Computer Science Engineering (CSE) Exam > Computer Science Engineering (CSE) Notes > Chapter 5 - FM Receivers , PPT, Semester, Engineering
Chapter 5 - FM Receivers , PPT, Semester, Engineering - Computer Science Engineering (CSE) PDF Download
INTRODUCTION
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Double-Conversion Superheterodyne FM Receivers
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FM DEMODULATION
Demodulation should provide an output signal whose amplitude is dependent on the instantaneous carrier frequency deviation and whose frequency is dependent on the rate of the carrier frequency change
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Slope Detector
1. simplest form of tuned-circuit frequency discriminator
2. Single ended slope has the most nonlinear voltage-versus-frequency characteristics à seldom used
3. Convert FM to AM - - > demodulate AM envelope with conventional peak detectors
4. Circuit consist of a tuned circuit and an AM detector
Disadvantages
1.Gain is reduced
2.Difficult to achieve a linear slope response curve
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Circuit Operation (Figure 5.3)
1. La & Ca (tuned circuit) produce o/put voltage (amplitude varies) which is proportional to the i/put freq. (FM in)
- - -> AM characteristic
2. Maximum o/put voltage occurs at resonant freq of tank circuit, fo and its o/put decrease proportionately as the i/put freq deviates below & above fo
3. IF center frequency (fc) falls in the center of the most linear portion of the voltage-versus-frequency curve (Figure 5.3(b))
4. When IF deviates above fc, output voltage increase and when IF deviated below fc, output voltage decrease.
5. The tuned circuit converts frequency variations to amplitude variations (FM-to-AM conversion).
6. Di, Ci and Ri - - -> simple peak detector that converts amplitude variations to o/put voltage (operate as AM Diode Detector)
• o/put voltage varies at a rate equal to i/put frequency
• Amplitude of o/put voltage depend on magnitude of freq changes
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Balanced slope detector
Figure 5.4: Balanced slope detector (a) schematic diagram (b) voltage-versus-frequency response curve.
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• made up of single-ended slope detector connected in parallel and fed 180o out of phase (phase inversion).
• the 2 tuned circuit perform the FM to AM conversions
• At resonant freq, the resultant output voltage is 0 V.
• As the IF deviates above the center freq, top tuned circuit produces higher voltage than the lower tank circuit (+Vout) and vice versa.
• Figure 5.4(b) shows the output versus frequency response curve.
• diode detector circuit (D1,R1,C1 & D2,R2,C2) recover the original signal.
• advantage : simple.
• disadvantages : poor linearity, difficult in tuning and lack of provisions for limiting.
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Foster Seeley Discriminator
• Also called phase shift discriminator (tuned-circuit frequency discriminator) à operation very similar to the balanced slope detector
• Is tuned by injecting a frequency equal to the IF center frequency and tuning C0 for 0 volts out.
• Output voltage is directly proportional to the magnitude and direction of the frequency deviation.
• Output voltage-versus-frequency deviation curve is more linear than that of a slope detector because there is only one tank circuit,
- - -> easier to tune.
• For distortion less demodulation, the frequency deviation should be restricted to the linear portion of the secondary tuned-circuit frequency response curve.
• Responds to amplitude as well as frequency variations and, therefore, must be preceded by a separate limiter circuit
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Figure 5.5: Foster Seeley discriminator (a) schematic diagram (b)vector diagram, fin = fo; (b) fin > fo; (c) fin < fo
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Principle Circuit Operation
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Figure 5.6 : Discriminator voltage-versus-frequency response curve.
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Ratio detector
• Advantage : relatively immune to amplitude variations in its input signal.
• Figure 5.7 shows the schematic diagram for a ratio detector.
• Same as the Foster-Seeley discriminator but with 3 limiting changes.
1. D2 has been reversed à current (Id) flow through the outermost loop of the circuit
2. Shunt capacitor, Cs charges to approximately the peak voltage across the secondary winding of T1. The reactance of Cs is low, and Rs simply provides a dc path for diode current.
3. Therefore, the time constant for Rs and Cs is sufficiently long so that rapid changes in the amplitude of the input signal due to thermal noise or other interfering signals are shorted to ground and have no effect on the average voltage across Cs.
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Figure 5.7 : Ratio detector (a) schematic diagram; (b) voltage-versus-frequency response curve
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• Consequently, C1 and C2 charge and discharge proportional to frequency changes in the input signal and are relatively immune to amplitude variations.
• Also, the output voltage from a ratio detector is taken with respect to ground, and for the diode polarities shown in Figure 5.7(a), the average output voltage is positive.
• At resonance, the output voltage is divided equally between C1 and C2 and redistributed as the input frequency is deviated above and below resonance.
• Therefore, changes in Vout are due to the changing ratio of the voltage across C1 and C2, while the total voltage is clamped by Cs.
• Figure 5.7(b) shows the output frequency response curve for the ratio detector shown in Figure 5.7(a). It can be seen that at resonance, Vout is not equal to 0 V but, rather, to one-half of the voltage across the secondary windings of T1 . Because a ratio detector is relatively immune to amplitude variations, it is often selected over a discriminator.
• However, a discriminator produces a more linear output voltage-versus-frequency response curve.
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PHASE LOCKED LOOP (PLL)
• FM demodulation can be accomplished quite simply with a phase-locked loop (PLL).
• PLL FM demodulator is probably the simplest and easiest to understand.
• A PLL frequency demodulator requires no tuned circuits and automatically compensates for changes in the carrier frequency due to instability in the transmit oscillator.
• Figure 5.8(a) shows the simplified block diagram for a PLL FM demodulator.
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Figure 5.8 (a) Block diagram for a PLL FM demodulator.
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• If the IF amplitude is sufficiently limited prior to reaching the PLL and the loop is properly compensated, the PLL loop gain is constant and equal to KV.
• after frequency lock had occurred the VCO would track frequency changes in the input signal by maintaining a phase error at the input of the phase comparator.
• Therefore, if the PLL input is a deviated FM signal and the VCO natural frequency is equal to the IF center frequency, the correction voltage produced at the output of the phase comparator and fed back to the input of the VCO is proportional to the frequency deviation and is, thus, the demodulated information signal.
• Therefore, the demodulated signal can be taken directly from the output of the internal buffer and is mathematically given as
Vout = Δf Kd Ka
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• Figure 5.8(b) shows a schematic diagram for an FM demodulator using the XR-2212. R0 and C0 are course adjustments for setting the VCO's free-running frequency. Rx is for fine tuning, and RF and Rc set the internal op-amp voltage gain (Ka). The PLL closed-loop frequency response should be compensated to allow un-attenuated demodulation of the entire information signal bandwidth.
• The PLL op-amp provides voltage gain and current drive stability
• PLL is the best frequency demodulator, because the filtering circuit removes noise and interference and its linear output reproduce the output signal
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Figure 5.8 (b) PLL FM demodulator using the XR-2212 PLL
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LIMITER
• If noise or other interference perturbs the amplitude of the carrier, then the carrier amplitude variations will cause distortion at the output.
• This distortion can be removed by passing through a limiter prior to the differentiator (demodulator)
• The limiter output must then be converted to a sinusoid by passing it through a band-pass filter with center frequency and sufficient bandwidth to pass the varying fundamental.
• The result is a constant amplitude sinusoid, which is differentiated and passed through the envelope detector to produce the desired signal.
• Improve S/N as much as 20 dB.
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Figure 5.9 Amplitude limiter input and output waveforms:
(a) input waveform; (b) output waveform
(a) input waveform; (b) output waveform
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FIGURE 5.10 Limiter output:
(a) captured by noise; (b) captured by signal.
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FM STEREO TRANSMITTER
• Broadcast of high fidelity stereo began in 1961 of FM radio
• The transmission of 2 channels of sound information (stereophonic) on a single carrier require compatibility with existing high-fidelity monophonic FM receiver
• FDM is used to combine the two audio
Figure 5.11
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FIGURE 5.12: Stereo FM transmitter using frequency-division multiplexing
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From Figure 5.12
• Both L & R channel will be added to the 2 adder circuits
1. 1st adder : (L + R) channel (50Hz ~ 15KHz) - -> total mono audio signal for compatibility
2. 2nd adder : (L – R) channel (50Hz ~ 15KHz)
♦ R channel is inverted and then added to adder circuit
• Delay network
1. (L – R) channel have delay which is introduced by signal path as it propagates via the balanced modulator
2. (L + R) channel must maintain its phase integrity with (L – R) channel to prevent phase error at further process.
• Balanced modulator
1. AM DSB-SC which modulate (L – R) channel with frequency subcarrier 38KHz
2. Produce ( L – R) sidebands with bandwidth 23KHz ~ 53KHz
• Multiplier x2
1. Multiply 19KHz oscillator by 2 to produce 38KHz subcarrier
• Linear combining network
1. Combine (L + R) channel, (L – R) channel, 19KHz stereo pilot and SCA (Subsidiary Communication Authorization) 60KHz ~ 74KHz
2. Produce composite baseband to be modulated with main station carrier
• SCA is used to broadcast uninterrupted music to private subscriber (not compulsory to be combined by fundamental of FM Broadcast)
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• The 19kHz pilot frequency fits between (L + R) and DSBSC (L – R) signals in the baseband frequency spectrum
• All three signals are added together to form the composite stereo broadband signal, which will modulate the main station carrier
• The pilot frequency is made small enough so that its FM deviation of the carrier is only ≈ 10% of the total ±75kHz maximum deviation allowed
• After the FM stereo is received and demodulated to baseband, the 19kHz pilot is used to phase lock an oscillator, which provides the 38 kHz subcarrier for the demodulation of the (L – R) signal.
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FM Baseband Spectrum
FIGURE 5.13 : Composite baseband spectrum
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FM STEREO RECEIVER
• All new FM broadcast receivers are being built with provision for receiving stereo, or two-channel broadcasts.
• The left (L) and right (R) channel signals from the program material are combined to form two different signals, one of which is the left-plus-right signal and one of which is the left-minus-right signal
• The (L - R) signal is double-sideband suppressed carrier (DSBSC) modulated about a carrier frequency of 38 kHz, with the LSB in the 23- to 38-kHz slot and the USB in the 38- to 53-kHz slot. The (L + R) signal is placed directly in the 0- to 15-kHz slot, and a pilot carrier at 19 kHz is added to synchronize the demodulator at the receiver.
• The output from the FM detector is a composite audio signal containing the frequency-multiplexed (L + R) and (L - R) signals and the 19-kHz pilot tone. This composite signal is applied directly to the input of the decode matrix.
FIGURE 5.14
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• The composite audio signal is also applied to one input of a phase-error detector circuit, which is part of a phase locked loop 38-kHz oscillator.
• The output drives the 38-kHz voltage-controlled oscillator, whose output provides the synchronous carrier for the demodulator.
• The oscillator output is also frequency divided by 2 (in a counter circuit) and applied to the other input of the phase comparator to close the phase locked loop.
• The phase-error signal is also passed to a Schmitt trigger circuit, which drives an indicator lamp on the panel that lights when the error signal goes to zero, indicating the presence of a synchronizing input signal (the 19-kHz pilot tone).
• The outputs from the 38-kHz oscillator and the filtered composite audio signals are applied to the balanced demodulator, whose output is the (L - R) channel.
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• The ( L + R) and (L - R) signals are passed through a matrix circuit that separates the L and R signals from each other. These are passed through de-emphasis networks and low-pass filters to remove unwanted high-frequency components and are then passed to the two channel audio amplifiers and speakers.
• On reception of a monaural signal, the pilot-tone indicator circuit goes off, indicating the absence of pilot tone, and closes the switch to disable the (L - R) input to the matrix.
• The (L + R) signal is passed through the matrix to both outputs. An ordinary monaural receiver tuned to a stereo signal would produce only the (L + R) signal, since all frequencies above 15 kHz are removed by filtering, and no demodulator circuitry is present.
• Thus the stereo signal is compatible with the monaural receivers.
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FIGURE 5.15 FM stereo and mono receiver
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FAQs on Chapter 5 - FM Receivers , PPT, Semester, Engineering - Computer Science Engineering (CSE)
| 1. What is an FM receiver? |  |
An FM receiver is a device that is used to receive and demodulate Frequency Modulated (FM) signals. It is commonly used in radio broadcasting systems to receive FM radio signals and convert them into audio signals that can be heard through speakers or headphones.
| 2. How does an FM receiver work? | |
An FM receiver works by first receiving the FM signal, which consists of a carrier wave that is modulated by the audio signal. The receiver then demodulates the FM signal to extract the audio signal. This is done by using a circuit called a frequency discriminator, which measures the difference in frequency between the carrier wave and the modulated signal. The output of the frequency discriminator is then amplified and processed to produce the audio signal.
| 3. What are the advantages of FM receivers? | |
FM receivers have several advantages over other types of radio receivers. Firstly, FM signals are less susceptible to interference and noise compared to AM signals, resulting in clearer audio reception. Additionally, FM receivers can provide stereo sound, which is not possible with AM receivers. FM receivers also have a wider bandwidth, allowing for better audio quality and the ability to transmit additional information such as RDS (Radio Data System) data.
| 4. What are the main components of an FM receiver? | |
The main components of an FM receiver include an antenna, a tuner, a frequency discriminator, and an audio amplifier. The antenna picks up the FM signal from the air and sends it to the tuner, which selects the desired frequency. The tuned signal is then passed through the frequency discriminator, which demodulates the FM signal. Finally, the demodulated audio signal is amplified by the audio amplifier and sent to the output device, such as speakers or headphones.
| 5. How can FM receivers be used in practical applications? | |
FM receivers have a wide range of practical applications. They are commonly used in radio broadcasting systems to receive FM radio signals. FM receivers can also be found in car radios, portable radios, and stereo systems. Additionally, FM receivers are used in wireless communication systems, such as wireless microphones and wireless headphones, where they receive FM signals from a transmitter.
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