COMNAV.PPT

AVIONICS SYSTEMSSHORT TERM TRAINING PROGRAM

COMMUNICATION SYSTEMS NEED1.Control of Airspace.2.Emergency.3.Command and Control.4.Data Transmission.

TYPES OF COMMUNICATION LINKS

1.Air Ground.2.Ground Ground.3.Air Air.4.Space Earth.

MODES OF COMMUNICATION1.Simplex - One Way Transmission. 2.Duplex - Two Way Transmission.

COMMUNICATION SYSTEMSMEDIUM1.Pair of Wires.2.Coaxial.3.Ether / Space ( HF to Wave EM Waves ).4.Fibre Optics.5.Satellite.

TYPES OF PROPAGATION1.Ground Wave.2.Space Wave.( LOS )3.Sky Wave ( Ionosphere ).4.Tropo Scatter 5.Duct.POLARISATON OF EM WAVES1. Linear.A.VerticalB.Horizontal.2. Elliptical / Circular

FREQUENCY BAND - APPLICATIONSBANDFREQ PROPOGATION APPLICATION RANGE MEDIUMVLF 12 GHz Space Wave ( LOS ) Ground Ground CommunicationOptical Fibre Optic Cables Point Point Communication

COMMUNICATION SYSTEMSMODULATIONTo convey information from the Source ( Transmitter ) to the Sink ( Receiver ) a Carrier is required.The Process of superimposing the information on the carrier is called Modulation.NEED1.Modulation For Efficient Radiation : Efficient electromagnetic radiation requires that physical length of the antenna be atleast 1/10 wavelength are so. But most signals including signals, have frequency components down to 100 Hz or lower necessitating antennae of dimensions 300 Km long or more if radiated directly. Utiising the frequency translation property of modulation these signals can be impressed upon a high frequency carrier, thereby permitting substantial reduction in size and hence practical realisation of efficient antenna. 2. Modulation to reduce Noise or Interference: Certain types of modulation have the useful property of suppressing both noise and interference at the cost of bandwidth. ( e.g. Frequency Modulation, Spread Spectrum)3. Modulation for Frequency Assignment: It is possible to select one of the several stations (channels) even while all stations are broadcasting by assigning different frequencies to different transmitting stations.4. Modulation for Multiplexing: Multiplexing techniques permit multiple signal transmission on single channel such that each signal can be separated out at the receiving end. ( e.g. Telemetry, Wide band tele communication Links )

COMMUNICATION SYSTEMS4. Modulation to Overcome Equipment Limitations : Modulation can be used to place a signal in that portion of the frequency spectrum where equipment limitations are minimum or where design requirements are easily met.TYPES OF MODULATION.MODULATIONCONTINUOUSPULSELINEAREXPONENTIAL ANALOGDIGITALAM,SSBFM,PM PAM, PPMPCM,DMDSB,VSB PWMAM- Amplitude Modulation SSB- Single Side BandDSB- Double Side Band VSB- Vestigial Side BandFM- Frequency Modulation PM- Phase ModulationPAM- Pulse Amplitude Modulation PPM- Pulse Position Modulation PWM- Pulse Width ModulationPCM- Pulse Code Modulation DM- Delta modulation In linear class of modulation there is a linear relationship between modulating signal and the carrier and the transmission band width is not more than twice the band width of the modulating signal . In exponential and other types of modulation there is no linear relationship between modulating signal and the carrier and the band width of the carrier can be many times more than the modulating signal bandwidth.

COMMUNICATION SYSTEMS e(t) = E0 Sin 2 fct + m E0 Cos 2 (fc - fm ) t 2 - m E0 Cos 2 (fc + fm ) t 2 BALANCED MODULATOR ( Ref fig 3 &4 )Balanced Modulators are used to obtain DSB or SSB with or with out carrier.DSBAt A1 e1= E0 Sin ct + m E0 Sin mt Sin ct At B1 e2 = E0 Sin ct - m E0 Sin mt Sin ct e1 - e2 = 2 m E0 Sin mt Sin ct = m E0 Cos ( c - m)t - m E0 Cos (c +m)t SSB ( Ref fig 5 ) Phase Shift Method. e1 = m E0 Sin mt Sin ct e2 = m E0 Sin ( mt + 90 ) Sin (ct + 90) e1+ e2 = m E0 Sin mt Sin ct + m E0 Cosmt Cos ct= m E0[ Cos(c - m)t - Cos (c + m )t ]+ 2 m E0[ Cos(c - m)t + Cos (c + m )t] 2= m E0 Cos(c - m)t

DEMODULATION 1.Reverse of Modulation using linear or non linear devices.2. Coherent or non coherent detection.

COMMUNICATION SYSTEMSDSB DETECTIONThe carrier must be supplied before the demodulator. Modulating signal is recovered by the modulated carrier to a half wave rectifier, whose output is then filtered to provide the desired modulating signal.SSB DETECTION - product Detectors,SSB signal consisting of two tone signal= A Cos (c + a )t + A Cos (c + b )tCarrier Voltage = B Cos c tProduct of the two signals= B Cos c t[A Cos (c + a )t + A Cos (c + b )t]= AB Cos a t + AB Cos (2c + b )t + AB Cos bt + 2 2 2 AB Cos (2c + b )t 2Square Law Detector.Eo =a ein 2ein = B Cos c t[A Cos (c + a )t + A Cos (c + b )t]a ein 2 =a [B Cos c t[A Cos (c + a )t + A Cos (c + b )t]2 = a [B2 Cos2 c t+A2 Cos2 (c + a )t + A2 Cos2 (c + b )t+2AB Cos c t Cos (c + a )t+ 2AB Cos c t Cos (c + b )t+ 2A2 Cos (c + a )t Cos (c + b )t]

COMMUNICATION SYSTEMS= a[B2 + B2 Cos 2 c t +A2 + A2 Cos 2 (c + a )t 2 2 2 2 + A2 + A2 Cos2 (c + b )t +AB Cos 2 (c + a )t 2 2+ AB Cos at + AB Cos (2c + b )t + AB Cos bt + A2 Cos (2c + a + b )t + A2 Cos (b - a )t ]If d-c and RF terms are neglected ( as they are filtered out in the demodulaion) the remaining terms are AB Cos at and AB Cos bt and A2 Cos (b - a )t. The first two of these are desired audio signals but the last is an undesired second order Inter Modulation product. The ratio of desired signal to IM product is AB = B A2 A thus to produce an IM product 40 dB down , the carrier must be 40 dB stronger than either of the two side bands. DSB Vs SSBThe energy contained in a modulated wave is the sum of energies of the separate frequency components, and therefore increased during modulation because of the energy added by sidebands. Thus when the carrier wave is completely modulated by a sinusoidal variation of the envelop amplitude i.e. m=1, there are two sideband components each having an amplitude one-half that of the carrier wave. Each of these hence contains one-fourth as much power as does the carrier, so that the two sidebands together make the power of completely modulated wave 50% greater than the carrier power. Inthis case , only one third of the total energy is in the sidebands, while two thirds is in the carrier. When the degree of modulation is less than 1.0, the sideband power will be

COMMUNICATION SYSTEMS proportionate to m2. As a result, the fraction of the energy that is contained in the intelligence bearing sideband decreases rapidly as the degree of modulation is reduced. For this reason a high degree of modulation is generally sought.The peak envelope voltage of AM wave is doubled when m=1, and consequently the peak power of AM is four times the peak to peak envelope power of the DSB wave. Since the average intelligence in both AM and DSB system are the same, we may conclude that under equal peak power limitations, DSB has a 6 dB advantage in intelligence power AM , independent of the waveform of the modulating signal.Figure 5 shows a comparison of AM and SSB signal, which result in equal signal to noise ratio in respective receive audio outputs . Coherent (in-phase) addition of two AM sideband voltage in detection is assumed. This graphic analysis illustrates why an SSB signal with a peak envelope power (PEP) equal to one half of the carrier power of an AM signal produces equal signal-to noise ratios from comparative receivers . It can be observed that both the systems have the same total sideband power which gives AM a 3 dB (1/0.707) advantage but this advantage is because of a 3 dB greater noise since the predetection bandwidth for AM must be twice as wide as the SSB bandwidth requirement. The term n for nboise voltage in fig 5 depends upon receiver noise figure , which is assumed to be the same for both receivers.A comparison of total average power radiated by the AM and SSB transmitters for equal signal-to-noise ratio is still more striking. Fig 5 shows that with the single tone sine wave

COMMUNICATION SYSTEMSModulation (100 % modulated in case of AM) the carrier power is twice the total sideband power. Therefore , the AM power is 1.5 units as compared with 0.5 units for SSB.The total average power for AM is three times the average power for SSB for equal signal to noise ratio.When power drain from the primary source or transmitter power power supply size is important , it is informative to compare AM with SSB transmitter performance on the basis of power input for AM class B modulator plus class C power amplifier:-Overall conversion efficiency, SSB-55%Overall conversion efficiency,AM carrier - 70%Overall conversion efficiency, AM modulator - 60%The SSB amplifier power input will be 0.5/0.55=0.91 units. The AM power input, consisting of class C amplifier input plus the class B modulator input, is the total of 1/0.70+(0.5/0.7)/0.60 = 2.62 units. Am power input is then equal to 2.62/0.91 or 2.88( 4.6 dB) times as much as the power input for SSB. This comparison holds good for other complex modulation also and results in a power input ratio decidedly in favour of SSB. Condition AM to SSBRatiodBCarrier power PEP2:13.0Average output power3:14.8Average input power2.88:14.6Peak voltage (2.0/0.707)2.83:19.0

COMMUNICATION SYSTEMSPHASE /FREQUENCY MODULATIONPhase Modulation ( PM)PM is that form of angle modulation in which the instantaneous phase angle i(t) is equal to the phase of the unmodulated carrier, plus a time varying component that is proportional to the applied modulating wave m(t) as shown by i(t) = 2 fc (t) + kp m(t)The term 2 fc (t) represents the phase of the unmodulated carrier and the constant kp represents the phase sensitivity of the modulator, expressed in radians per volt. This assumes that m(t) is a voltage wave form. The phase modulated wave S(t) is described in the time domain by S(t) =Ac cos [ (2 fc (t) + kp mi(t)]Frequency Modulation (FM)FM is that form of modulation in which the instantaneous frequency fi (t) is equal to the constant frequency of the unmodulated carrier wave plus a time varying component that is proportional to the applied modulating wave m(t) as shown by fi (t) = fc +kf m(t)The term fc represents the frequency of the unmodulated carrier and the constant kf represents the frequency sensitivity of the modulator expressed in hertz per volt. This assumes that m(t) is a voltage waveform. Integrating w.r.t. time and multiplying the result by 2 , we get i(t) = 2 fc (t) + 2 kf (integral 0 to t of) 0t m(t)dtWhere the phase angle of the modulated carrier wave is assumed to

COMMUNICATION SYSTEMS be zero at t=0 for convenience. The frequency modulated wave is therefore described in the time domain by S(t) =Ac cos [2 fc (t) + 2 kf 0t m(t)dt ]Figure 6 illustrates the phase and frequency modulated waves. It can be seen that a distinction can be made between PM and FM waves only when compared with the modulating wave. In fact, PM and FM are inseparable in the sense that any variation of phase of a sinusoidal carrier is accompanied by a frequency variation, and similarly, any frequency change necessarily involves phase change as instantaneous angular frequency is time derivative of phase angle. This means that an FM wave can be generated by just integrating m(t) and then using it as an input to a phase modulator as shown in fig 7a. Conversly , a PM wave can be generated by first differentiating m(t) and then using it as the input to a frequency modulator, as shown in fig 7b.Single Tone frequency ModulationThe FM wave S(t) defined earlier is a nonlinear function of the modulating wave m(t) , and hence frequency modulation is a nonlinear modulation process. Consequently, unlike amplitude modulation the spectrum of an FM wave is not related in manner to that of an FM wave .Consider a sinusoidal modulating wave defined by m(t) = Am cos (2 fm t ) The instantaneous frequency of the resulting FM wave is defined by fi (t) = fc + kf Am cos (2 fm t ) = fc + f cos (2 fm t )

COMMUNICATION SYSTEMSwhere f = kf AmThe quantity f is called the frequency deviation, representing the maximum departure of the instantaneous frequency of the FM wave from the carrier frequency fc. A fundamental characteristics of an FM wave is that the frequency deviation f is proportional to the peak amplitude of the modulating amplitude of the modulating wave and is independent of the modulation frequency.The instantaneous phase angle of the FM wave is i(t) = 2 0t fi(t)dt = 2 fc (t) + 0t f cos (2 fm t ) = 2 fc (t) + f sin (2 fm t ) fmThe ratio of the frequency deviation f to the modulation frequency fmis called the modulation index of the FM wave. We may write = f fmand i(t) = 2 fc (t) + sin (2 fm t ) The FM wave is given by S(t) =Ac cos [2 fc (t) + sin (2 fm t ) ]Depending on the modulation value index , we may distinguish two cases of frequency modulation,1. Narrow band FM for which is small2. Wide band FM for which is largeBoth are compared to one radian. The result is that transmission band width of a narrow band FM is closely equal to twice the message bandwidth , whereas in the case of wide band FM it is well in excess

COMMUNICATION SYSTEMSof this value.Narrow-band Frequency ModulationExpanding the eqn for FM wave we getS(t) =Ac cos (2 fc t) cos [ sin (2 fm t ) ] - Ac sin (2 fc t) sin [ sin (2 fm t ) ]Assuming that the modulation index ( narrow-band)is small compared to one radian, we may use the following approximation cos [ sin (2 fm t ) ] 1 sin [ sin (2 fm t ) ] sin (2 fm t )The eqn for narrow band is then given byS(t) =Ac cos (2 fc t) - Ac sin (2 fc t) sin (2 fm t ) This eqn defines a narrow band FM wave produced by a sinusoidal wave Am cos (2 fm t ). This narrow band FM can be generated by using an integrator followed by a phase modulator, as shown in fig 8. This phase modulator involves splitting the carrier wave Ac sin (2 fc t) in to two paths. One path contains a 90 phase -shifting network, and the other a product modulator which generates a DSB suppressed carrier wave. A combination of these two signals produces an FM wave with a maximum phase deviation up to 30 with good linearity.The narrow-band FM can be further expanded as S(t) =Ac cos (2 fc t) + Ac{cos [2 ( fc + fm ) t ] - cos [2 ( fc - fm ) t ] }The corresponding AM wave expression is given byS(t) =Ac cos (2 fc t) + m Ac{cos [2 ( fc + fm ) t ] + cos [2 ( fc - fm ) t ] }

COMMUNICATION SYSTEMSwhere m is the modulation index of AM wave. Comparing the eqns for AM wave and the narrow-band FM wave we see that in the case of sinusoidal modulating wave the basic difference is that the sign of the lower side band frequency in the case of narrow - band FM wave is reversed. Thus a narrow - band FM requires essentially the same transmission band width (i.e 2fm) as the AM wave. The narrow-band FM wave with a phasor diagram is represented in fig.9 where the carrier has been used as a reference. The resultant of the two side frequency phasors is always at right angles to the carrier phasor. The effect of this is to produce a resultant phasor representing athe narrow-band FM wave which is approximately of the same amplitude as the carrier phasor but out of phase with respect to it. This phasor diagram should be compared with that of fig 9b which represents a AM wave. In this case we see that the resultant phasor representing the AM wave has an amplitude which is different from that of the carrier phasor, but is always in phase with it.Wideband Frequency ModulationThe frequency components contained in the wave represented by modulating wave can be determined by expanding the right hand side of the eqn for Fm wave and evaluating the the resulting expression we getS(t) = Acn=- Jn () cos[2 ( fc + nfm ) t ] -fm t fmwhere Jn () is the Bessal function of the first kind and nth order with argument .The spectrum of an FM wave contains a carrier component and an infinite sets of side frequencies located symmetrically on either side of the carrier at frequency separations of fm, 2 fm, 3 fm . Etc. in this respect it is unlike that which prevails in AM system, since in an AM system a sinusoidal modulating signal gives rise to one pair of side frequencies. The amplitude of the carrier component varies with according to Jo ().

COMMUNICATION SYSTEMSTransmission BandwidthIn theory, an FM wave contains an infinite number of side frequencies so that the bandwidth required to transmit such a signal is similarly infinite in extent. In practice however we find that the FM wave is effectively limited to a finite number of significant side frequencies compatible with a specified amount of distortion.In the case of an FM wave generated by a single tone modulating wave of frequency fm, the side frequencies that are separated from the carrier frequency fc by an amount greater than the frequency deviation f decreases rapidly towards zero, such that the bandwidth always exceeds the total frequency excursion, but nevertheless limited. Specifically , for large values of the modulation index , the bandwidth approaches, and is only slightly greater than, the total frequency excursion 2 f . On the other hand , for small values of the spectrum of FM wave is sufficently limited to the carrier frequency fc and one pair of side frequencies at fc fm, so that the bandwidth approaches 2 fm. Thus an approximate rule for the transmission bandwidth of an FM wave generated by single-tone modulating wave of frequency fm can be written asB = 2 f + 2 fm = 2 fm ( 1+ f) = 2fm ( 1 + ) fmThis is known as Carson Rule bandwidth.Generation of FM . Indirect Method : The input modulating wave m(t) is first integrated and then used to phase modulate a crystal oscillator. In order to minimize the distortion which is inherent in the phase modulator, the maximum phase deviation or modulation index is kept small, thereby resulting in a narrow band FM wave. This signal is next multiplied in frequency by means of frequency multiplier so as to produce the desired wideband FM wave. The implementation of narrow band modulator is shown in fig 10.

COMMUNICATION SYSTEMSDirect Method : In the simplest form of such modulator, a varactor diode is reverse biased by a fixed d-c voltage superimposed with the input modulating signal. Its dynamic capacitance forms part of the tank circuit for an oscillator. In such a modulator , the varactor diode capacitance should vary as the inverse square of the instantaneous reverse bias. But as the capacitance does not vary as desired over a large range, it results in predominant second order distortion and resulting intermodulation noise becomes intolerable as the channel capacity is increased.Push Pull Modulator : This inter modulation problem can be overcome by the use of Push Pull Modulator. In this the resulting even order products gets cancelled. Fig 11 shows the basic configuration. Two oscillator frequencies f1 & f2 are mixed to produce the required output frequency fo. The two oscillators are frequency modulated in push-pull by the incoming modulating signal. The oscillators are arranged in such a way that at any time if f1increases f2 and vice-versa. The resulting frequency deviation of fo is the sum of the individual deviation of f1 & f2 . In such a case , the percentage deviation of each oscillator is less than if only one oscillator has been used. The advantages are the following :-1. In push-pull technique, the even order products cancel out . The third order products are inherently small. The third order products decrease, as the inverse square f the oscillator frequency. So a moderate increase in this frequency is enough.2. It is easy to obtain a good linearity with two oscillators operating with a relatively high frequency, with about half of the total frequency deviation.3. The modulation sensitivity is increased without sacrificing stability.4. Improves the SNR by 3 dB.

COMMUNICATION SYSTEMSFM DemodulatorThe simple block diagram of of FM demodulator is shown in fig 12. It consists of Limiter, Discriminator driver, Discriminator and an output amplifier. Limiter compresses amplitude variations of the incoming FM signal. Discriminator driver provides a sufficient input to the Discriminator. After the FM demodulator the signal is amplified by by a post detection amplifier for proper output. Pulse Code Modulation ( PCM)PCM is the process of converting an analogue in to a digital ( Binary ) signal. The analogue signal is first sampled at atleast twice the highest frequency ( Nyquist Rate ) component contained in the signal. The sampled value of the analogue signal is then converted in to equivalent Binary Digits comprising ones and zeros. This is done by analogue -to - Digital Converter ( A to D Converter ). The binary digits ( bits ) or signal is then made to modulate an analogue carrier for transmission.When it becomes necessary to superimpose a binary waveform on a carrier, then AM, PM or FM may be used. As a matter of practice straight forward AM is rarely used. Phase and frequency modulation are commonly used. Because of the special ( two level) nature of the carrier modulating signal, phase modulation is referred to asPhase-Shift Keying ( PSK) and frequency modulation is known as Frequency Shift Keying (FSK).In FSK the binary signal is used to generate a waveformXc(t) = Ac cos (c )t in which the plus or the minus sign depends on whether the bit is a 1 or 0. The transmitted signal is then of amplitude Ac and has an angular frequency c + or c - , with a constant angular frequency offset from the carrier frequency c - commonly called the frequency deviation .

COMMUNICATION SYSTEMSA synchronous method of demodulation is shown in fig 14. This scheme has the advantage that , it may be readily be adopted to yield optimum performance in the presence of noise. Observe that two synchronous local carriers are required here, at angular frequencies c + and c - . If the received signal is Ac cos(c + )t, then thr output of the difference amplifier will be VD = Ac - Ac [ cos 2 t + cos 2ct - cos 2(c + )t]If the received signal is Ac cos(c - )t, the output of the difference amplifier will beVD = - Ac + Ac [ cos 2 t + cos 2ct - cos 2(c - )t] A low pass filter may be used to effect an adequate separation of the d.c terms in above eqns and to allow a determnation of whether a 1 or 0 has been transmitted.Phase Shift Keying ( PSK)In Phase Shift Keying the binary signal which takes on the valuesX(t) = +V or X(t) = -V is used to generate a waveform( Fig 15) Xc (t) = Ac cos [ct + (t)]In which Ac is a fixed amplitude and = 0 for say X(t) = +V and = 1 for X(t) = -V. the discontinuous transitions shown at the beginning and end of each bit interval whenever a transition from a 0 to 1 or 1 to 0 occurs are actually smoothed out during transmission. Above eqn may be written in the alternative formXc(t) = X(t) Ac cos ct Vso that Xc(t) is Ac cos ct or - Ac cos ct for X(t) = +V or X(t)= -V. This wave form may be generated as in fig 15 by applying waveform X(t) and the carrier cos ct to a balanced modulator.

COMMUNICATION SYSTEMSThe received signal has the form X(t) Ac cos( ct + ) in which is a V phase angle depends on the effective length of path between transmitter and receiver. Demodulation must be performed synchronously ; hence we require the waveform cos( ct + ) at the receiver. A synchronising circuit which can extract the waveform cos( ct + ) from the received signal itself is shown in fig 15. The output of the square-law device is Ac cos( ct + ) since X(t) will always be 1. VNow Ac cos( ct + )= Ac / 2 cos 2( ct + ). Hence a bandpass filter may be used as indicated to separate the waveformcos 2( ct + ). The frequency divider divides frequency by 2 yielding cos ( ct + ) as required. The input signal is then multiplied by the locally recovered carrier cos ( ct + ). The product iscos ( ct + )Xc(t)= 1 X(t) Ac + 1 X(t) Ac cos 2( ct + ) 2 V 2 VOur interest is in in X(t). If we X(t) were a band limited signal , then we might recover X(t) precisely through the use of a low pass filter. However, in principle at least, the wave form X(t) is not band limited because of the abrupt transitions in its waveform. Hence a low pass filter will introduce some distortion in X(t) double frequency carrier in the above eqn. It is to be noted, however, that we are not really interested in recovering X(t) but only knowing whether X(t) = +V or -V in each bit interval. If bit interval extends over many cycles of the carrier cos ct then it will be easy to find a low pass filter which will effect an adequate separation of the terms in the above eqn to allow such a determination to be made.

COMMUNICATION SYSTEMSNOISENoise is defined as any unwanted signal.Types of Noise1. Cosmic/Galectic : There is a continuous background of noise like electromagnetic radiation which arrives from outer space. This extra terrestrial noise comes from our own galaxy, from extragalactic sources, discrete radio stars. This noise energy is prominent at lower frequencies up to lower UHF bands and decreases with increasing frequency.2. Atmospheric Noise : A single lightning stroke radiates considerable RF noise power. There are at any one moment an average of 1800 thunderstorms in progress in different parts of the world. From all these storms about 100 lightening flashes take place every second. The combined effect of all the lightening strokes gives rise to a noise spectrum which is especially large at broadcast and short wave radio frequencies. Noise that arise from lightening-stroke radiation is called atmospheric noise. The spectrum of atmospheric noise falls off rapidly with increasing frequency and is usually of little consequence above 50 MHz. 3. Man made : Noise from automobile ignition, electric motors, high tension lines and fluorescent lights are examples of this type of noise. This type of noise also decreases with increasing frequency.4. Solar Noise : The sun is a strong emitter of electromagnetic radiation, the intensity of which varies with time. The minimum level of solar noise is due to blackbody radiation at a temperature of about 6000K. The solar noise power increases approximately as the square of the frequency. This is unlike most other noise mechanisms which produce less power with increasing frequency. The solar noise level can increase orders of magnitude over that of the quite , or undisturbed, sun when its surface is disturbed by solar storms ( sunspots and flares). The enhanced noise from the disturbed is

COMMUNICATION SYSTEMScomplex, and its mechanism is not well understood. It might last for a fraction of a second, or it might last for days. In general, the greater the intensity of the enhanced solar noise the shorter its duration.5. Receiver Noise: This type of noise is present in all receiving systems and can be further categorised as followsA. Thermal Noise. The electron in any conductor at a temperature other than absolute zero are always in random motion. Such random motion of electrons give rise to an electrical noise voltage and whose maximum power is given by kTBn where k is the Boltzmans constant, T is the absolute temperature measured in degrees Kelvin and Bn is the bandwidth over which noise is measured. This noise power is known as thermal noise because because of its dependence on the temperature T. it is present in allo conductors of finite resistance at non zero temperature. Its spectrum is independent of RF frequency band and for this reason it is called white noise. Thermal noise is also known as Johnson Noise.B. Shot noise : Electrons are emitted at random time intervals from the cathode of a vacuum tube diode and strike the plate at a non uniform rate. The random arrival of electrons give rise to a random noise current called shot noise. Shot noise occurs not only in simple diodes but in grid controlled tubes, travelling wave tubes, klystrons, magnetrons, crystal diodes, transistors and other devices which carry current.C. Flicker Noise : There exists in semi conductors at low frequencies a noise mechanism whose spectral density is inversely proportional to frequency. This is called flicker noise or 1/f noise and occurs in semiconductor devices such as transistors as well as in crystal diodes. Because of the inverse relationship between flicker-noise power and frequency flicker noise will be the predominant effect in semiconductor devices at lower frequencies.

COMMUNICATION SYSTEMSD. Crystal Noise : An unexcited crystal diode will produce thermal noise just as any other resistance will at thermal equilibrium. When current passes through a crystal , the resulting noise is a complicated combination of thermal noise in the spreading resistance, shot noise in the barrier and the flicker noise in the semi conductor.Noise FigureAn ideal receiver adds no noise of its own to the signal being amplified. All practical receivers, however , generate noise to some extent. A measure of the noise produced by a practical receiver compared with the noise of an ideal receiver is the noise figure or noise factor. The noise figure of a linear system is defined asF = Sin / Nin Sout/NoutorF = Nout kTBnGwhere Sin = available input signal power Nin = available input noise power ( equal to kTBn) Sout = available output signal power Nout = available noise output power. Available Power refers to the power which would be delivered to a matched load.. The available gain G is equal to Sout / Sin. The noise figure is commonly expressed in decibels.If we assume that n is the additional noise introduced by the network ( receiver) the noise figure may also be written asF = kTBnG + n kTBnGF = 1+ n kTBnGThe temperature T which appears in the noise figure definition is the temperature at the input to the net work. This temperature has been standardised as To = 290K , being close to the ambient temperature.

COMMUNICATION SYSTEMSNoise Figure of Networks in Cascade.: Consider two networks in cascade, each with the same noise bandwidth Bn, but with different noise figures and available gain. Let F1, G1 be the noise figure and available gain, respectively of the first network, and F2, G2 be similar parameters for the second network. The problem is to find Fo, the over-all noise figure of the two circuits in cascade. From the definition of noise figure the output noise No of the two circuits in cascade isNo = Fo G1 G2kTo Bn = noise from network 1 at the output of network 2 + noise n2 introduced by network 2No = kTo BnF1 G1 G2 + n2 = kTo BnF1 G1 G2 + ( F2 -1) kTo BnG2OrFo = F1 + F2 -1 G1The contribution of the second network to the over-all noise figure may be made negligible if the gain of the first network is large. It is not sufficient that only the first stage of a low-noise receive have a small noise figure. The succeeding stage must also have a small noise figure, or else the gain of the first stage must be high enough to swamp the noise of the succeeding stages. If the first network is not an amplifier butis a network with a loss ( as in crystal mixer) the gain G1should be interpreted as a number less than unity.The noise figure of N cascade may be shown to be Fo = F1 + F2 -1 + F3 - 1 + . + FN - 1___________ G1 G1G2 G1G2..GN-1 Effective Noise TemperatureThe concept of noise figure was originally formulated to describe the performance of relatively noisy receivers or networks. The use of the noise-figure with its standard temperature To = 290K is not as convenient with low-noise devices such as is the effective noise temperature.

COMMUNICATION SYSTEMSF = 1+ n kTBnGwhere n is the noise produced by the network itself and all other quantities were defined previously. The effective noise temperature of the network is defined as that temperature Te at the input of the network which would account for for the noise n at the output. Therefore n = kTnBnG andF = 1+ Te ToTe = ( F-1}ToThe effective noise temperature of n network in cascade is simplyTe = T1 + T2 + T3__ + ... + TN__________ G1 G1G2 G1G2.GN-1Low Noise Amplifiers1. Maser ( Cryogenic Liquid Cooled ) 2. Parametric Amplifier3. Tunnel Diode Amplifier4. FET 5. G As Solid State Semiconductor.

COMMUNICATION SYSTEMSANTENNAAntenna is an interface/ transducer between the transmitter ( Power Amplifier ) and the free space. It converts the varying electrical current in to Electro Magnetic energy for propagation in free space. By reciprocity theorem it converts EM energy in to varying electrical current in a conductor.Types of Antenna1. Tuned ( Dipole)2. Untuned ( Whip )3. Directional ( Parabolic, Antenna arrays, Rhombic etc)4. Omni Directional / Isotropic ( Sense Antenna, Vertical Antenna)5. Narrow Band / Beam Width ( Slot Antenna, Phased Array )6. Broad Band/ Beam ( Log Periodic )Antennae for Communication SystemsL.F - Long Wire RadiatorsM.F - /4 Vertical AntennaH.F - Dipole Arrays, Rhombic, Log PeriodicV.H.F - Dipole- Yagi ArrayMicro Wave - Bill Board, Parabolic.Gain Of AntennaDirective Gain G = Max Radiation Intensity Average Radiation Intensity Power Gain G = Maximum Radiation Intensity from Antenna Radiation intensity from Isotropic Antenna ( for same power input)

COMMUNICATION SYSTEMSTypes of Aircraft Antenna1. Blade ( V/U H f Communication Transceiver )2. Wire ( H F Communication Transceiver }3. Loop ( Radio Compass )4. Slot / Phased Array ( Doppler Navigation System, Radar )5. Dipole ( Radar Altimeter 06. Parabolic ( Radar )

Special Problems In Aircraft Antenna1. Aerodynamic Characteristics.2. Space.3. Weight.4. Power.5. EMI / EMC6. Location.7. Blind Spots

COMMUNICATION SYSTEMSELECTRO MAGNETIC COMPATABILITY (EMC)ELECTROMAGNETIC INTERFERENCE (EMI)EMC is the ability of a system to function as designed in their operational environs without affecting other systems and without being affected by other systems.EMI is interference in the functioning of a system due to electromagnetic radiation and can occur within a system or between two systems.SourceMediumObjectExternal EM radiationAirAircraftEMPIonised AirCommn. FacilityLighteningSoilTank

Sources of EMI in an AircraftApertures - Cockpit, Small Windows, Open Access HolesCracks around the doors/platesRivetted joints, rusted jointsNaked / Unscreened wires, Improper connections.Measures to prevent EMI in an Aircraft.1. EMP hardening.2. Frequency Selection/ separation.3. Polarisation ( Vert/ Hor, RCP/LCP )4. Screening / Bonding5. Proper Grounding.

COMMUNICATION SYSTEMSTELEMETRYTelemetry is the technique of remotely measuring operational data/parameters, inclusive of possible processing.SourcedataTrans-ducerSignalConditionerModu-latorMuxTxTransmissionMediumRxDe-muxDemodSignal ConditionerDisplay /Processor

Types of DataStatus - On / Off , Level StatesVarying Values - Voltage, Frequency , Flow

Analogue Vs DigitalAnalogue1. Analogue primary generators are of simpler construction than digital generators.2. Greater clarity of indication.3. Discernebility of change tendencies4. Cost saving in small-medium sized system.Digital1. Ideal for transmission of binary and numerical data.2. Simultaneous transmission of different kinds of data.3. Greater resolving power / accuracy.4. Elimination of interference in the transmission path.5. Digital indicators and digital recorders are preferred for most numerical values.6. Convenient for further processing.Transmission MediumsWires, Coax cables and free space ( EM Waves )ModulationFM - FSK, FM- FDM PBW / CBWPCM-TDM SUB FRAME / FRAMEError Detection & Correction - Parity Check and Vitterbi Coding.Frame synchronisation by Unique Word.

COMMUNICATION SYSTEMSAIRBORN COMMUNICATION TRANSCEIVERFrequency Synthesisers The air to ground VHF communications band has 720 frequencies that must be generated. The accuracy of the carrier frequency must be 0.001% to prevent radio transmitters from interfering with each other. The device that performs this task is called frequency synthesiser. The best that an LC oscillator can provide is only 0.1% or one part in thousand. But crystal oscillators that are compensated for temperature effects can provide stabilities of 0.00001% or 0.1 part per million (ppm). The chief disadvantage of the crystal oscillator is that there must be one crystal for each generated frequency. Even though there are methods of reducing the number of crystals to 50 or 60 by using crystal frequencies in combinations, still it is an impractical number of crystals. Because several airborne systems require a large number of accurate frequencies, systems of frequency generation called frequency synthesisers were developed. That use one crystal oscillator but provide unlimited number of frequencies. Fig 16 shows the block diagram of phase-locked-loop synthesiser. A special LC oscillator called a Voltage Controlled Oscillator ( VCO) is used as the source of the RF signal. The VCO is a conventional LC oscillator in which varactor is used to tune the oscillator.The output frequency is divided by an integer using digital circuits called a programmable divider. The division is by an integer that can be changed easily or programmed . If the division is N, then the output of the programmable divider is Fout = Fin N where Fout is the output frequency from the progrmable divider and Fin is the input frequency. The phase and frequency detector in the loop has two inputs, Fa and Fb and provides a d-c output whose polarity

COMMUNICATION SYSTEMSDepends on whether input FA or Fb is higher in frequency. If Fa is made to be the reference crystal Oscillator out put, then when ever Fb is higher or lower in frequency as compared with ref frequency an appropriate d-c voltage output from the phase/freq detector is fed back to the VCO through a loop filter and the VCO frequency is made to change so as to bring the difference between reference oscillator and Fb to zero. This requires Fa = Fb = Fref = Fosc Nor Fosc = N FrefTherefore , the oscillator frequency may be set to any multiple of Fref by changing the value of N.Communication Transceiver An airborne communication system can use a separate transmitter and receiver or may combine both functions into one unit. There are cost,power and weight advantages to be gained by sharing circuits between transmit and receive functions Fig 17 shows the block diagram of a Airborn communication transceiver. The most effective circuit sharer is the frequency synthesiser. When receiving the synthesiser provides local oscillator frequency, and whle transmitting the carrier frequency. The antenna under quiescent (receive) condition is connected to the receive chain starting with Varactor tuned RF amplifier for high gain, a mixer for hetrodyning and frequency transformation to IF. After IF amplification the signal is detected amplified and fed to the ear phone. An AGC feedback is provided to IF stage to prevent large variations as the input signal strength increases. A squelch circuit is provided to eliminate any residual noise when no audio output is available. To transmit PTT ( T/R )switch is pressed and this connects the antenna,freq synthesise , mike to the tansmitter chain. The speech is amplitude modulated (DSB) and transmitted.