The block diagram of super-heterodyne receiver is shown in figure below. The signal after receiving from antenna is amplified and then fed to mixer or converter. The RF amplifier passes only the required frequencies in the band to avoid image problems at the later stages. In mixer, it is mixed or heterodyned with signal from a local oscillator. The local oscillator frequency is chosen so that the output of the mixer is an intermediate frequency (IF) ( 455 KHz in normal commercial receivers). The output of mixer is then amplified using an IF amplifier. The IF amplifier also filters out all the signals falling out of pass band of IF amplifier. The IF signals are then fed to detector or demodulator to get audio signal which is the amplified using audio amplifier and then fed to speaker. The AGC is used to maintain constant signal output by increasing IF amplifier's gain if the output of amplifier is low. (Radio-Electronics, 2013)
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The advantage of using intermediate frequency is that system requires fixed frequency filter rather than variable frequency filters and implementation of filters at intermediate frequency is much simpler and economical than in case of high frequencies. The sharp filters at lower frequency can be used to provide better receiver selectivity. It is also easy to amplify the intermediate frequency than the high frequency. The receiver gain and selectivity is determined by IF stages leading to consistence gain and selectivity of super heterodyne receiver over the entire tuning range. (Butler, 1988)
The superheterodyne receiver after mixing can also pick up the image frequency that gets generated when the signal with frequency that is more than twice the value of intermediate frequency is available. For example, assuming LO frequency as 1550 KHz, the receiver will generate IF frequency of 455 KHz when input frequency is 2005 KHz (2005 - 1550 = 455 KHz) or 1100 KHz (1555 - 1100 = 455 KHz )
The 2005 KHz is the image frequency. The image frequency reception is undesirable feature and needs to be taken care of. (Butler, 1988)
In order to take care of image problem two stage conversion is used using two intermediate frequencies. The first frequency is higher than the second frequency. In a typical FM receiver the first IF used is 10.7 MHz and lower IF is 455 KHz. The first high frequency allows higher levels of image rejection as the image frequency lies twice that of IF away from the main frequency. The father the image frequency the easier rejection is possible. The use of lower IF frequency at second stage allows marrow band filtering to be used for removing adjacent channel signals. Thus the double superheterodyne receiver provided better selectivity and image rejection. (Radio-Electronics, 2013)
Write a brief summary of how noise can affect an AM/FM Superhet receiver, what are the differences in particular to how the demodulator stage works, and comment on the choice of component values used.
The noise received along with signal can affect the performance of the receiver if the level of noise as compared to the signal is substantial or signal to noise ratio is below the value required by the demodulator or discriminator circuit. If the level of noise goes below the threshold value of the demodulator the output of the receiver is cut off or will get only noise. The noise affects more in AM receivers than in FM receivers due to inherent difference in nature of modulation.
The AM demodulator is basically an envelope detector that detects the variations in carrier's amplitude. In simplest form it consists of a rectifier followed by a low pass filter. The principle is depicted in Fig below:
Basics of AM demodulation / detection
Figure AM demodulation (Radio-Electronics, 2013)
Another method of AM detection is the use of synchronous demodulation. In this the incoming signal is mixed with local oscillator frequency close to the carrier frequency and the resultant side band signal appears as demodulated signal.
The FM demodulator use phase locked loop technique for demodulation. In this the input signal is fed to a phase comparator which is fed on other side using a voltage controlled oscillator (VCO) of similar frequency as that of carrier. The output of the phase compactor is filtered and fed back to VCO for controlling the VCO frequency. The resulting output of phase comparator after filtering is the demodulated signal. The output of detector in PLL appears as demodulated signal. One of the common form of detector is ratio detector that uses frequency sensitive phase shift network along with diode in series to get the demodulated output. Another method is first passing the signal through differentiating network and then using envelop demodulator like in AM demodulator to extract the signal. (Clarke & Hess, 1971)
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One of the critical components used in demodulator is the output filter. The filter allows the low frequency component to pass through but sinks all the high frequency components. The typical filter consists of parallel resistance and capacitance circuit. The increase in time constant value of RC circuit will reduce level of harmonics of feed through RF but at the same time increase negative peak clipping. Thus the values chosen should be compromise of two depending on the requirement.
Discuss the need for automatic gain control in AM receivers and Automatic frequency control in FM receivers, how are these 'control' voltages produced and used?
The signal received by the radio suffers from large variations due to atmospheric changes. The changes will affect the output signal after demodulation causing variation of levels of audio in the signal. In order to prevent these variations AGC is used. The AGC in AM receiver is used to maintain the output of IF amplifier constant despite variations in signal level at the input of the receiver. The AGC circuit samples a part of detected circuit and converts it into dc signal which is used as negative feedback to control the gain of various stages to keep output constant. The AGC can be simple AGC that is used for all levels of output signal. Another type of AGC called delayed AGC is used only when the output signal goes below a threshold level. (Tomasi, 2004)
The FM radio is less prone to signal fluctuations as compared to AM system due to the very nature of modulation. The limiter after IF amplifiers are used to remove the amplitude variations in the FM signal. Thus in case of FM receivers the AGC is used as optional. The limiters used after IF amplifier provides functions of AGC in FM receivers as well and the output of limiter is fed back to IF/RF amplifiers for gain control. (Chitode, 2008)
The AGC circuit consists of a peak detector at the output of last IF amplifier. The output is taken and then averaged and sent to the IF/RF amplifier for gain control as DC voltage. The use of this voltage to change the gain can be done using several methods. In a forward acting type of AGC the trans-conductance of the amplifier is controlled by changing the voltage between base and collector. The gain may be varied by controlling the emitter current of the amplifiers so that input impedance of the amplifier changes thereby limiting its gain. (Clarke & Hess, 1971)
Discuss using diagrams where appropriate how analogue signals are converted to digital signals, discuss the need for non-uniform quantisation and the process of conversion back to an analogue signal.
The conversion of analogue signals into digital signals is a three stage process: sampling, quantization and coding.
As per Nyquist sampling theorem, in order to reproduce the analogue signals faithfully after conversion into digital signals, the analogue signals are converted into digital signals by taking samples of analogue signal at the rate twice the highest signal frequency. Thus voice signal of bandwidth 0-4KHz is samples at 8K times per second. This process is called sampling and is shown in Figure below for a sine wave analogue signal (Freeman, 2004):
Figure : Sampling of sinusoidal signal (Freeman, 2004, p.264)
After taking sample of analogue signal at regular intervals each sample is quantized or converted into digital but streams of 1's and 0's. The numbers of bits selected determines the accuracy of quantization process and determines the quantization noise during conversion back to analogue signal. For example, the number of bits used for quantization of voice signals in case of PCM system is 8 out of which the first bit represents the polarity of the signal. The use of remaining 7 bits gives the quantization steps of 27= 128 steps. After converting the sample to digital form the bits are coded into suitable form for transportation over the medium. (Freeman, 2004)
The numbers of bits used for voice quantization are 7 or 8 leading to quantization steps of 27 = 128 and 28 =256 respectively. The voice has very wide dynamic range and depends on talker to talker. If quantization is done in linear fashion it will require large number of discrete steps to quantize it properly. In order to solve this problem the companding is used where more steps are used for quantizing small signal and fewer steps are used for larger signal. This allows lower value signal to be encoded properly without increasing the number of bits required for quantization. This process is called companding and used in generation of PCM signals. (Freeman, 2004)
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At the receiver end the signal received is decoded into bits which are then converted to impulse type signal and then passed through a low pass filter to get the original analogue signal. The total process is shown in Figure 2 below.
Figure : (Encyclopædia Britannica, Inc., 2013)
Part B - Digital Communication
Frequency Division Multiplexing
In the frequency division multiplexing (FDM) the total spectrum available is divided into number of non-overlapping sub bands and each band is used to carry a separate signal. The sub-bands are then combined to form a single signal that is transported over the transmission medium. This allows number of channel to be transmitted using a single medium. The analogue cable TV system used FDM for transmission of multiple channels over a single co-axial cable.
Methodology of FDM
In FDM, the carrier frequency is divided into number of sub carriers. Each subcarrier has certain bandwidth allocated to it so that it does not clash with the bandwidth of adjacent carrier. The data signal them modulates the carrier using modulation technique such as amplitude, frequency or phase modulation. The modulated carrier has number of components and the data is carried in the side bands located on each side of the carrier frequency. The signal is filtered so that one side band is available and bandwidth allocation for carriers is made so that each band does not interfere with the other band. These filtered modulated data side bands are then combined and sent over the communication channel. At the receive end each band is filtered out using a band pass filter and the signal is demodulated to get the original signal using a local carrier frequency.
The FDM techniques have been used most widely in telephony systems till such time digital technique over took the analogue system. In this system each voice channel is allocated a bandwidth of 4 KHz. Each signal modulates a carrier and the output of the modulator is filtered to restrict the bandwidth of modulated signal in given range. The multiplexing scheme follows some definite steps.
In the first step of multiplexing 12 voice channels are combined in a group called basic group. Each voice channel n modulates carrier fc = 60 + 4n KHz where n = 1 , 2, â€¦.12. The lower sideband of modulated signal are retained after filtering and then combined to get the output that is in frequency band 60 to 108 KHz
CCITT Standard Super group
In second step of FDM five basic groups each of 60 to 108 KHz are combined to form a super group. The super group is formed by using the nth group to modulate the carrier of frequency fc = 312+ 48n KHz where n = 1 , 2, â€¦.5. The carrier frequencies are 420 KHz, 468 KHz, 516 KHz, 564 KHz and 612 KHz. This transports the super group into the band 312 KHz to 552 KHz. Each super group carries 60 voice channels.
Standard Master group
Next step is formation of standard master group. This is done by allowing each super group to modulates carrier of frequency fc = 812+ 240n KHz where n = 1 , 2, â€¦.5. This process gives a master group in the frequency range of 812 to 2044 KHz and contains 300 voice channels.
Standard Super Master group
The Standard Super Master group is composed of 3 master groups occupying frequency range of 8.516 MHz to 12.388 MHz and carrier 900 voice channels. The subcarrier frequencies used are 10.56 MHz, 11.88 MHz and 13.2 MHz.
These carrier frequencies are generated using harmonics of frequencies of 4 KHz, 12 KHz and 124 KHz oscillators. The standard group utilizes harmonics of 4 KHz , the super group utilizes harmonics of 12 KHz between 420 KHz and 612 KHz and the harmonics of 124 KHz are used for master group ranging from 1052 KHz to 2.044 MHz.
The groupings are different from system to system. The FDM hierarchy recommended by CCITT for European system is as follows (P.Gnanasivam, 2005):
12 channels per group Rec. G.232 (Spectrum 60-108 KHz)
5 groups per 60 channel supergroup Rec. G.233 (Spectrum 312-552 KHz)
5 super groups per 300 channel mastergroup (Spectrum 812-2044 KHz)
3 master groups per 900 channel jumbogroup
The North American system follows a little different system and has following hierarchy:
12 channels per group
5 groups per 60 channel supergroup
10 super groups per 600 channel mastergroup
6 master groups per 3600 channel Junbogroup
The hierarchy used in UK is shown in Figure below:
Figure Hierarchy of FDM system in UK (Flood, 1997, p.107)
The L1 system in USA uses 600 channel master group using frequency band of 60 KHz to 2.788 MHz. A L3 system uses 1800 channels using three master groups in frequency range of 564 KHz to 8.84 MHz. It also uses another super group of 312 to 552 KHz. (Flood, 1997)
In order to maintain the system level and regulate frequencies a pilot signal is transmitted along with the FDM signal. The level of pilot allows regulation of level within +/- 0.5 dB range and allows constant frequency generation across the spectrum used. A stable and reliable master frequency source is used to generate FDM components and the receiver is synchronized with the transmitter frequency for proper operation of the system. (Flood, 1997)
The long range submarine cable system being very expensive needed less bandwidth for transmission and accordingly CCITT allowed the use of channel spacing of 3 KHz instead of normal 4 KHz for this transmission. The system used good quality filters with pass band of 250 Hz to 3.05 KHz for each voice channel. This allowed transmission of 16 channels in a basic group of 60 KHz to 108 KHz against 12 channels in normal method.
The sound programming requires larger bandwidth for good quality voice transmission and accordingly CCITT specified following channel for sound programs (Flood, 1997):
50 Hz to 6.4 KHz (occupies bandwidth of two telephony channels)
50 Hz to 10 KHz (occupies bandwidth of 3 telephony channels)
50 Hz to 15 KHz (occupies bandwidth of 6 telephony channels)
In case of stereophonic transmission system uses two 15KHz bandwidth circuits occupying the whole group band.
Orthogonal Frequency-Division Multiplexing
The latest use of FDM technique is in Orthogonal Frequency-Division Multiplexing (OFDM). The OFDM is a multicarrier modulation where the individual carriers overlap each other and at the same time, each carrier is orthogonal to adjacent carrier. This allows the carrier to overlap without causing ISI (inter symbol interference) and doing away with the requirement of guard band between the carriers as shown in Figure . This leads to enormous savings in bandwidth required for transmission of the signal.
Each set of orthogonal subcarriers in frequency domain are modulated to give one OFDM symbol. Each OFDM symbol is the separated in time domain by a guard band. This guard band prevents the ISI through multipath delay in the channel.
Figure : Concept of the OFDM signal: (a) conventional multicarrier technique, and (b) orthogonal multicarrier modulation technique (Prasad, 2004, p.12)
The OFDM is used in multimedia communication, latest mobile technologies such as LTE ( Long term evolution), WiMax etc.