTER SIGNALSbeing used as the message) then the display of Figure 4 will not be possible.pilot carrierFor synchronous demodulators a local, synchronous carrier is required. See the Lab Sheetentitled Product demodulation, for example. As an aid to the carrier acquisition circuitry atthe receiver a small amount of pilot carrier is often inserted into the DSBSC at thetransmitter (see Figure 1). Provision for this is made in the model of Figure 3.TIMS Lab Sheet copyright tim hooper 1999, amberley holdings pty ltd ACN 001-080-093 2/2 15. PRODUCT DEMODULATIONmodulesbasic: for the demodulator MULTIPLIER, PHASE SHIFTER, VCObasic: for the signal sources ADDER, MULTIPLIER, PHASE SHIFTERoptional basic: AUDIO OSCILLATORpreparationThe product demodulator is defined by the block diagram of Figure 1/2modulatedsignal incarriersourcephase shiftermessageoutFigure 1: a product demodulatorThe carrier source must be locked in frequency to the carrier (suppressed or otherwise) ofthe incoming signal. This will be arranged by stealing a carrier signal from the source ofthe modulated signal. In practice this carrier signal must be derived from the received signalitself, using carrier acquisition circuitry. This is examined in other Lab Sheets - forexample, Carrier acquisition - PLL.Being an investigation of a demodulator, this experiment requires that you have availablefor demodulation a choice of signals. These can come from the TIMS TRUNKS system (ifavailable), an adjacent TIMS bay, or your own TIMS system. The latter case will beassumed . You will need to know how to generate separately AM and DSBSC signalsbased on a 100 kHz () carrier and derived from a sinusoidal message (). See the LabSheets AM - amplitude modulation and DSBSC - generation.Since an SSB signal so derived is itself just a single sinewave, at either ( ), it can besimulated by the sinusoidal output from a VCO. Set it to say 102 kHz.Remember that in the experiment to follow the message will be a single sine wave. This isvery useful for many measurements, but speech would also be very revealing. If you do nothave a speech source it is still possible to speculate on what the consequences would be.experimentThe block diagram of Figure 1 is shown modelled by TIMS in Figure 2. Not shown is thesource of input modulated signal, which you will have generated yourself. It will use the100 kHz source from MASTER SIGNALS. This will also be the source of stolen carrier. 16. Emona-TIMS Product demodulation L-04 rev 1.3The sinusoidal message at the transmitter should be in the range 300 to 3000 kHz, say, tocover the range of a speech signal. The 3 kHz LPF in the HEADPHONE AMPLIFIER iscompatible with this frequency range.INstolen carrierOUTFigure 2: the TIMS model of Figure 1synchronous carrierInitially use a stolen carrier; that is, one synchronous with the received signal.DSBSC inputNotice that the phase of the stolen carrier plays a significant role. It can reduce the messageoutput amplitude to zero. Not very useful here, but most desirable in other applications.Think about it.SSB inputNotice that the phase of the stolen carrier has no effect upon the amplitude of the messageoutput. But it must do something ? Investigate.Since this system appears to successfully demodulate the SSB signal, could it be called anSSB demodulator ? Strictly no ! It cannot differentiate between an upper and a lowersideband. Thus, if the input is an independent sideband (ISB) signal, it would fail. Considerthis.AM inputCompare with the case where the input was a DSBSC. What difference is there now ?An envelope detector will give a distorted output when the depth of modulation (m) of theAM signal exceeds unity. What will happen to the output with a product demodulator ?Investigate.non-synchronous carrierRepeat all of the above, but with a non-synchronous carrier from a VCO. Observe theconsequences, especially with a small frequency error (say a few Hertz). DSBSC and SSBdiffer quite remarkably especially noticeable with speech.Refer to the TIMS User Manual for fine tuning details of the VCO. In summary: coarse tuning is accomplished with the front panel fo control (typically with no inputconnected to Vin). for fine tuning set the GAIN control of the VCO to some small value. Tune with aDC voltage, from the VARIABLE DC module, connected to the Vin input. Thesmaller the GAIN setting the finer is the tuning.TIMS Lab Sheet copyright tim hooper 1999, amberley holdings pty ltd ACN 001-080-093 2/2 17. AM - AMPLITUDE MODULATION - 1/2modulesbasic: ADDER, MULTIPLIERoptional basic: AUDIO OSCILLATORpreparationAn amplitude modulated signal is defined as:AM = E (1 + m.cost) cost ........ 1= A (1 + m.cost) B cost ........ 2= [low frequency term a(t)] x [high frequency term c(t)] ........ 3Here:E is the AM signal amplitude from eqn. (1). For modelling convenience eqn. (1) has beenwritten into two parts in eqn. (2), where (A.B) = E.m is a constant, which, as will be seen, defines the depth of modulation. Typically m < 1.Depth of modulation, expressed as a percentage, is 100.m. There is no inherentrestriction upon the size of m in eqn. (1). and are angular frequencies in rad/s, where /(2.) is a low, or message frequency, sayin the range 300 Hz to 3000 Hz; and /(2.) is a radio, or relatively high, carrierfrequency. In TIMS the carrier frequency is generally 100 kHz.block diagramEquation (2) can be represented by the block diagram of Figure 1.( )( )gGmessagesinewavem(t)a(t)c(t)DCvoltage carriersinewaveFigure 1: generation of AM 18. Emona-TIMS AM - amplitude modulation - I L-05 rev 1.3modelCH1-BCH1-ACH2-AVARIABLE DC 100kHz MASTER SIGNALext trigthe message(say 1kHz)AM outFigure 2: model of Figure 1If no AUDIO OSCILLATOR is available the 2 kHz message from MASTER SIGNALS canbe used instead (although this is a special case, being synchronous with the carrier).experimentTo make a 100% amplitude modulated signal adjust the ADDER output voltagesindependently to +1 volt DC and 1 volt peak of the sinusoidal message. Figure 3 illustrateswhat the oscilloscope will show.tim eFigure 3 - AM, with m = 1, as seen on the oscilloscopeThe depth of modulation m can be measured either by taking the ratio of the amplitude ofthe AC and DC terms at the ADDER output, or applying the formula:m P Q= ........ 4P +Qwhere P and Q are the peak-to-peak and trough-to-trough amplitudes respectively of the AMwaveform of Figure 3. Note that Q = 0 for the case m = 1.To vary the depth of modulation use the G gain control of the ADDER.Notice that the envelope, or outline shape, of the AM signal of Figure 3 is the same as thatof the mes