ECE 4670 Communication Systems Laboratory …

1 ECE 4670 Communication SystemsLaboratoryExperiments ManualSpring 2010 Mark A. WickertElectrical Engineering DepartmentUniversity of Colorado at Colorado SpringsColorado Springs, CO 80933-7150 1988 1990, 1995, 2002, & 2010 by Mark A. Communications System Balanced Modulator .. Gain Level Shifter .. KHZ Filter .. Gain 455 KHZ Filter .. Active Lowpass Filter .. Active Highpass Filter .. Amplifier/Envelope Detector .. Amplifier with Speaker Output .. 122 Laboratory System Characteristics .. Spectrum Analyzer Measurements .. Network Analyzer Measurements .. Box Filter Characterization .. Analysis .. Analysis .. Pulse Train.

2 Cosinusoidal Pulse Train .. (PN) Sequence Generators .. and AM Modulation and Demodulation .. Demodulation of DSB .. Envelope Detector for an AM Signal .. Trapezoid and Modulation Index .. Superheterodyne Receivers .. Translation .. Selectivity .. of Local AM Broadcast Stations .. Modulation and Demodulation .. Frequency Deviation Constant .. Modulation Index - .. with Other than Sinusoidal Signals .. Demodulation using Slope Detection .. Order Phase-Lock Loops .. Subsystem Characterization .. Exercises Part I .. Order PLL with Lead-Lag Loop Filter .. Exercises Part II .. Order PLL with Integrator/Lead Compensation Loop Filter.

3 Exercises Part III .. Investigations .. : Data Sheets .. 76 ECE 4670 Communication Laboratory ExperimentsiiiChapter 1 Communications System BoxThe Communication Systems Laboratory experiments require the use of several different types ofsignal processing blocks. In terms of hardware each of these blocks is typically an active electroniccircuit, in some cases requiring many components. The Communication system box contains eightdifferent electronic subassemblies that are used for generation and detection of modulated intent of the system box is that its use will allow a reduction in experiment set-up time, therebygiving more time to learn and understand the overall system signal processing blocks incorporated into the system box are the following.

4 2 - analog balanced modulators (multipliers) 1 - unity gain level shifter 1 - 455 KHz bandpass filter 1 - high gain 455 KHz amplifier 1 - second-order active lowpass filter with variable cutoff frequency 1 - second-order active highpass filter with variable cutoff frequency 1 - variable gain summing amplifier with envelope detector output 1 - audio amplifier with speaker outputThe front panel layout of the system box is shown in Figure A brief description of eachblock including the circuit schematics is given in the following Analog Balanced ModulatorThe system box contains two analog balanced modulator circuits. Each modulator circuit containsa four quadrant multiplier chip (1496) as shown in the schematic of Figure These circuitsserve as analog multipliers and thus are used for DSB and AM modulation and demodulationas well as frequency translation in superheterodyne receivers.

5 TheVcinput is typically used forthe carrier signal. TheVsinput is used for the analog message signal during modulation and ANALOG BALANCED MODULATOR SpeakerSMultiplierDC OffsetControlVcInputVsInVoOutSMultiplier DC OffsetControlVcInputVsInVoOutSMultiplier DC OffsetControlVcInputVsInVoOutSMultiplier DC OffsetControlVcInputVsInVoOut10k10k10k-+ 10k10k10k-+Level ShifterOffsetInputOutput-+InputInputSumm ing Amp/Env. DetAudio AmplifierVolumeInputHigh Gain 455 Amplifier455 kHz BPFGainOutputInput455 kHz BPFI nputOutput455 kHz Filter2ndOrderHPFfc2ndOrderLPFfcActive FiltersInputOutputOutputInputPower-12vGr ound+12vFigure : Communication System Box Front Panel LayoutECE 4670 Communication Laboratory UNITY GAIN LEVEL SHIFTERF igure : Balanced Modulator Schematicreceived signal during demodulation.

6 The DC offset control is used to vary the amplitude of theDC component added to the message input. In many applications this control is used to null thecarrier spectral component seen at the modulator Unity Gain Level ShifterThe unity gain level shifter circuit consists of an op-amp inverting summing amplifier with twoinputs. Both inputs are connected in a unity gain configuration. Level shifting is accomplishedby connecting one input to a potentiometer that is connected between the +12v and -12v powersupply. The schematic for this circuit is shown in Figure primary use of this circuit is to remove the DC component present on the balanced mod-ulator output. In most applications the DC component is nulled to zero.

7 A case where a non-zeroDC component may be desired is in a phase-lock loop (PLL) where a balanced modulator used asa phase detector must drive a voltage controlled oscillator (VCO). Here it is desired to haveVdccorrespond to the VCO quiescent operating 4670 Communication Laboratory 455 KHZ FILTERF igure :Unity Gain Level Shifter 455 KHZ FilterThe 455 KHz bandpass filter is a fixed tuned ceramic filter with the input/output terminalsbrought out to the front panel of the system box. The frequency selectivity of a ceramic filter isdue to mechanical vibrations resulting from a piezoelectric effect1. The equivalent circuit of abasic two terminal ceramic filter resonator is shown in Figure along with a typical impedanceversus frequency response curve.

8 The resonant frequency,fr, is given byfrD12 pL1C1( )The so-called anti-resonant frequency,fa, is given byfaD"2 sL1C1C0C1CC0# 1( )By cascading resonators that are either in a ladder connection or a cascade connection, as shownin Figure , the desired filter frequency response characteristics can be Communication system box uses one of two types of ceramic filters manufactured by Mu-rata Erie North America, Inc. The terminology used to describe the frequency response charac-teristics of these filters is shown in Figure An explanation of each term given in Figure provided in Table The type of filter used in the Communication system boxes is either anSFU455A or a CFU455D2, where both filters have center frequencies at 455 KHz.

9 The detailedspecifications for each of these filters are given in Figures and respectively. Note that thespecifications assume either 3K or source and load impedances, depending on the filter the impedances are lower than those specified then the center frequency is lowered, the insertionloss increases, and the ripple increases. If on the other hand the impedances are higher than thosespecified, then the center frequency is higher, with again increased insertion loss and passbandripple. In the experiments these filters will be used to construct a superheterodyne Erie North America Inc., Ceramic Filters Catalog No. 59-10, 4670 Communication Laboratory 455 KHZ FILTERTwo-terminalequivalent circuitImpedance versusfrequency responseTypical frequencyresponse at 455 kHzFigure : Two-Terminal Ceramic Filter ResonatorLadder connectionof resonatorsCascade connectionof resonatorsFigure : Resonator ConnectionsECE 4670 Communication Laboratory 455 KHZ FILTERF igure : Ceramic Filter Frequency Response TerminologyECE 4670 Communication Laboratory 455 KHZ FILTERT able : Ceramic Filter Terminology ChartNumbersTerminologySymbolUnitExplana tion of Term1 Center FrequencyHzThe frequency in the center of the pass-band-width.

10 However, the center frequency for some products is expressed as the point where the loss is at its lowest BandwidthBWHzSignifies a difference between the two frequen-cies where the attenuation becomes 3 dB fro the level of the minimum loss LossILdBExpressed as the input/output ratio at the point of minimum loss. (The insertion loss for some products is expresses as the input/output ratio at the center frequency.) Insertion in dBIf there are peaks and valleys in the pass-band-width, the ripple expresses the difference between the maximum peak and minimum Bandwidth20 dB BWHzThe bandwidth at a specified level of attenua-tion. Attenuation may be expresses as the ratio of signal strength to the output signal strength in Attenuation dBThe level of the signal strength at a specified frequency outside the ResponseSRdBThe difference in decibels between the inser-tion loss and the spurious signal in the Impedance OhmInternal impedance value of the input and out-put of the ceramic dBThe ability of a filter to pass signals of one fre-quency and reject all others.