An About Spread Spectrum Communications Essay


Spread spectrum is a technique whereby an already modulated signal is modulated a 2nd clip in such a manner as to bring forth a wave form, which interferes in a barley noticeable manner with any other signal operating in a same frequence set. Therefore, a receiving system tuned to have a specific AM or FM broadcast would likely non detect a presence of a spread spectrum signal operating over the same frequence set. Similarly, the receiving system of the spread spectrum would non detect the presence of the AM or FM signal. Therefore, we say that interfering signals are crystalline to distribute spectrum signals and spread spectrum signals are crystalline to interfering signals.To supply the “transparency” described above the spread spectrum technique is to modulate an already modulated wave form, either utilizing amplitude transition or wideband frequence transition, so as to bring forth a really broadband signal. For illustration, an ordinary AM signal utilizes a bandwidth of 10KHz. See that a spread spectrum signal is runing at the same bearer frequence as the AM signal and has the same power P, as the AM signal but a bandwidth of 1 MHz.

Then, in the 10 KHz bandwidth of the AM signal, the power of the 2nd signal is Ps x ( 104/106 ) = Ps/100. Since the AM signal has a power of Ps, the interfering spread spectrum signal provides noise, which is 20 dubnium below the AM signal. The widest application at this clip is its usage in military communications systems where dispersed spectrum serves two maps.

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The first is that it allows a sender to convey a message to a receiving system without the message being detected by a receiving system for which it is non intended i.e. the sender is crystalline to an unfriendly receiving system. To accomplish this transparence the spread spectrum transition decreases the familial power spectral denseness so that it lies good below the thermic noise degree of any unfriendly receiving system. The 2nd major application of spread spectrum is found, when, as a affair of fact, it turns out non to be possible to hide the transmittal. Police radio detection and rangings can use spread spectrum to avoid sensing by radio detection and ranging sensor employed by drivers. In such a instance the operator of an unfriendly receiving system might try to get down conveying an interfacing signal to barricade communicating between sender and receiving system.

Here once more dispersed spectrum act to cut down the effectual power of the intervention so that communicating can treat with minimum intervention.In the commercial communicating field, spread spectrum has many applications, a major application being the transmittal of a spread spectrum signal on the same bearer frequence as an already bing microwave signal. By pass oning in the mode extra signal can be transmitted over the same set thereby increasing figure of user. In extra, dispersed spectrum is used in satellite communicating and is being considered for usage in local country webs.The object of our undertaking is to Design & A ; Implement a “Digital Speech Security System” ( DSSS ) which can supply protection against externally generated interfering ( thronging ) signals with finite power. The thronging signal may dwell of reasonably powerful broadband noise or battalion wave form that is detected at the receiving system for the intent of interrupting communications. Protection against thronging wave forms is provided by intentionally doing the information – bearing signal occupy a bandwidth necessary to convey it. This has the consequence of doing the familial signal presume a noise like visual aspect so as to intermix into the background.

The familial signal is therefore enabled to propagate through the channel undetected by any one who may be listening.The extra security to speech can be provided by inverting two spots at sender every bit good as at receiving system station. Conventionally, address can be protected from unauthorised interception or eavesdropping by parallel techniques utilizing scramblers. There are clip, frequence and bandwidth scramblers in diplomatic usage.

In a Digital Speech Security System ( DSSS ) , the parallel signal is converted into Pulse Code Modulated ( PCM ) digital signal from utilizing an analog-to-digital convertor. This digital signal is combined straight with the end product from a pseudo-random noise ( PN sequence ) generator to obtain an encrypted address before transmittal. This technique is known as direct sequence spread spectrum technique. Such a system has belongings that for the interceptor, the standard message appears like noise & A ; therefore prevents him from listen ining.

However, the coveted party can decode the message with local reproduction of the pseudo-random noise ( PN sequence ) available with him.

Pulse codification transition

Pulse codification transition ( PCM ) was developed by AT & A ; T in 1937 at their Paris laboratories Alex H. PCM is the preferable method of communicating within the public switched telephone web. Pulse codification transition is the lone one of the digitally encoded transition techniques shown in Figure 2.

1 that is used for digital transmittal.With PCM, the pulsations are of fixed length and fixed amplitude. PCM is a binary system where a pulsation or deficiency of a pulsation within a prescribed clip slot represents either a logic 1 or logic 0 status. PWM, PPM, and PAM are digital but seldom binary, as pulsation does non stand for a individual binary figure ( spot ) .Figure 2.2 shows a simplified block diagram of a single- channel, simplex ( one manner merely ) PCM system.

The bandpass filter limits the frequence of the input analog signal to the standard voice – set frequence scope of 300 Hz to 3000Hz. The sample and keep circuit sporadically samples the parallel input signal and convert those samples to a multilevel PAM signal. The parallel – to digital convertor ( ADC ) converts the PAM samples to parallel PCM codifications, which are converted to consecutive informations in the parallel- to-serial convertor so outputted onto the transmittal line. The transmittal line repeaters sporadically regenerate the PCM codifications.In the receiving system, the serial-to-parallel convertor converts consecutive informations from the transmittal line to parallel PCM codifications. The digital-to-analog convertor ( DAC ) converts the parallel PCM codification to multilevel PAM signals.

The clasp circuit and low base on balls filter convert the PAM signal back to its original parallel signifier.Figure 2.2 besides shows several redstem storksbills and samples pulsations. An incorporate circuit that performs the PCM encryption and decrypting map is called a codec ( coder/decoder ) .


1 PCM Sampling:

The map of a trying circuit in a PCM sender is to sporadically try the continually altering parallel input signal and convert those samples to a series of pulsations that can more easy be converted to binary PCM codification. For ADC to accurately change over a signal to a binary codification, the signal must be comparatively changeless. If non, before the ADC can finish the transition, the input would alter and the ADC would be continually trying to follow the parallel alterations and may ne’er stabilise on any PCM codification.There are two basic techniques used to execute the sample and hold map: natural and flat-top sampling. Natural sampling is shown in Figure 2.3.

Natural sampling is when the tops of the sampled parallel wave form retain their natural form. In Figure 2.3 ( a ) , the FET parallel switch merely grounds the input wave form when the sample pulsation is high. When the sample pulsation is low, nevertheless, the input signal is allowed to go through unchanged through the end product amplifier to the input of the analog-to-digital convertor. The wave form for a of course sampled signal resembles a series of every bit separated pulsations with rounded tops, as shown in Figure 2.3 ( B ) .With natural sampling, the frequence spectrum of the sampled end product is different from that of an ideal sample.

The amplitude of the frequence constituents produced from narrow, finite-width pulsations decreases for the higher harmonics in a ( sin ten ) /x mode. This alters the information frequence spectrum necessitating the usage of frequence equalisers ( compensation filters ) before recovery by a low-pass filter.The most common method used for trying voice signals in PCM systems is flattop sampling, which is accomplished in a sample-and-hold circuit. The intent of a sample-and-hold circuit is to sporadically try the continually altering parallel input signal and convert those samples to a series of constants-amplitude PAM degrees. Flat-topped trying alters the frequence spectrum and introduces an mistake called aperture mistake, which prevents the recovery circuit in the PCM receiving system from precisely reproducing the original parallel signal.

The magnitude of mistake depends on how much the linear signal alterations while the sample I being taken.The conventional diagram of a sample-and-hold circuit. The FET acts as a simple parallel switch.

When turned ON, Q1 provides a low-impedance way to lodge the parallel sample electromotive force across capacitance C1. The clip that Q1 is ON is known as aperture or acquisition clip. Basically, C1 is the clasp circuit. When Q1 is OFF, C1 dose non hold a complete way to dispatch through and, hence, shops the sampled electromotive forces. The storage clip of the capacitance is called the A/D transition clip, because it is during this clip that the ADC converts the sample electromotive force to a PCM codification. The acquisition clip should be really short to guarantee that a minimal alteration occurs in the linear signal while it is being deposited across C1.

If the input to the ADC is altering while it is executing the transition, aperture deformation consequences. Therefore, by holding a short aperture clip and maintaining the input to the ADC severally changeless, the sample-and-hold circuit can cut down aperture deformation. Flat-topped trying introduces less aperture deformation than natural sampling and requires a slower analog-to-digital convertor.Figure 2.

4 ( B ) shows the input analog signal, the sampling pulsation, and the wave form developed across C1. It is of import that the end product electric resistance of electromotive force follower Z1 and the on opposition of Q1 be every bit little as possible. This ensures that the RC charging clip invariable of the capacitance is unbroken really short, leting the capacitance to bear down or dispatch quickly during the short acquisition clip. The rapid bead in the capacitance electromotive force instantly following each sample pulsation is due to the redistribution of the charge across C1. The interelectrode electrical capacity between the gate and drain of the FET is placed in series with C1 when the FET is away, therefore moving as a capacitive electromotive force splitter web. Besides, note the gradual discharge across the capacitance during the transition clip. This is called sag and is caused by the capacitance dispatching through its ain escape opposition and the input electric resistance of electromotive force follower Z2. Therefore, it is of import that the input electric resistance of Z2 and the escape opposition of C1 be every bit high as possible.

Basically, electromotive force followings Z1 and Z2 isolate the sample-and-hold circuit ( Q1 and C1 ) from the input and end product circuitry.

2.2 Sampling Rate:

The Nyquist trying theorem establishes the minimal sampling rate ( degree Fahrenheit ) that can be used for a given PCM system. For a sample to be reproduced accurately at the receiving system, each rhythm of the parallel input signal ( fa ) must be sampled at least twice. Consequently, the minimal sampling rate is equal to twice the highest audio input frequence. If degree Fahrenheit is less than two times fa, deformation will ensue. The deformation is called aliasing or foldover deformation.

Mathematically, minimal Nyquist sample rate is fs a‰? 2fawhere degree Fahrenheit = minimal Nyquist sample rate ( Hertz )fa = highest frequence to be sampled ( Hertz )Basically, a sample-and-hold circuit is an AM modulator. The switch is a nonlinear device that has two inputs: the sampling pulsation and the input analog signal. Consequently, nonlinear commixture ( heterodyning ) occurs between these two signals. Figure 2.5 ( a ) shows the frequency-domain representation of the end product spectrum from a sample-and-hold circuit. The end product includes the two original inputs ( the sound and the cardinal frequence of the trying pulsation ) , their amount and difference frequences ( fs A± fa ) , all the harmonics of degree Fahrenheit and fa ( 2fs, 2fa,3fs,3fa, etc ) , their associated cross merchandises ( 2fs A± fa, 3fs A± fa, etc. ) .Because the sampling pulsation is a repetative wave form, it is made up of a series of harmonically related sine moving ridges.

Each of these sine moving ridgesis amplitude modulated by the parallel signal and produces amount and difference frequences symmetrical around each of the harmonics of degree Fahrenheit. Each amount and difference frequence generated is separated is particular from its several Centre frequence by fa. Equally long as degree Fahrenheit is atleast twice fa, none of the side frequences from one harmonic will slop into the sidebands of another harmonic and aliasing does non happen. Figure 2.

5 ( B ) shows the consequences when an parallel input frequence greater than fs /2 modulates fs. The side frequences from one harmonic crease over into the sideband of another harmonic. The frequence that folds over is an assumed name of the input signal ( hence, the names “ aliasing” or “foldover deformation ” ) .

If an alias side frequence from the first harmonics creases over into the audio spectrum, it can be removed through filtering or any other techniques.The input bandpass filter shown in Figure 2.2 is called an antialiasing or antifoldover filter. Its upper cutoff frequence is chosen such that no frequence greater than one-half of the sampling rate is allowed to come in the sample and keep circuit, therefore extinguishing the possibility of foldover deformation happening.With PCM, the parallel input signal is sampled, so converted to a consecutive binary codification. The binary codification is transmitted to the receiving system, where it is converted back to the original parallel signal. The binary codifications used for PCM are n-bits codifications, where N may be any positive whole number greater than 1.

The codifications presently used for PCM are sign-magnitude codifications, where the most important spot ( MSB ) is the mark spot and the staying spots are used for magnitude. Table-2.1 shows an n-bit PCM codification where N equals 3. The most important spot is used to stand for the mark of the sample ( logic1=positive and logic 0=negative ) . The two staying spots represent the magnitude. With two magnitude spots, there are four codifications possible for positive Numberss and four codifications possible for negative Numberss.

Consequently, there is a sum of eight possible codifications ( 23=8 ) .

2.3 Folded Binary Code:

The PCM codification shown in Table 2.

1 is called a folded binary codification. Except for the mark spot, the codifications on the bottom half of the tabular array are a mirror image of the codifications on the top half. ( If the negative codifications were folded over on the top of the positive codifications, they would fit absolutely. ) Besides, with folded double star there are two codifications assigned to zero Vs: 100 ( +0 ) and 000 ( -0 ) . For this illustration, the magnitude of the minimal measure size is 1V. Therefore, the maximal electromotive force that may be encoded with this strategy is +3V ( 111 ) or -3V ( 011 ) . If the magnitude of a sample exceeds the highest quantisation interval, overload deformation ( besides called extremum modification ) occurs.

Delegating PCM codifications to absolute magnitudes is called quantising. The magnitude of the minimal measure size is called declaration, which is equal in magnitude to the electromotive force of the least important spot ( Vlsb or the magnitude of the minimal measure size of the ADC ) . The declaration is the minimal electromotive force other than 0V that can be decoded by the DAC at the receiving system. The smaller the magnitude of the minimal measure size, the better ( smaller ) the declaration and the more accurately the quantisation interval will resemble the parallel sample.In Table 2.1, each three-bit codification has a scope of input electromotive forces that will be converted to that codification. For illustration, any electromotive force between +0.

5 and +1.5 will be converted to the codification 101. Any electromotive force between +1.5 and +2.5 will be encoded as 110. Each codification has a quantisation scope equal to + or – one-half the declaration except the codifications for +0V and -0V.

The 0-V codifications each have an input scope equal to merely one-half the declaration, but because there are two 0-Vcodes, the scope for 0V is besides + or – one-half the declaration. Consequently, the maximal input electromotive force to the system is equal to the electromotive force of the highest magnitude codification plus one-half of the electromotive force of the least important spot.Figure 2.6 shows an parallel input signal, the sampling pulsation, the corresponding PAM signal, and the PCM codification. The linear signal is sampled three times.

The first sample occurs at t1 when the parallel electromotive force is precisely +2V. The PCM codification that corresponds to try 1 is 110. Sample 2 occurs at clip t2 when the parallel electromotive force is -1V. The corresponding PCM codification is 001. To find the PCM codification for a peculiar sample, merely split the electromotive force of the sample by the declaration, convert it to an n-bit binary codification, and add the mark spot to it.

For sample 1, the mark spot is 1, bespeaking a positive electromotive force. The magnitude codification ( 10 ) corresponds to a binary 2. Two times 1V peers 2V, the magnitude of the sample.

  1. Linear transportation map ;
  2. Quantization ; ( degree Celsius ) Qe

Sample 3 occurs at clip t3. The electromotive force at this clip is about +2.6V.

The folded PCM codification for +2.6V is 2.6 /1 = 2.6. There is no codification for this magnitude. If consecutive estimate ADCs are used, the magnitude of the sample is rounded away to the nearest valid codification ( 111 or +3V for this illustration ) . This consequences in an mistake when the codification is converted back to parallel by the DAC at the receiver terminal. This mistake is called quantisation mistake ( Qe ) .

The quantisation mistake is tantamount to the linear white noise ( it alters the signal amplitude ) . Similar to resound, the quantisation mistake may add or deduct from the existent signal. Consequently, quantisation mistake is called quantisation noise ( Qn ) and its maximal magnitude is one-half the electromotive force of the minimal measure size ( Vlsb /2 ) . For this illustration, Qe = V/2 or 0.

5V.The input-versus-output transportation map for a additive analog-to-digital convertor ( sometimes called a additive quantizer ) . As the figure shows for a additive parallel input signal ( i.

e. , a incline ) , the quantal signal is a stairway. Therefore, as shown in Figure 2.7 ( degree Celsius ) , the maximal quantisation mistake is the same for any magnitude input signal.the same parallel input signal used in Figure2.8 being sampled at a faster rate. As the figure shows, cut downing the clip between samples ( increasing the sample rate ) produces a PAM signal that more closely resembles the original parallel input signal.

However, it should besides be noted that increasing the sample rate does non cut down the quantisation mistake of the samples.

2.4 Coding Efficiency:

Coding efficiency is a numerical indicant of how a PCM codification is utilised. Coding efficiency is the ratio of the minimal figure of spots required to accomplish a certain dynamic scope to the existent figure of PCM spots used. Mathematically, coding efficiency is


5 Signal to quantization noise ratio:

The three spot PCM coding strategy described in the preceding subdivision is a additive codification. That is, the magnitude between any two consecutive codifications is the same. Consequently, the magnitude of their quantisation mistake is besides the same.

The maximal quantisation noise is the electromotive force of the least important spot is divided by 2. Therefore, the worst possible signal voltage-to-quantization noise electromotive force ratio ( SQR ) occurs when the input signal is at its minimal amplitude ( 101 or 001 ) . Mathematically, the worst instance electromotive force SQR isFor a maximal amplitude input signal of 3 V ( either 111 or 011 ) , the maximal quantisation noise is besides the electromotive force of the least important spot divided by 2. Therefore, the electromotive force SQR for a maximal input signal status isFrom the predating illustration it can be seen that even though the magnitude of mistake remains changeless throughout the full PCM codification, the per centum of mistake does non ; it decreases as the magnitude or the input signal additions. As a consequence, the SQR is non changeless.The predating look for SQR is for electromotive force and presumes the maximal quantisation mistake and a changeless amplitude parallel signal ; therefore it is of small practical usage and is shown merely for comparing intents.

In world and as shown in Figure 2.6, the difference between the PAM wave form and the parallel input wave form varies in magnitude. Therefore, the signal to quantization noise ratio is non changeless. By and large, the quantisation mistake or deformation caused by digitising an parallel sample is expressed as an mean signal power to average noise power ratio. For additive PCM codifications ( all quantisation intervals have equal magnitudes ) , the signal power to quantisation noise power ratio ( besides called signal to distortion ratio or signal to resound ratio ) is determined as follows:where,R = opposition ( ohm )V = rms signal electromotive force ( Vs )Q = quantisation interval ( Vs )v2/R = mean signal power ( Watts )If the oppositions are assumed to be equal,

2.6 Coding Methods:

There are several coding methods used to quantise PAM signals into 2n degrees.

These methods are classified harmonizing to whether the cryptography operation proceeds a degree at a clip, a figure at a clip, or a word at a clip.

LEVEL-AT-A-TIME Cryptography:

This type of coding compares the PAM signal to a incline wave form while a binary counter is being advanced at a unvarying rate. When the incline wave form peers or exceeds the PAM sample, the counter contains the PCM codification.

This type of coding requires a really fast clock if the figure of spots in the PCM codification is big. Level-at-a-time cryptography besides requires that 2n consecutive determinations be made for each PCM codification generated. Therefore, level-at-a-time cryptography is by and large limited to low velocity application. Nonuniform cryptography is achieved by utilizing a nonlinear map as the mention incline.

DIGIT-AT-A-TIME Cryptography:

This type of coding determines each figure of the PCM codification consecutive.

Digit-at-a-time cryptography is correspondent to a balance where known mention weights are used to find an unknown weight. Digit-at-a-time programmers provide a via media between velocity and complexness. One common sort of Digit-at-a-time-coder, called a feedback programmer, uses a consecutive estimate registry ( SAR ) . With this type of programmer, the full PCM codification word is determined at the same time.

WORD-A-TIME Cryptography:

A Word-at-a-time programmers are brassy encoders and are more complex ; nevertheless, they are more suited for high-velocity applications.

One common type of word-at-a-time programmer uses multiple threshold circuits. Logic circuit sensed by a PAM input signal and bring forth the approximative PCM codification. This method is once more impractical for big values of N.


3.1 Definition of Spread Spectrum:

A transmittal technique in which a pseudo-noise codification, independent of the information informations, is employed as a transition wave form to “spread” the signal energy over a bandwidth much greater than the signal information bandwidth. At the receiving system the signal is “despread” utilizing a synchronised reproduction of the pseudo-noise codification.


2 Basic Principle of Spread Spectrum System:

The Principal types of Spread Spectrum are Direct Sequence ( DS ) , and Frequency Hopping ( FH ) . An over position of these systems is hereby given:pseudo displacement of the stage imposter displacement of the frequenceconsistent demodulation noncoherent

Direct Sequence Spread Spectrum ( DSSS )

A pseudo-noise sequence pnt generated at the modulator, is used in concurrence with an M-ary PSK transition to switch the stage of the PSK signal imposter indiscriminately, at the come offing rate Rc ( =1/Tc ) a rate that is integer multiple of the symbol rate Rs ( =1/Ts ) .The familial bandwidth is determined by the bit rate and by the base set filtering. The execution limits the maximal bit rate Rc ( clock rate ) and therefore the maximal spreading. The PSK transition strategy requires a consistent demodulation.

PN codification length that is much longer than a information symbol, so that a different bit form is associated with each symbol.

Frequency Hoping Spread Spectrum

A Pseudo-noise sequence pnt generated at the modulator is used in conjuction with an M-ary FSK transition to switch the bearer frequence of the FSK signal pseudurandomly, at the skiping rate Rh. The familial signal occupies a figure of frequences in clip, each for a period of clip Th ( = 1/Rh ) , referred as dwell clip. FHSS divides the available bandwidth into N channels and hops between these channels harmonizing to the PN sequence.

At each frequence hop clip the PN generator feeds the frequence synthesizer a frequence word FW ( a sequence of n french friess ) which dictates one of 2n frequence place fhl. Transmitter and receiving system follows the same frequence hop form.The familial bandwidth is determined by the lowest and highest hop place by the bandwidth per hop place ( a?†fch ) . For a given hop, instantaneous occupied bandwidth is the conventional M-FSK, which is typically much smaller than Wss. So the FHSS signal is a narrowband signal, all transmittal power is concentrated on one channel. Averaged over many hops, the FH/M-FSK spectrum occupies the full spread spectrum bandwidth. Because the bandwidth of an FHSS system merely depends on the tuning scope, it can be hopped over a much wider bandwidth than an DSSS system.Since the hops by and large result in stage discontinuity ( depending on the peculiar execution ) a noncoherent demodulation is done at receiving system.

With slow hopping there are multiple informations symbol per hop and with fast hopping there are multiple hops per informations symbol.

3.3 Basic rule of Direct Sequence Spread Spectrum

For BPSK transition the edifice blocks of a DSSS system are:

Input signal:

  • Binary informations dt with symbol rate Rs = 1/Ts ( =bitrate Rb for BPSK )
  • Pseudo-noise codification pnt with bit rate Rc = 1/Tc ( an whole number of Rs )


In the sender, the binary informations dt ( for BPSK, I and Q for QPSK ) is‘directly ‘ multiplied with the PN sequence pnt, which is independent of the binary informations, to bring forth the familial baseband signal txb:txb = dt. pntThe consequence of generation of dt with a PN sequence is to distribute the baseband bandwidth Rs of dt to a baseband bandwidth of Rc.


The spread spectrum signal can non be detected by a conventional narrowband receiving system.

In the receiving system, the baseband signal rxb is multiplied with the PN sequence pnr.

  • If pnr = pnt and synchronized to the PN sequence in the standard informations, than the cured binary informations is produced on dr. The consequence of generation of the spread spectrum signal rxb with the PN sequence pnt used in the sender is to despread the bandwidth of rxb to Rs.
  • If pnr a‰ pnt, than there is no dispreading action.

    The signal dr has a spread spectrum. A receiving system non cognizing the PN sequence of the sender can non reproduce the transmitted information.


4 Performance in the presence of intervention:

To simplify the presence of intervention, the spread spectrum system is considered for baseband BPSK communicating ( without filtrating ) .The standard signal rxb of the familial signal txb plus an linear inteferance I ( noise, other users, jammer, … .

.. ) :rxb = T xb + I = dt. pnt + ITo retrieve the original informations dt the standard signal rx0 is multiplied with a locally generated PN sequence pnr that is an exact reproduction of that used in the sender ( that is pnr = pnt and synchronized ) The multiplier end product is hence given by:dr = rxb. pnt = dt. pnt.

pnt + I. pntThe informations signal dt is multiplied twice by the PN sequence pnt, where as the unwanted inteferance I is multiplied merely one time.Due to the belongings of the PN sequence:pnt + pnt = +1 for all TThe multiplier end product becomes:dr = dt + I. pntThe informations signal dr is reproduced at the multiplier end product in the receiving system, except for the inteferance represented by the linear term I. pnt. Generation of the inteferance by the locally generated PN sequence, means that the distributing codification will impact the inteferance merely as it did with the information bearing signal at the sender.

Noise and inteferance, being uncorrelated with the PN sequence, becomes noise-like, addition in bandwidth and lessening in power denseness after the multiplier.After dispreading, the informations constituent dt is narrow set ( Rb ) whereas the inteferance constituent is wideband ( Rc ) . By using the dr signal to a baseband ( low-pass ) filter with a set breadth merely big plenty to suit the recovery of the informations signal, most of the inteferance constituent I is filtered out. The consequence of inteferance is reduced by treating addition ( Gp ) .

Narrowband inteferance:

The narrowband noise is spread by the generation with the PN sequence pnr of the receiving system. The power denseness of the noise is reduced with regard to the despread informations signal. Merely 1/Gp of the original noise power is left in the information baseband ( Rs ) .

Spreading and dispreading enables a bandwidth trade for treating addition against narrow set interfering signals. Narrow set inteferance would disenable conventional narrow set receiving systems.The kernel behind the inteferance rejection capableness of a spread spectrum system: the utile signal ( informations ) gets multiplied twice by the PN sequence, but the inteferance signal get multiplied merely one time.

Wideband intervention:

Generation of the standard signal with the PN sequence of the receiving system gets a selective despread of the informations signal ( smaller bandwidth, higher power denseness ) . The inteferance signal is uncorrelated with the PN sequence and is spread.Beginning of wideband noise:

  • Multiple Spread Spectrum user: multiple entree mechanism.

  • Gaussian Noise: There is no addition in SNR with dispersed spectrum: The big channel bandwidth ( Rc alternatively of Rs ) increase the standard noise power with Gp:

Ninfo = N0. BWinfo a Nss = N0. BWss = Ninfo.GpThe spread spectrum signal has a lower power denseness than the straight transmitted signal.

3.5 Mutiple Entree:

Code division multiple entree ( CDMA ) is a methode of multiplexing ( radio ) users distinct ( extraneous ) codifications.

All users can convey at the same clip, and each is allocated the full available frequence spectrum for transmittal. CDMA is besides known as Spread-Spectrum multiple entree ( SSMA ) .CDMA dose non necessitate the bandwidth allotment of FDMA, nor the clip synchronism of the single users needed in TDMA. A CDMA user has full clip and full bandwidth available, but the quality of the communicating decreases with an increasing figure of users ( BER ) .In CDMA each user:

  • Has it ‘s ain PN codification
  • Uses the same RF bandwidth
  • Transmits at the same time ( asynchronous or synchronal )

Correlation of the standard baseband spread spectrum signal rxb with the PN sequence of user 1 merely despreads the signal of user 1. The other user green goodss noise Nu for user 1.

3.6 Jamming:

The end of a jammer is to upset the communicating of his antagonist.

The ends of the communicator are to develop a jam- immune communicating system under the undermentioned premises:

  • A Complete impregnability is non possible
  • A The jammer has a anterior cognition of most system parametric quantities, frequence sets, timing, traffic, … .


  • A The jammer has no a anterior cognition of the PN spreading codification

Protection against thronging wave forms is provided by intentionally doing the information-beating signal occupy a bandwidth far in surplus of the minimal bandwidth necessary to convey it. This has the consequence of doing the familial signal presume a noise-like visual aspect so as to intermix into background.The familial signal is therefore enabled to propagate though the channel undetected by anyone who may be listening. Spread spectrum is a method of “camouflaging” the information-bearing signal.

3.6 Evaluation of SS:


  • Signal concealment ( lower power denseness, noise-like ) , non intervention with conventional systems and other SS systems.

  • Secure communicating ( privateness )
  • Code Division Multiple Access CDMA ( multi-user )
  • Mitigation ( rejection ) of multipath, hold merely the direct way
  • Protection to knowing intervention ( Thronging )
  • Rejection of unwilled intervention ( narrowband )
  • Low chance of sensing and interception ( LPI )
  • Avability of licence-free ISM ( Industrial, Scientific and Medical ) frequency-bands.


  • No improve in public presentation in the presence of Gaussian noise
  • Increased bandwidth ( frequence use, wideband receiving system )
  • Increased complexness and computational burden.



1 Design:

In this undertaking, the operation & A ; building of DSSS is described. The block diagram of the strategy is shown in figure. The system is divided into two subdivisions:

  • Sender
  • Receiver

Transmitter subdivision has two parts, Pulse Code modulator ( ADC ) & A ; Encryptor.Receiver subdivision besides consists of two parts Decryptor & A ; Pulse Code Demodulator ( DAC ) .

4.2 Principle:

The voice input given to microphone is amplified by audio power amplifier and Federal to ADC.

The ADC used in building provides 8-bit equivalent of sampled address in parallel signifier. This is converted into consecutive spot stream utilizing a Multiplexer. Modulo-2 adder is used to obtain the enciphered address by adding the ( PCM ) signal with the end product of Pseudo-random figure generator. The encrypted signal is transferred to the receiving system through wire.

In Receiver, this signal is decrypted by utilizing Modulo-2 adder, by adding this signal with the same PN sequence that is used at sender. Then, the decrypted signal is fed to pulsate codification modulator which performs address demodulation. Then this signal is amplified and fed to talker.

4.6 Working:

Sender Section

In first phase to change over the voice signal into electrical signal, we use the capacitor mic.Condenser mic is a inactive transducer so it requires opposite supply which is given by 3.3K resistance and so to match the signals, with following phase, we use capacitance of 5.6Kpf.

This phase is a set base on balls filter used to band restrict the signal so that we can make up one’s mind the trying rate to change over the signal into digital signifier. In this phase, we have utilizing one of four OP-AMPS ( LM 324 ) . Hence, this is an active filter. Bandwidth of this filter is chosen 400Hz to 3.

4 KHz. We have decided the values of constituent for this phase by the expression for the first of the filterFc = 1/ ( 4?»RC )In the following phase, we are utilizing secondary OP-AMP of this IC, merely as an amplifier. The end product of this phase is coupled to following OP-AMP, to hike the current capacitance. Hence, we configure this phase as Buffer Amplifier.

The end product of this phase is the input for sample and hold phase to try the information ( signal ) , we use Ninety-nine 4066 which is a quartz parallel switch and to keep the value of this sample, we use 1.5Kpf capacitance. To protect the capacitance from dispatching during transition period, we use OP-AMP in Buffer manner because it provides the highest input electric resistance, so, capacitance holds the sampled value during transition clip for proper transition. Then, the end product of this phase is feeded to Analog to Digital Converter IC ADC 0804 which converts parallel signal into 8-bit digital signal. To protect the ADC from high electromotive force, we use a zener rectifying tube of 5Vs at its input phase because the input coming from old phase may make a value greater than 5V.Here, we are runing the ADC at 640 KHz frequence which is the optimal frequence of this IC.

At this frequence, IC provides the transition clip of 100 microseconds. The 640 KHz frequence is decided by the resistance and capacitance connected with pin 19 and pin 4 of IC. The scaling factor of IC is decided by the electromotive force at pin 9 so that we can alter the addition of ADC by altering biasing at pin 9. Now, to supply the start pulsation to get down transition, we generate a cheque by spliting standard 1 MHz clock by 4-bit counters.

The end product of this phase bring forth a clock of about 8 KHz. Since IC requires a low to high passage, the start transition, we invert the clock by utilizing simple HEX-INVERTER 7404.Now, this parallel information is converted into consecutive watercourse by 4051 Multiplexer. Because of informations at parallel input must stay unchanged during parallel to consecutive transition, we hold a information in a Latch 74574. The end product of pin 3 of IC4051 gives the consecutive informations.

Now, to distribute the spectrum, we multiply the information by sequence of 1 ‘s or 0 ‘s by utilizing IC 4077 which is EX-NOR IC. Now, to name the codification for generation, we use a clock of 1MHz. For blending the end products of multiple channels use the IC 4071. To bring forth a standard clock, we use Ninety-nine 4060 which is an oscillator plus splitter IC, so we can choose the needed clock by choosing proper crystal and proper spliting value by this IC. Here, we are utilizing 4 MHz crystal which is used to bring forth 1 MHz frequence with the aid of IC Cadmium 4060.

Receiver Section

In receiving system phase, the encoded signals are multiplied with 4077.

Here, besides, the same codification sequences are multiplied in the same manner as a transmitter side. Now, to avoid false sensing, we average the signal by utilizing simple RC planimeter and IC 4093 which is a Schmitt trigger NAND gate. To avoid any mistake during consecutive to parallel transition, we delay the switching clock of displacement registry by little sum of clip so it samples the available informations at right place. Now, the parallel information is taken from displacement registry 4015 which is latched in IC 74574 for proper transition into linear signal. To change over digital informations into parallel informations, we use DAC 0800 which is simple displacement D to A convertor. The end product of DAC contains the harmonic deformation because of quantisation mistake ; we smooth the signals by utilizing simple inactive Low Pass Filter and so, fed the signal for power elaboration to power amplifier IC LM 386.

Before feeding the signal into this IC, we use a variable registry so that we can utilize set the volume of the talker. Because DAC require double supply to work, we use two separate transformers to bring forth the supply.


1. Integrated Circuits:

S. No. IC No. Specification Pin Configuration Qty.

LM 324 Quad Op-Amp IC 14 Pin DIP 2
2. Cadmium 4066 Quad Analog Switch 14 Pin DIP 2
3. ADC0804 8-bit A to D Converter 20 Pin DIP 2
4. 7404 Hex Inverter 14 Pin DIP 1
5. 74574 8-bit D-Latch 20 Pin DIP 3
6. Cadmium 4051 8-Channel Analog Multiplexer 16 Pin DIP 5
7. Cadmium 4060 14-Stage Counter/ Divider/Oscillator 16 Pin DIP 1

Cadmium 4520 Double Binary Counter 16 Pin DIP 1
9. Cadmium 4077 Quad EX-NOR Gate 14 Pin DIP 2
10. Cadmium 4071 Quad 2 input OR Gate 14 Pin DIP 1
11. Cadmium 4093 Quad 2 input NAND Schmitt Trigger 14 Pin DIP 1
12. Cadmium 4015 Double 4 spot inactive Shift Register 16 Pin DIP 1
13. DAC 0800 8-bit D to A Converter 16 Pin DIP 1

LM 386 Low Voltage Audio Power Amplifier 8 Pin DIP 1
15. 7805 5V fixed regulated IC 3 Pin Flat Pack. 2

2. Resistors:



Resistance Value Specification Qty
1. 3.

3 KI©

A? Watt 2
2. 470 KI© A? Watt 2
3. 10 KI© A? Watt 20
4. 6. 8 KI© A? Watt 2
5. 10 MI© A? Watt 2
6. 37 KI© A? Watt 2
7. 4.


A? Watt 2

3. Preset ( Variable Resistor ) :

S.No Resistance Value Qty

2 KI© 2
2. 50 KI© 2
3. 20 KI© 2
4. 10 KI© 1

4. Capacitors:



Capacitance Value Specification Qty
1. 5.6 nF Ceramic Capacitor 2
2. 4.7 nF Ceramic Capacitor 2
3. 1 I?f, 63 Volt Electrolytic Capacitor 5

1.5 nF Ceramic Capacitor 2
5. 150 pF Ceramic Capacitor 2
6. 15 pF Ceramic Capacitor 1
7. 0.

01 I?F

Ceramic Capacitor 3
8. 0.1 I?F Ceramic Capacitor 2

220 I?F,10 V Electrolytic Capacitor 1
10. 1000 I?F, 16 Volt Electrolytic Capacitor 3

5. Diodes:



Diode Name Specification Qty
1. Inch 4007 PN Junction rectifying tube 6
2. Zener Diode Provides 5V Regulation 2

6. Oscillators:

S.No Oscillator Name Specification Qty
1. Crystal Oscillator Operating at 4 MHz 1



S.No Transformer Value Specification Qty
1. 9-0-9, 500mA Step Down Transformer 3

8. Microphone:

S.No Microphone Name Specification Qty

Condenser Mic. Directivity- OmnidirectionalS/N Ratio – approximately 40 dubniumsDistortion – Low ( 1 % )O/p Impedance – High ( 100I© ) 2

9. Speaker:

S.No Speaker Name Specification Qty
1. Loudspeaker 8I© , 50 W ( Max.

) , 10W ( Normal )


10. Switchs:

S.No. Switch Name Qty.

1. Push to ON Switch 24

11. Connecting Wires:

We have used thread wires.


C.B. Construction

It is an of import procedure in the fiction of electronic equipment. The design of PCBs ( Printed Circuit Boards ) depends on circuit demands like noise unsusceptibility, working frequence and electromotive force degrees etc. High power PCBs require a particular design scheme.The fiction procedure to the printed circuit board will find to a big extent the monetary value and dependability to the equipment. A common mark aimed is the fiction of little series of extremely dependable professional quality PCBs with low investing cost.

The mark becomes particularly of import for usage tailored equipment in the country of industrial electronics.The layout of a PCB has to integrate all the information of the board before one can travel on the graphics readying. This means that a construct, that clearly defined all the inside informations of the circuit and partially besides of the concluding equipment is prerequisite before the existent ballad out can get down. The elaborate circuit diagram is really of import for the layout interior decorator but he must besides be familiar with the design construct and with the doctrine behind the equipment.


The two most popular PCB types are:

1. Single Sided Boardss

The individual sided PCBs are largely used in amusement electronics where fabrication costs have to be kept at a lower limit.

However in industrial electronics cost factors can non be neglected and individual sided boards should be used wherever a peculiar circuit can be accommodated on such boards.

2. Double Sided Boardss

Double-sided PCBs can be made with or without plated through holes. The production of boards with plated through holes is reasonably expensive. Therefore plated through holes boards are merely chosen where the circuit complexnesss and denseness does non go forth any other pick.

6.2 Chronology:

The undermentioned stairss have been followed in transporting out the undertaking.

  • Study the books on the relevant subject.

  • Understand the working of the circuit.
  • Fix the circuit diagram.
  • Fix the list of constituents along with their specification. Estimate the cost and secure them after transporting out market study.
  • Plan and fix PCB for mounting all the constituents.
  • Fix the constituents on the PCB and solder them.

  • Test the circuit for the coveted public presentation.
  • Trace and rectify mistakes if any.
  • Give good coating to the unit.

  • Fix the undertaking study.

6.3 DESIGN Specification:


The chief intent of printed circuit is in the routing of electric currents and signal through a thin Cu bed that is bounded steadfastly to and insulating base stuff some clip called the substrate. This base is manufactured with an built-in bounded bed of thin Cu foil which has to be partially etched of other wise remove to get at a pre designed form to suite the circuit connexions or whatever other application is noted.

The term printed circuit board is derived from the original method where by a printed form is used as the mask over wanted countries of Cu. The PCB provides an ideal mopboard upon which to piece and keep steadfastly most of the little constituents.From the builder ‘s point of position, the chief attractive force of utilizing PCB is its function as the mechanical support for little constituents. There is less demand for complicate and clip devouring metal work of human body contraceptive method except possibly in supplying the concluding enclosure. Most consecutive frontward circuit designs can be easy covered in to publish wiring layer the idea required to transport out the inversion cab footed high visible radiation an possible mistake that would otherwise be missed in conventional point to indicate wiring.

The finished undertaking is normally orderly and truly a work of art.Actual size PCB layout for the circuit shown is drawn on the Cu board. The board is so immersed in FeCl3 solution for 12 hours.

In this procedure merely the exposed Cu part that is etched out by the solution.Now the gasoline washes out the pigment. Now the Cu layout on PCB is rubbed with a smooth sand paper easy and lightly such that merely the oxide beds over the Cu is removed.

Now the holes are drilled at the several topographic points harmonizing to constituent layout as shown in figure.


When planing the layout one should detect the minimal size ( component organic structure length and weight ) . Before get downing to plan the layout we need all the needed constituents in manus so that an accurate appraisal of infinite can be made.

Other infinite consideration might besides include from instance of mounted constituents over the printed circuit board or to entree way to present constituents.It might be necessary to turn some constituents round to a different angular place so that terminuss are closer to the connexions of the constituents. The graduated table can be checked be positioning the constituents on the squared paper. If any connexion crosses, so one can reroute to avoid such status.All common or earth lines should ideally be connected to a common line routed around the margin of the layout. This will move as the land plane. If possible attempt to route the outer supply line to the land plane.

If possible attempt to route the other supply lines around the opposite border of the layout to through the centre. The first set is rupturing the circuit to extinguish the crossing over with out changing the circuit item in any manner.Plan the layout looking at the topside to this board. First this should be translated reverse subsequently for the etching pattern big countries rate recommended to keep good Cu adhesive it is of import to bear in head ever that Cu path breadth must be harmonizing to the recommended lower limit dimensions and allowance must be made for increased breadth where expiration holes are needed. From this facet, it can go small slippery to negociate the path to link little transistors.There are fundamentally two ways of Cu interconnectednesss pattern in the under side to the board. The first is the remotion of merely the sum of Cu necessary to insulate the junction to the constituents to each other ensuing in the big countries of Cu. The 2nd is to do the interconnectedness form looking more similar conventional point wiring by routing unvarying breadth of Cu from constituent to constituent.

( C ) Etching Procedure:

Etching procedure requires the usage of chemicals acid immune dishes and running H2O supply. Ferric chloride is largely used solution but other etching stuffs such as ammonium per sulfate can be used. Azotic acid can be used but in general it is non used due to toxicant exhausts.The form prepared is glued to the Cu surface of the board utilizing a latex type of adhesive that can be cubed after usage. The form is laid steadfastly on the Cu utilizing a really crisp knife to cut round the form carefully to take the paper matching to the required Cu form countries. Then use the resist solution, which can be a sort of ink proportion for the intent of keeping smooth clean lineations every bit far as possible. While the board is drying, trial all the constituents.Before traveling to following phase, look into the whole form and cross cheque against the circuit diagram. Check for any free metal on the Cu. The etching bath should be in a glass or enamel phonograph record. If utilizing crystal of ferric- chloride these should be exhaustively dissolved in H2O to the relative suggested. There should be 0.5 lt. of H2O for 125 gram of crystal.Waste liquid should be exhaustively deflated and dried in H2O land. Never pour down the drain. To forestall atoms of Cu impeding farther etching, agitate the solutions carefully by gently writhing or swaying the tray.The board should non be left in the bath a minute longer than is needed to take merely the right sum of Cu. Inspite of there being a resistive coating there is no protection against etching off through exposed Cu borders. This leads to over etching. Have running H2O ready so that etched board can be removed decently and rinsed. This will hold etching instantly.Drilling is one of those operations that calls for great attention. For most intents a 1mm drill is used. Drill all holes with this size foremost those that need to be larger can be easy drilled once more with the appropriate larger size.


From the greatest assortment of electronic constituents available, which runs into 1000s of different types, it is frequently a vexing undertaking to cognize which is right for a given occupation.There could be damage such as hairline cleft on PCB. If there are, so they can be repaired by soldering a short nexus of bare Cu wire over the affected portion.The most popular method of keeping all the points is to convey the wires far apart after they have been inserted in the appropriate holes. This will keep the constituent in place ready for soldering.Some constituents will be well larger.So it is best to get down mounting the smallest first and come oning through to the largest. Before get downing, be certain that no farther boring is likely to be necessary because entree may be impossible subsequently.Next will likely be the resistance, little signal rectifying tubes or other similar size constituents. Some capacitances are besides really little but it would be best to suit these after wards. When suiting each group of constituents mark off each one on the constituents as it is fitted and if we have to go forth the occupation we know where to recommence.Although transistors and integrated circuits are little points there are good grounds for go forthing the bonding of these until the last measure. The chief point is that these constituents are really sensitive to heat and if subjected to drawn-out application of the bonding Fe, they could be internally damaged.All the constituents before mounting are rubbed with sand paper so that oxide bed is removed from the tips. Now they are mounted harmonizing to the constituent layout.

( Tocopherol ) Bonding: –

Soldering is an debasing procedure whereby a little sum of soft metal is made to run between the two metals to be joined, thereby blending or debasing them. Flux is to be applied to the tips and so the constituents are soldered. All solder metals have low runing the metal to be joined. Following points must be remembered while bonding:

  • Components must non be loose.
  • Avoid formation of Solder Bridge.
  • Over warming of constituent or IC ‘s may damage it for good.

( F ) Precaution:

  • All connexion leads should every bit little as possible to cut down unwanted inductive reactance.
  • The bonding Fe being used for soldering of semiconducting materials should be of low electromotive force.
  • While soldering semiconducting materials, heat sinks should be used.
  • While soldering solder should non distribute over the full circuit, and solder tip should be crisp and smooth.
  • While mounting constituents values should be seeable.
  • Semiconductors and other polarized constituents should be mounted with right mutual oppositions.


7.1 Sender:

7.2 Receiver:


A Digital Speech Security System brings out the assorted operations involved in the procedure. Front panel trial points provided can be used to demo wave forms of redstem storksbills, PCM signals, encrypted address, receives o/p wave form before and after filtrating. Even though for presentation intent, a sinusoidal signal of 2V p-p is preferred, Speech signal from a mike amplified can besides be applied and the o/p can be maintained utilizing speaker unit. The faculty can be used as a presentation theoretical account for communicating technology pupils.


This undertaking can be enhanced for long distance or wireless communicating for bettering B.W. of by modulating the encrypted signal by either QPSV or BPSV.This system can be made multi user by utilizing CDMA ( Code Division Multiple Access ) , where multiple signal occupy the BW are to be transmitted at the same time without interfacing with one another. As each user has its ain codification which perform the direct sequence particular spectrum transition. This multiple entree system has alone advantage over other multiple entree system that the passage clip of users day of the months symtrds do non hold to co-occur with those of other users. For carry throughing this alone characteristic of CDMA, we have to put in a computing machine system loaded with exchanging package.


  • Taub Schilling – Principles of Communication Systems
  • Wayne Tomasi – Advance Electronics & A ; Communication System
  • R.G.Gupta – Audio & A ; Video Systems
  • K. R. Botkar – Integrated Circuits
  • Ramakant Gaikwad – Operational Amplifiers
  • Timothy Pratt – Satellite Communication
  • M. Morris Mano – Digital Logic & A ; Computer Design
  • CMOS Datasheet – 40XX Series
  • CMOS Datasheet – 74XX Series

Web Sites

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