74 4.8 FM Reeivers FM reeivers, like their AM ounterparts, are superheterodyne reeivers. Figure 4.11 shows a simplified blok diagram for a double onversion superheterodyne FM reeiver Figure 4.11: Double onversion FM reeiver The preseletor, RF amplifier, first and seond mixers, and IF amplifier setions of a FM reeiver perform idential funtions as they did in AM reeivers Automati gain ontrol (AGC) is used to prevent mixer saturation when strong RF signals are reeived The peak detetor ommon to AM reeivers is replaed in FM reeivers by a limiter, frequeny disriminator and deemphasis network Limiter is used to remove amplitude variations aused by noise (whih is one of the drawbaks of AM reeivers) Frequeny disriminator extrats the information from the modulated wave Deemphasis network ontributes to an improvement in signal-to-noise ratio The first IF is a relatively high frequeny (often 10.7 MHz) for good image frequeny rejetion The seond IF is a relatively low frequeny (often 455 khz) that allows the IF amplifiers to have high gain 4.9 FM Demodulators FM demodulators are frequeny-dependent iruits designed to produe an output voltage that is proportional to the instantaneous frequeny at its input. The overall transfer funtion for an FM demodulator is nonlinear but when operated over its linear range is
75 V K d = (4.23) f The output from an FM demodulator is v out ( t) = K f (4.24) d Where v out (t) = demodulated output signal (volts) K d f = demodulator transfer funtion (volts per hertz) = differene between input frequeny and the entre frequeny of the demodulator (hertz) Several iruits are used for demodulating FM signals. The most ommon are the slope detetor, balaned slope detetor and PLL demodulator. The slope detetor and the balaned slope detetor are forms of tuned-iruit frequeny disriminators 4.9.1 Tuned Ciruit Frequeny Disriminators Tuned-iruit frequeny disriminators onvert FM to AM and then demodulate the AM envelope with onventional peak detetors. Slope detetor Figure 4.12 (a) shows the shemati diagram for single ended slope detetor Figure 4.12: Slope detetor (a) shemati diagram; (b) voltage-versus-frequeny urve The tuned iruit ( La & Ca ) produes an output voltage that is proportional to the input frequeny The maximum output voltage ours at the resonant frequeny f 0 and its output dereases proportionately as the input frequeny deviates above or below f 0 The iruit is designed so that the IF entre frequeny f falls in the entre of the most linear portion of the voltage-versus-frequeny (shown in Figure 4.12 (b)) o When IF deviates below f, the output voltage dereases o When IF deviates above f, the output voltage inreases
76 The tuned iruit, therefore, onverts frequeny variations to amplitude variations D, C & R make up a simple peak detetor to demodulate the AM signals i i i Balaned slope detetor Figure 4.13 (a) shows the shemati diagram for a balaned slope detetor Figure 4.13: Balaned slope detetor (a) shemati diagram; (b) voltage-versus-frequeny Balaned slope detetor is simply two single-ended slope detetors onneted in 0 parallel and fed 180 out of phase Phase inversion aomplished by entre tapping seondary windings of T 1 Tuned iruits L, C & L, C ) perform FM-to-AM onversion ( a a b b ( D1, C1, R1 & D2, C2, R2 Balaned peak detetors ) remove the information from the AM envelope L a & Ca is tuned to frequeny f a that is above the IF entre frequeny f L b & Cb is tuned to frequeny fb that is below the IF entre frequeny f Operation: The IF entre frequeny f falls exatly halfway between the resonant frequenies of the two tuned iruits At f, the output voltages from the tuned iruits are equal in amplitude but opposite in polarity. I.e. the retified voltage aross R & R 1 2, when added, produe an output voltage V out = 0. When IF deviates above the resonane, the top tuned iruit produes higher output voltage than the lower tuned iruit, and Vout goes positive When IF deviates below the resonane, the output voltage from lower tuned iruit is larger than the voltage from top tuned iruit, and V out goes negative
77 Slope Detetor Simple iruit Poor linearity, diffiult to tune Need to use separate limiter stage to ompensate amplitude variation Balaned Slope Detetor More omplex iruit Better linearity and tuning Does not need limiter stage 4.9.2 PLL FM Demodulator FM demodulation an also be ahieved with PLL. A PLL frequeny demodulator requires no tuned iruits and automatially ompensates for hanges in the arrier frequeny due to instability in the transmit osillator. Figure 4.14 shows simplified blok diagram for a PLL FM demodulator Figure 4.14: Blok diagram for PLL FM demodulator After the frequeny lok had ourred, the VCO traks frequeny hanges in the input signal by maintaining the phase error at the input of the phase omparator If the input is a deviated FM signal and the VCO natural frequeny is equal to the IF entre frequeny, the orretion voltage produed at the phase omparator is proportional to the frequeny deviation V d f I.e. orretion voltage produed is proportional to the modulating/ information signal If the IF amplitude is suffiiently limited before reahing the PLL and the loop is properly ompensated, the PLL loop gain K v is onstant Therefore the demodulated signal an be taken diretly from the output and is mathematially expressed as V = fk K (4.25) out d a f = frequeny deviation, K d = phase omparator gain, K a = amplifier gain
78 4.10 FM Stereo Until 1961, all ommerial FM transmissions were monophoni. That is, a single 50 Hz to 15-kHz audio hannel made up the entire voie and musi information spetrum. This single audio hannel modulated a arrier and was transmitted through a 200-kHz bandwidth FM hannel. With mono transmission, eah speaker assembly at the reeiver reprodues exatly the same information. I.e. the entire information signal sounds as though it is oming from the same diretion (i.e. no diretivity to the sound) In 1961, Federal Communiations Commission (FCC) authorized stereophoni transmission. With stereo transmission, the signal is spatially divided into two 50-Hz to 15-kHz audio hannels (a left and a right).musi originated on the left side is reprodued on the left speaker and musi originated on the right side is reprodued only on the right speaker. Therefore, it is possible to reprodue musi with a unique diretivity and spatial dimension. Also, it is possible to separate sound by tonal quality, suh as perussion, strings, horns, and so on. FM stereo frequeny spetrum is shown below: Figure 4.15: FM stereo spetrum It ompromises the 50-Hz to 15-kHz stereo hannel plus an additional stereo hannel frequeny division multiplexed (FDM) into a omposite baseband signal with a 19-kHz pilot and Subsidiary Communiations Authorization (SCA) hannel. I.e. there are 3 hannels in FM stereo: 1. The left (L) plus the right (R) audio hannels (L+R) 2. The left plus the inverted right audio hannels (L-R) 3. SCA hannel The L+R stereo hannel oupies the 50-Hz to 15-kHz passband (mono ombination) The L-R audio hannel amplitude modulates a 38-kHz subarrier and produes the L-R stereo hannel oupying the 23-kHz to 53-kHz passband SCA hannel, used to broadast musi to private subsribers suh as department stores, restaurants and offies, oupies the 60-kHz to 74-kHz spetrum Idential information in ontained L+R and L-R exept for their phase
79 4.10.1 FM Stereo Transmission Figure 4.16 shows a blok diagram for stereo FM transmitter Figure 4.16: Stereo FM transmitter The L and R audio hannels are ombined in a matrix network to produe the L+R and L-R audio hannels The L-R audio hannels then modulates a 38-kHz subarrier and produes a 23- khz to 53-kHz L-R stereo hannel The L+R stereo hannels must be delayed to maintain phase integrity with the L- R stereo hannel for demodulation purpose. A 19-kHz pilot is transmitted rather than 38-kHz subarrier beause it is easier to reover the pilot in the reeiver This omposite / whole baseband signal is fed to the FM transmitter where it modulates the main arrier 4.10.2 FM Stereo Reeption FM stereo reeivers are idential to standard FM reeivers up to the output of the audio detetor stage. The output of the disriminator is the total baseband spetrum that was shown in Figure 4.15. The following figure shows a blok diagram for an FM reeiver that has mono and stereo outputs For mono setion of the signal proessor, the L+R stereo hannel, whih ontains all the original information from both the L and R audio hannels, is simply filtered, amplified and then fed to both the L and R speakers
80 Figure 4.17: FM stereo and mono reeiver For stereo setion, the L+R and L-R stereo hannels and the 19-kHz pilot are separated from the omposite baseband with filters The L+R stereo hannel is filtered by a lowpass filter with an upper utoff frequeny of 15 khz The 19-kHz pilot is filtered with a high-q bandpass filter, multiplied by 2, amplified and then fed to the L-R demodulator The L-R double sideband signal is separated with a broadly tuned bandpass filter and then mixed with the reovered 38-kHz arrier in a balaned modulator to produe L-R audio hannel The matrix network ombines the L+R and L-R signals in suh a way as to separate the L and R audio signals, whih are fed to their respetive speaker Figure 4.18: Stereo matrix network deoder
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