Meghe Group of Institutions Department for Technology Enhanced Learning 1

Department of Electronics & Telecommunication VI SEM Radio Receiver UNIT- V Introduction to Radio Receiver DTEL 2UNIT- V

DTEL Syllabus UNIT-V: Radio Receivers and its measurements: TRF receiver, superhetrodyne receiver. Detailed study of block schematic and circuits of mixer, RF-stage, I.F stage detector, Automatic gain control (AGC), FM radio receivers. 3 DTEL UNIT- V

Syllabus cont.. Receiver Measurements: Sensitivity, selectivity, image frequency rejection etc. communication Receiver, block schematic and its special features 4 DTEL UNIT- V

Generalized Receiver Extracting the source information from received modulated signal – Corrupted by noise – Often it is a replica of the message signal Two main classes of receiver – TRF(Tuned Radio Frequency) Not popular because hard to design and implement – Superhetrodyne Most receiver uses this technique 5 DTEL UNIT- V

TRF (Tuned Radio frequency) receiver Tuned radio frequency amplifier detector A.F. amplifier Modulatin g signal 6 DTEL UNIT- V

Drawbacks of TRF Receiver TRF receivers are simple to design and allow the broadcast frequency 535 KHz to 1640 KHz. But at the higher frequency, it produces difficulty in design. It has poor audio quality. 7 DTEL UNIT- V

Super Heterodyne Receiver RF amplifier Local oscillator mixer IF amplifier detector AF amplifier Modulatin g signal Ganged tuning fs fo IF=fo- fs The shortcomings of the TRF receiver are overcome by the invention of the super heterodyne receiver. A super heterodyne receiver converts all incoming radio frequency (RF) signals to a lower frequency known as an intermediate frequency (IF). 8 DTEL UNIT- V

This IF is constant, for AM receivers it is 456 to 465 KHz and for FM receivers it is 9 MHz to 12 MHz. The output of mixer will produce sum and difference frequencies: fo + fs and fo – fs For all incoming fs IF is constant and constant frequency difference is maintained between local oscillator and RF circuit through capacitor tuning in which all capacitors are connected together and operated by one control nob. It is called ganged tuning. 9 DTEL UNIT- V

Automatic Frequency Control (AFC) and Automatic Gain Control (AGC) in Superheterodyne Receiver AGC or AVC (Automatic Volume Control) is a system by means of which the overall gain of radio receiver is varied automatically with the variations in the strength of received signals, to maintain the output constant. AFC circuit is used to adjust and stabilize the frequency of local oscillator. 10 DTEL UNIT- V

Image Frequency Definition: In radio reception using heterodyning in the tuning process, an undesired input frequency that is capable of producing the same intermediate frequency (IF) that the desired input frequency produces. It is given by signal frequency plus twice the intermediate frequency fsi = fs + 2fi 11 DTEL UNIT- V

Image Frequency Rejection The rejection of an image frequency by a single tuned circuit is the ratio of the gain at the signal frequency to the gain at the image frequency. It is given by, = 1+Q 2 ρ 2 Where, ρ=(fsi / fs) - (fs / fsi) 12 DTEL UNIT- V

Selectivity:- Radio receiver should have good selectivity and selectivity of radio receiver is its ability to differentiate desired signal from unwanted signals. 13 DTEL UNIT- V

Selectivity is obtained by using tuned circuits, which are tuned to desired frequency. The quality factor of these LC circuits determines the selectivity. It is given by, Q=XL/R For better selectivity ‘Q’ should be high. 14 DTEL UNIT- V

Sensitivity:- Ability to amplify weak signals. Broadcast receivers/ radio receivers should have reasonably high sensitivity so that it may have good response to the desired signal but should not have excessively high sensitivity otherwise it will pick up all undesired noise signals. It is function of receiver gain and measures in decibels. 15 DTEL UNIT- V

Fidelity:- 1. Radio receiver should have high fidelity or accuracy. 2. It is determined by the high frequency response. Therefore it should have high frequency response over entire audio frequency range. 3. Ex. In an A.M. broadcast the maximum audio frequency is 5 KHz hence receiver with good fidelity must produce entire frequency up to 5KHz. 16 DTEL UNIT- V

AM/FM Radio System Principles: – Frequency Spectrum Sharing (many transmitters using one medium) – Demodulating desired signal and rejecting other signals transmitted at the same time 17 DTEL UNIT- V 17 DTEL UNIT- V

AM/FM Radio System The source signal is audio Different sources have different spectrum – Voice (speech) – Music – Hybrid signals (music, voice, singing) 18 DTEL UNIT- V

AM/FM Radio System Different audio sources have different bandwidth “W” – Speech- 4kHz – High quality music- 15kHz – AM radio limits “baseband” bandwidth W to 5kHz – FM radio uses “baseband” bandwidth W to 15kHz 19 DTEL UNIT- V

AM/FM Radio System Radio system should be able to receive any type of audio source simultaneously. Different stations with different sources transmit signals simultaneously. Different listeners tune to different stations simultaneously. 20 DTEL UNIT- V

AM/FM Radio System The different radio stations share the frequency spectrum over the air through AM and FM modulation. Each radio station, within a certain geographical region, is designated a carrier frequency around which it has to transmit Sharing the AM/FM radio spectrum is achieved through Frequency Division Multiplexing (FDM) 21 DTEL UNIT- V

Example of AM Radio Spectrum Different radio stations, different source signals Carrier spacing- 10kHz (AM) Bandwidth (3-5kHz) 22 DTEL UNIT- V

AM/FM Radio System For AM radio, each station occupies a maximum bandwidth of 10 kHz Carrier spacing is 10 kHz For FM radio, each station occupies a bandwidth of 200 kHz, and therefore the carrier spacing is 200 kHz 23 DTEL UNIT- V

AM/FM Radio System Transmission Bandwidth: is the bandwidth occupied by a message signal in the radio frequency spectrum is also the carrier spacing AM: FM: (Carson’s Rule) 24 DTEL UNIT- V

AM/FM Radio Receiver Design of AM/FM radio receiver The radio receiver has to be cost effective Requirements: – Has to work with both AM and FM signals – Tune to and amplify desired radio station – Filter out all other stations – Demodulator has to work with all radio stations regardless of carrier frequency 25 DTEL UNIT- V

AM/FM Radio Receiver For the demodulator to work with any radio signal, we “convert” the carrier frequency of any radio signal to Intermediate Frequency (IF) Radio receiver design can be optimized for that frequency IF filter and a demodulator for IF frequency 26 DTEL UNIT- V

AM/FM Radio Spectrum Recall that AM and FM have different radio frequency (RF) spectrum ranges: – AM: 540 kHz – 1600 kHz – FM: 88 MHz – 108 MHz Therefore, two IF frequencies – AM: 455 kHz – FM: 10.7 MHz 27 DTEL UNIT- V

AM/FM Radio Receiver A radio receiver consists of the following: – A Radio Frequency (RF) section – An RF-to-IF converter (mixer) – An Intermediate Frequency (IF) section – Demodulator – Audio amplifier 28 DTEL UNIT- V

AM/FM Radio Receiver This is known as the “Superheterodyne” receiver Two stages: RF and IF (filtering and amplification) The receiver was designed by Armstrong 29 DTEL UNIT- V

AM/FM Radio Receiver RF Section – Tunes to the desired RF frequency, – Includes RF bandpass filter centered around – The bandwidth – Usually not narrowband, passes the desired radio station and adjacent stations 30 DTEL UNIT- V

AM/FM Radio Receiver The minimum bandwidth of RF filter: Passes the desired radio channel, and adjacent channels 31 DTEL UNIT- V

AM/FM Radio Receiver RF-IF converter: – Converts carrier frequency  IF frequency How can we convert signals with different RF frequencies to the same IF frequency? 32 DTEL UNIT- V

AM/FM Radio Receiver Local oscillator with a center frequency is a function of RF carrier frequency 33 DTEL UNIT- V

AM/FM Radio Receiver RF-to-IF receiver includes: – An oscillator with a variable frequency (varies with RF carrier frequency) – By tuning to the channel, you are tuning the local oscillator and RF tunable filter at the same time. 34 DTEL UNIT- V

AM/FM Radio Receiver All stations are translated to a fixed carrier frequency for adequate selectivity. 35 DTEL UNIT- V

AM/FM Radio Receiver Two frequencies are generated at the output of product modulator: The higher frequency component is eliminated through filtering We are left with IF frequency 36 DTEL UNIT- V

AM/FM Radio Receiver One problem with this receiver: “Image Signal” Image signal has a center frequency: 37 DTEL UNIT- V

AM/FM Radio Receiver If an “image signal” exists at the input of the “RF-to-IF” converter, then the output of the converter will include the desired signal + image signal 38 DTEL UNIT- V

AM/FM Radio Receiver Example: Incoming carrier frequency 1000 kHz, Local oscillator = =1455 kHz Consider another carrier at 1910 kHz If this is passed through the same oscillator, will have a =455 kHz component Therefore, both carriers will be passed through RF-to-IF converter 39 DTEL UNIT- V

AM/FM Radio Receiver Therefore, RF filter should be designed to eliminate image signals The frequency difference between a carrier and its image signal is: RF filter doesn’t have to be selective for adjacent stations, have to be selective for image signals Therefore, 40 DTEL UNIT- V

AM/FM Radio Receiver IF filter: – Center frequency – Bandwidth approximately same as transmission bandwidth, – For AM: – For FM: 41 DTEL UNIT- V

AM/FM Radio Receiver Depending on the type of the received signal, the output of “IF filter” is demodulated using AM or FM demodulators. For AM: envelope detector For FM: frequency discriminator 42 DTEL UNIT- V

43 DTEL UNIT- V Summary TRF receiver Super heterodyne receiver AFC and AGC Image Frequency Image Frequency rejection Sensitivity and Selectivity FM receiver

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