ELECTRONIC COMMUNICATIONS SYSTEMS PART 4 1 Fall 2001

ELECTRONIC COMMUNICATIONS SYSTEMS PART 4 -1 Fall 2001 ENZO PATERNO 1

AMPLITUDE MODULATION RECEPTION Fall 2001 ENZO PATERNO 2

AM DEMODULATION • REVERSE PROCESS OF AM MODULATION • AM RECEIVER RE-CREATES THE ORIGINAL INFORMATION • RECEIVER MUST BE ABLE TO BANDLIMIT THE TOTAL RADIO-FREQUENCY SPECTRUM TO A DESIRED BAND OF FREQUENCIES (i. e. TUNING THE RECEIVER) Fall 2001 ENZO PATERNO 3

AM RECEIVER 1 st STAGE LAST STAGE Fall 2001 ENZO PATERNO 4

AM RECEIVER • RF SECTION: DETECTS, BANDLIMITS AND AMPLIFIES RF SIGNALS • MIXER/CONVERTER: DOWN-CONVERTS THE RECEIVED RF FREQUENCIES TO INTERMEDIATE FREQUENCIES (IFs) • IF SECTION: AMPLIFICATION AND SELECTIVITY (TUNING) • AM DETECTOR: DEMODULATES THE AM WAVE AND CONVERTS IT BACK TO THE ORIGINAL SIGNAL • AUDIO SECTION: AMPLIFIES THE RECOVERED INFORMATION Fall 2001 ENZO PATERNO 5

AM RECEIVER - SELECTIVITY • SELECTIVITY: RECEIVER PARAMETER TO MEASURE THE ABILITY OF THE RECEIVER TO ACCEPT A GIVEN BAND OF FREQUENCIES AND REJECT ALL OTHERS (FOR EXAMPLE IN AM BROADCASTING - EACH CHANNEL IS ALLOCATED A 10 k. Hz BANDWITH - BP FILTER IS OF 10 k. Hz BW TO PASS ONLY THE SPECIFIC CHANNEL) • SHAPE FACTOR (SF): USED TO MEASURE SELECTIVITY (IDEAL SF = 1) BW 60 d. B BELOW MAXIMUM SIGNAL LEVEL EXAMPLE: BW 3 d. B BELOW MAXIMUM SIGNAL LEVEL Fall 2001 ENZO PATERNO 6

AM RECEIVER - SENSITIVITY • SENSITIVITY: RECEIVER PARAMETER TO MEASURE THE MINIMUM RF SIGNAL LEVEL THAT CAN BE DETECTED AT THE INPUT TO THE RECEIVER AND STILL PRODUCE A USABLE DEMODULATED INFORMATION SIGNAL (ALSO CALLED RECEIVER THRESHOLD) • SENSITIVITY IS USUALLY STATED IN MICROVOLTS OF RECEIVED SIGNAL (i. e. COMMERCIAL BROADCAST AM RECEIVER IS 50 u. V, TWO-WAY MOBILE RADIO IS BETWEEN 0. 1 u. V and 10 u. V) • SENSITIVITY OF AN AM RECEIVER DEPENDS ON THE NOISE POWER PRESENT AT THE INPUT TO THE RECEIVER (LOWER NOISE, LOWER SENSITIVITY LEVELS) Fall 2001 ENZO PATERNO 7

AM RECEIVER - FIDELITY • FIDELITY: IS A MEASURE OF THE ABILITY OF A COMMUNICATIONS SYSTEM TO PRODUCE AT THE OUTPUT OF THE RECEIVER, AN EXACT REPLICA OF THE ORIGINAL SOURCE INFORMATION • ANY FREQUENCY, PHASE, OR AMPLITUDE VARIATIONS THAT ARE PRESENT IN THE DEMODULATED WAVEFORM THAT WERE NOT IN THE ORIGINAL INFORMATION SIGNAL ARE CONSIDERED DISTORTION. THE THREE DISTORTIONS ARE: AMPLITUDE, FREQUENCY AND PHASE Fall 2001 ENZO PATERNO 8

COHERENT RECEIVERS • COHERENT RECEIVERS: SYNCHRONOUS RECEIVER WHERE THE FREQUENCIES GENERATED IN THE RECEIVER AND USED FOR DEMODULATION ARE SYNCHRONIZED TO OSCILLATOR FREQUENCIES GENERATED IN THE TRANSMITTER (THE RECEIVER MUST HAVE SOME MEANS OF RECOVERING THE RECEIVED CARRIER AND SYNCHRONIZING TO IT) COHERENT DEMODULATION IS WHERE A LOCALLY GENERATED CARRIER IS SYNCHRONIZED TO THE TRANSMITTER’S CARRIER FREQUENCY. THIS TECHNIQUE IS RARELY USED IN PRACTICE. Fall 2001 ENZO PATERNO 9

NONCOHERENT RECEIVERS • NONCOHERENT RECEIVERS: ASYNCHRONOUS RECEIVERS WHERE EITHER NO FREQUENCIES ARE GENERATED IN THE RECEIVER OR FREQUENCIES USED IN THE DEMODULATION ARE COMPLETELY INDEPENDENT FROM THE TRANSMITTER’S CARRIER FREQUENCY THREE NONCOHERENT METHODS OF AM MODULATION ARE: • • • Fall 2001 RECTIFIER DEMODULATION ENVELOPE DETECTION SQUARE-LAW DETECTION ENZO PATERNO 10

NONCOHERENT RECEIVERS • NONCOHERENT DETECTION: ALSO CALLED ENVELOPE DETECTION BECAUSE THE INFORMATION IS RECOVERED FROM THE RECEIVED WAVEFORM BY DETECTING THE SHAPE OF THE MODULATED ENVELOPE Fall 2001 ENZO PATERNO 11

SUPERHETERODYNE RECEIVER NONCOHERENT RECEIVER Fall 2001 ENZO PATERNO 12

SUPERHETERODYNE RECEIVER • HETERODYNE: TO MIX TWO FREQUENCIES TOGETHER IN A NONLINEAR DEVICE OR TO TRANSLATE ONE FREQUENCY INTO ANOTHER USING NONLINEAR MIXING • RF SECTION: THE PRESELECTOR IS A BROAD-TUNED BANDPASS FILTER WITH AN ADJUSTABLE CENTER FREQUENCY THAT IS TUNED TO THE DESIRED CARRIER FREQUENCY. MAIN PURPOSE OF THE PRESELECTOR IS TO PREVENT UNWANTED RADIO FREQUENCY (IMAGE FREQUENCY) FROM ENTERING THE RECEIVER. RF FOR COMMERCIAL AM BROADCAST IS: 535 k. Hz TO 1605 k. Hz Fall 2001 ENZO PATERNO 13

SUPERHETERODYNE RECEIVER • MIXER/CONVERTER: THIS SECTION INCLUDES A RADIO-FREQUENCY OSCILLATOR STAGE (LOCAL OSCILLATOR) AND A MIXER/CONVERTER STAGE (THE FIRST DETECTOR) WHICH IS A NONLINEAR DEVICE USED TO CONVERT RF’S TO IF’S (RF-TO-IF FREQUENCY TRANSLATION). IF SIGNALS IN COMMECIAL AM BROADCAST ARE BETWEEN 450 k. Hz AND 460 k. Hz. THE MOST COMMON IF USED IS 455 k. Hz • IF SECTION: A SERIES OF IF AMPLIFIERS AND BP FILTERS (CALLED IF STRIP). MOST OF THE RECEIVER GAIN AND SELECTIVITY IS ACHIEVED IN THE IF SECTION Fall 2001 ENZO PATERNO 14

SUPERHETERODYNE RECEIVER • DETECTOR SECTION: CONVERTS THE IF SIGNALS BACK TO THE ORIGINAL SOURCE INFORMATION (ALSO CALLED THE AUDIO DETECTOR OR THE SECOND DETECTOR). IT CAN BE AS SIMPLE AS A DIODE OR AS COMPLEX AS A PLL OR BALANCED MODULATOR • AUDIO AMPLIFIER SECTION: COMPRISES SEVERAL CASCADED AUDIO AMPLIFIERS AND ONE OR MORE SPEAKERS. Fall 2001 ENZO PATERNO 15

LOCAL OSCILLATOR RF SIGNALS ARE COMBINED WITH THE LOCAL OSCILLATOR FREQUENCY IN A NONLINEAR DEVICE THE LOCAL OSCILLATOR IS DESIGNED SUCH THAT ITS FREQUENCY OF OSCILLATION IS ALWAYS ABOVE OR BELOW THE DESIRED RF CARRIER BY AN AMOUNT EQUAL TO THE IF CENTER FREQUENCY. THUS: PRESELECTOR AND LOCAL OSCILLATOR CENTER FREQUENCIES ARE GANG TUNED SO TO ALWAYS SATISFY THE CONDITION SHOWN ABOVE Fall 2001 ENZO PATERNO 16

LOCAL OSCILLATOR • WHEN THE LOCAL OSCILLATOR FREQUENCY IS TUNED ABOVE THE RF, IT IS CALLED HIGH-SIDE INJECTION (HIGH BEAT INJECTION) • WHEN THE LOCAL OSCILLATOR FREQUENCY IS TUNED BELOW THE RF, IT IS CALLED LOW-SIDE INJECTION (LOW BEAT INJECTION) • IN AM BROADCAST_BAND RECEIVERS, HIGH-SIDE INJECTION IS ALWAYS USED Fall 2001 ENZO PATERNO 17

LOCAL OSCILLATOR TRACKING IS THE ABILITY OF THE LOCAL OSC. IN A RECEIVER TO OSCILLATE EITHER ABOVE OR BELOW THE SELECTED RF CARRIER BY AN AMOUNT EQUAL TO THE INTERMEDIATE FREQUENCY THROUGHOUT THE ENTIRE RADIO FREQUENCY BAND FOR HIGH-SIDE INJECTION: LOCAL OSCILLATOR TRACKS ABOVE THE INCOMING RF CARRIER BY THE AMOUNT: FOR LOW-SIDE INJECTION: LOCAL OSCILLATOR TRACKS BELOW THE INCOMING RF CARRIER BY THE AMOUNT: Fall 2001 ENZO PATERNO 18

EXAMPLE AM SUPERHETERODYNE RECEIVER USING HIGH-SIDE INJECTION, WITH LOCAL OSCILLATOR OF 1355 k. Hz. INCOMING RF WAVE CONSISTS OF A CARRIER (900 k. Hz) AND USF (905 k. Hz) & LSF (895 k. Hz). Fall 2001 ENZO PATERNO 19

EXAMPLE Fall 2001 ENZO PATERNO 20

IMAGE FREQUENCY AN IMAGE FREQUENCY IS ANY FREQUENCY OTHER THAN THE SELECTED RADIO FREQUENCY CARRIER THAT IF ALLOWED TO ENTER A RECEIVER AND MIX WITH THE LOCAL OSCILLATOR, WILL PRODUCE A CROSS-PRODUCT FREQUENCY THAT IS EQUAL TO THE IF. AN IMAGE FREQUENCY IS EQUIVALENT TO A SECOND RADIO FREQUENCY THAT WILL PRODUCE AN IF THAT WILL INTERFERE WITH THE IF FROM THE DESIRED RADIO FREQUENCY. ONCE AN IMAGE FREQUENCY HAS BEEN MIXED DOWN TO IF, IT CANNOT BE FILTERED OUT OR SUPPRESSED. Fall 2001 ENZO PATERNO 21

IMAGE FREQUENCY FOR A RADIO FREQUENCY TO PRODUCE A CROSS PRODUCT EQUAL TO THE IF, IT MUST BE DISPLACED FROM THE LOCAL OSCILLATOR FREQUENCY BY A VALUE EQUAL TO THE IF: FOR HIGH-SIDE INJECTION: GIVING: Fall 2001 ENZO PATERNO 22

IMAGE FREQUENCY RELATIVE FREQUENCY SPECTRUM FOR THE RF, IF, LOCAL OSCILLATOR, AND IMAGE FREQUENCIES FOR A SUPERHETERODYNE RECEIVER USING HIGH-SIDE INJECTION FOR BETTER IMAGE-FREQUENCY REJECTION, A HIGH INTERMEDIATE FREQUENCY IS PREFERRED Fall 2001 ENZO PATERNO 23

RECTIFIER DETECTOR C RECTIFIER + + R LOWPASS FILTER R B THE NEGATIVE PART OF THE AM WAVE IS SUPPRESSED AT THE OUTPUT OF THE DIODE. THUS, THE OUTPUT ACROSS THE RESISTOR IS A RECTIFIED VERSION OF THE AM WAVE. IN ESSENCE, THE AM SIGNAL IS MULTIPLIED BY K(t) [TRAIN OF PULSE] Fall 2001 ENZO PATERNO 24

RECTIFIER DETECTOR Fall 2001 ENZO PATERNO 25

DSB-SC / DSB-FC DISADVANTAGES • DSB-FC (AM): CARRIER POWER IS 2/3 TOTAL POWER • DSB-FC (AM): CARRIER CONTAINS NO INFORMATION • DSB: USES TWICE AS MUCH FREQUENCY AS USED IN SSB • ONLY SIDEBANDS CONTAIN THE INFORMATION USB INFORMATION = LSB INFORMATION TRANSMITTING BOTH SIDEBANDS IS REDUNDANT • DSB-FC: IS BOTH POWER AND BW INEFFICIENT • DSB-SC: BW INEFFICIENT Fall 2001 ENZO PATERNO 26

SSB TECHNIQUES • AM SSBFC: CARRIER @ FULL POWER & 1 SIDEBAND • AM SSBSC: NO CARRIER & 1 SIDEBAND (NO ENVELOPE) • AM SSBRC: 10% CARRIER (PILOT) & 1 SIDEBAND • AM ISB: SINGLE CARRIER MODULATED BY TWO INDEPENDENT MODULATING SIGNALS. TRANSMITTER CONSISTS OF TWO SSB-SC MODULATORS (DSB SIGNAL WITH TWO INDEPENDENT SSBs). FINALLY CARRIER IS REINSERTED AS IN SSBRC. USED FOR STEREO AM: LEFT CHANNEL = LSB RIGHT CHANNEL = USB • AM VSB: CARRIER & FULL 1 st SB & PART OF 2 d SB Fall 2001 27 ENZO PATERNO

DSB/SSB POWER DISTRIBUTION DSBFC: SSBFC: DSBSC: SSBSC: SSBRC: Fall 2001 ENZO PATERNO 28

AMPLITUDE MODULATION SYSTEMS DSBFC AM SSBSC AM SSBRC AM ISB AM VSB AM Fall 2001 ENZO PATERNO 29

DSBFC WAVE Fall 2001 ENZO PATERNO 30

SSBFC WAVE PEAK CHANGE IN THE ENVELOPE IS HALF THAT OF THE DSB WAVE (ONLY ONE SIDEBAND) 100% MODULATED SSBFC WAVE WITH A SINGLE FREQUENCY MODULATING WAVE Fall 2001 ENZO PATERNO 31

SSBSC WAVE THE WAVEFORM IS NOT AN ENVELOPE; IT IS A SINE WAVE AT A SINGLE FREQUENCY EQUAL TO THE CARRIER FREQUENCY PLUS/MINUS THE MODULATING SIGNAL FREQUENCY Fall 2001 ENZO PATERNO 32

WAVE IS SIMILAR TO A DSBSC WAVE BUT WITH A REPETITION RATE TWICE THAT OF THE MODULATING SIGNAL FREQUENCY ISB WAVE . Fall 2001 ENZO PATERNO 33

VARIOUS AM WAVES DSBFC DSBSC SSBSC Fall 2001 ENZO PATERNO 34