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### Lmece407

2. 2. Experiment No 1 OBJECT By the use of the slotted line a) To determine the unknown frequency b) To determine the Voltage Standing Wave Ratio (VSWR) and Reflection Coefficient. APPARATUS Transmitter Mod MW-TX, One slotted line MW-5. Loads of different values (OC,SC,75Ω,50Ω,100Ω) RF cable (Zo=75Ω) Voltmeter THEORY When power is applied to transmission line, voltage & current appear. If Z L=ZO, load absorbs all power & none is reflected. If ZL≠ZO, some power is absorbed & rest is reflected. We have one set of Voltage & Current waves traveling towards load & a reflected set traveling back to generator. These sets of traveling waves, in opposite directions, set up an interference pattern called Standing Waves. Maxima (antinodes) & minima (node) of Voltage & Current occur at fixed positions. The slotted line is used to measure voltage and current directly on the various sections of a coaxial line, as by the slot you can enter the electrical and magnetic fields between the two connectors constituting the coaxial line. In presence of standing wave, the voltage (or current) maximum and minimum value can bee seen; the distance between a maximum and the adjacent minimum is equal to one fourth the wave length; the speed factor of the line is equal to1 because the dielectric is air. Once the speed factor is known, by measuring the distance between two minima and multiplying it by two, it is possible to obtain the frequency of the signal applied to the slotted line, if this is unknown. The standing wave ratio (SWR) is equal to the ratio to the maximum to the minimum value; in fact, on the maximum, the direct and reflected wave value (of voltage and current) are added and on the minimum are subtracted. If the reflected wave does not exist, voltage and current keep constant along all the line and their ratio is equal to the characteristic impedance Zo; the SWR is equal to 1. Such a line is called a flat line. The output power of the generator, tuned to the lowest frequencies (for example 701.5 MHz), must be regulated to the maximum, connect the output of the generator to the
3. 3. slotted line with 75 Ω cable, 1 m long, connect 75 Ω to the other end of the slotted line: the line is thus terminated on its characteristic impedance. If the machining is perfect, by moving the probes along the slotted line the signal amplitude will keep almost constant any way there may be variations which are due to the connectors or to slight variation of the probes alignment. Change the termination of 75 Ω with a 50 Ω and measure the voltage along the line: it has stronger minimum and maximum values than the last ones. Check if the distance between minimum and maximum is equal to ¼ the wavelength, in other words by varying the frequency and repeating measurement, you can observe how the distance between max an min is longer or shorter if you decrease or increase the frequency repeat the exercise with termination of 100 ohm Note that, with the help of slotted line, you can distinguish if the load is greater or smaller than the characteristic impedance of the line, In fact, with 100 ohm the voltage minimum is at ¼ wave length from the load, while on the load there is a maximum; with 50 ohm, the voltage minimum is on the load. PROCEDURE 1. Connect the generator (transmitter) to the slotted line through RF cable. 2. Terminate the line by attaching a load (ZL) on other end of line. 3. Insert probes of voltmeter in the slots provided on the trailer of the slotted line. 4. Turn on the generator and excite the cable with RF waves. 5. Move the trailer on the slotted line. Positions of maximum & minimum voltage appear alternately on the slotted line. 6. Note down the max & min values of voltage. 7. Also note down the positions of the voltage minima and voltage maxima on the scale 8. Determine VSWR by the following formula: Measured VSWR= V max / V min 9. Determine the calculated VSWR by the formula:
4. 4. VSWR = 1 + Г 1-Г where Г= ZL – Z0 ZL + Z0 10. Calculate the unknown frequency with the help of the following formula. λ / 2 =distance between consecutive V maxima or minima f=c/λ 11. Repeat same procedure for different loads (ZL). OBSERVATIONS Frequency of incident wave = CALCULATIONS RESULT PRACTICAL NO 2
7. 7. Is there any significant difference between the plots, now? _____________________________________________________________________You will find that the addition of a second reflector has little effect on the gain and directivity of the antenna, irrespective of the spacing between the two reflectors. ADDING DIRECTORS 28. Remove the second reflector element from the boom. 29. Launch a new signal strength vs. angle 2D polar graph window. 30. Acquire a new plot at 1500 MHz. 31. Observe the polar plot 32. Mount a parasitic element about 5cm in front of the driven 33. element and adjust its length to 8.5cm. 34. Acquire a second new plot at 1500 MHz. 35. Observe the polar plot. Is there any significant difference between the two plots? _____________________________________________________________________ 36. Move the director to about 2.5 cm in front of the driven element. 37. Acquire a third new plot at 1500 MHz 38. Observe the polar plot. How does the new plot compare with the previous two? ____________________________________________________________________ 39. Launch another new signal strength vs. angle 2d polar graph window. 40. Acquire a new plot at 1500 MHz. 41. Add a second director 5 cm in front of the second. 42. Acquire a second new plot at 1500 MHz. 43. Add a third director 5 cm in front of the second. 44. Acquire a third new plot at 1500 MHz. 45. Add a fourth director 5 cm in front of the third. 46. Acquire a fourth new plot at 1500 MHz How do the gains and directivities compare? _____________________________________________________________________ 47. Launch another new signal strength vs. angle 2D polar graph window. 48. Acquire a new plot at 1500 MHz. 49. Move the reflector to 2.5 cm behind the driven element. Acquire a second new plot at 1500 MHz. Does the driven element – reflector spacing have much effect on the gain or directivity of the antenna? _____________________________________________________________________ RESULT The addition of a second parasitic dipole element close to the driven dipole gives rise to a change in directivity and an increase in gain in a preferred direction. It also showed that the length of the parasitic element had an effect on the direction of maximum gain. If the parasitic element is the same length, or longer than the driven element the gain is in a direction from parasitic element to driven element. The parasitic element acts as a reflector. If the parasitic element is shorter than the driven element the gain is in a direction from driven element to parasitic element. The parasitic element acts as a director.
8. 8. PRACTICAL NO 3 OBJECT To study the effect of thickness of conductors upon the bandwidth of dipole. APPARATUS Electronica Veneta (turntable) with stand Field meter SFM 1 EV Microwave generator 75ohm coaxial cable Basic dipole antenna short thick conductors (8mm) Basic dipole antenna Short Thin dipole (3mm) THEORY Dipole: It consists of two poles that are oppositely charged. Dipole antenna: The simple dipole is one of the basic antennas. It is an antenna with a center-fed driven element for transmitting or receiving radio frequency energy. This is the directed antenna i.e. radiations take place only forward or backward. Its characteristic impedance is 73Ω. Half wave dipole: Half wave dipole is an antenna formed by two conductors whose total length is half the wave length. In general radio engineering, the term dipole usually means a half-wave dipole (center-fed). Thin and thick dipole Theortically the dipole length must be half wave; this is true if the wavelength/conductor’s dia ratio is infinite. Usually there is a shortening coefficient K (ranging from 0.9-0.99)according to which the half wavelength in free space must be multiplied by K in order to have the half wave dipole length, once the diameter of the conductor to be used is known.(refer fig) Bandwidth: The range of frequencies in which maximum reception is achieved. Effect of thickness: By increasing the conductor diameter in respect to the wavelength, the dipole characteristic impedance will increase too in respect to the value of 73Ω. On the other hand outside the center frequency range, the antenna reactance varies more slowly in a thick than in a thick antenna. This means, with the same shifting in respect to the center frequency, the impedance of an antenna with larger diameter is more constant and consequently the SWR assumes lower values. Practically the BANDWIDTH is wider.
9. 9. PROCEDURE 1. Construct a dipole with arms of 3mm diameter (short) and mount on the central support of the tuntable. 2. Set the antenna and instruments as shown in figure. 3. Set the generator to a determinate output level and to the center frequency of the antenna under test. 701.5 MHz for measurements with short (thick or thin dipole) 4. Adjust the dipole length and sensitivity of the meter to obtain the maximum reading (10 th LED glowing) 5. Now decrease the frequency up to the value such that the 10 th LED keeps on glowing. Note the value as f2. 6. Now increase the frequency up to the value such that the 10 th LED keeps on glowing. Note the value as f1. 7. Note down the difference between these two frequencies, this will be the bandwidth
10. 10. 8. Calculate the wavelength for the resonance frequency of around 700 MHz for short dipole using the formula f/c=λ 9. The ratio used for calculating the shortening coefficient is 2/λ where d=dia of conductor 11. From graph obtain a shortening coefficient K. 12. Calculate the physical length of Dipole and compare with the measured length. Physical length of half wavelength dipole= x K 13. Construct a dipole with arms of 8mm diameter (short). 14. Repeat the same procedure for thick dipole. OBSERVATIONS & CALCULATIONS Resonant frequency= MHz f/c=λ= 300/ = cm Measured length of short dipole thin= 220mm Measured length of short thick dipole=195mm THIIN dipole: d= The ratio used for calculating the shortening coefficient is (with a dia of 3mm) =d2/λK= From graph we obtain a coefficient. of 0.960 for the thin dipole Physical length of half wavelength dipole= x K= THICK dipole: d= The ratio used for calculating the shortening coefficient is with a diameter of 8mm =d2/λK= From graph we obtain a coefficient of 0.947 for thick dipole Calculated physical length of half wavelength dipole= x K= (These values refer to a dipole in air. Actually the dipole under consideration is not totally in air because for mechanical reasons, its internal part is in a dielectric. This slightly increases the resonance frequency.) RESULT With the same shifting in respect to the center frequency, the impedance of an antenna with larger diameter is more constant and consequently the SWR assumes lower values. Practically the BANDWIDTH is wider. In other words increasing the thickness of conductor has an effect upon the bandwidth of the dipole. Thicker the conductor larger would be the bandwidth.
11. 11. PRACTICAL NO 4 OBJECT OBUnderstand the terms ‘baying’ and ‘stacking’ as applied to antennas. UnTo investigate stacked and bayed yagi antennas. ToTo compare their performance with a single yagi. THEORY Yagi antennas may be used side-by-side, or one on top of another to give greater gain or directivity. This is referred to as baying, or stacking the antennas, respectively. PROCEDURE (A)Baying Two Yagis 1. Connected up the hardware of AntennaLab. 2. Loaded the Discovery software. 3. Loaded the NEC-Win software. 4. Ensure that a Yagi Boom Assembly is mounted on the Generator Tower. 5. Building up a 6 element yagi. The dimensions of this are: Length Spacing Reflector 11 cm 5cm behind driven element Driven Element 10 cm Zero (reference) Director 1 8.5 cm 2.5 cm in front of DE Director 2 8.5 cm 5 cm in front of D1 Director 3 8.5 cm 5 cm in front of D2 Director 4 8.5 cm 5 cm in front of D3 6. Plot the polar response at 1500 MHz. 7. Without disturbing the elements too much, remove the antenna from the Generator Tower. 8. Identify the Yagi Bay base assembly (the broad grey plastic strip with tapped holes) and mount this centrally on the Generator Tower.
12. 12. 9. Mount the 6 element yagi onto the Yagi Bay base assembly at three holes from the centre. 10. Assemble an identical 6 element yagi on the other Yagi Boom Assembly and mount this on the Yagi Bay base assembly at three hole the other side of the centre, ensuring that the two yagis are pointing in the same direction (towards the Receiver Tower). 11. Identify the 2-Way Combiner and the two 183mm cables. 12. Connect the two 183mm cables to the adjacent connectors on the Combiner and their other ends to the two 6 element yagis. 13. Connect the cable from the Generator Tower to the remaining connector on the Combiner. 14. Acquire a new plot for the two bayed antennas onto the same graph as that for the single 6 element yagi. 15. Reverse the driven element on one of the yagis and acquire a third plot OBSERVATIONS Does reversing the driven element make much difference to the polar pattern for the two bayed yagis? ___________________________________________________________________________ How does the directivity of the two bayed yagis compare with the single yagi plot (with the driven element the correct way round)? ___________________________________________________________________________ How does the forward gain of the two bayed yagis compare with the single yagi plot (with the driven element the correct way round)? ___________________________________________________________________________ Now, move the two yagis to the outer sets of holes on the Yagi Bay base assembly. Ensure that you keep the driven elements the same way round as you had before to give the correct phasing. Superimpose a plot for this assembly. How do the directivity and forward gain of the wider spaced yagis compare with the close spaced yagis? ___________________________________________________________________________
13. 13. (B) Stacking Two Yagis 1. Identify the Yagi Stack base assembly (the narrow grey plastic strip with tapped holes) and mount this on the side of the Generator Tower. 2. Mount the 6 element yagi onto the Yagi Stack base assembly at one set of holes above the centre. 3. Plot the polar response at 1500 MHz. 4. Mount the other 6 element yagi on the Yagi Stack base assembly at the uppermost set of holes, ensuring that the two yagis are pointing in the same direction (towards the Receiver Tower) 5. Identify the 2-Way Combiner and the two 183mm coaxial cables. 6. Connect the two 183mm cables to the adjacent connectors on the Combiner and their other ends to the two 6 element yagis. 7. Connect the cable from the Generator Tower to the remaining connector on the Combiner. 8. Superimpose the polar plot for the two stacked antennas onto that for the single 6 element yagi. 9. Reverse the driven element on one of the yagis and superimpose a third plot. 10. Change the position of the lower yagi to the bottom set of holes on the Yagi Stack base assembly. Ensure that the driven elements are correctly phased and superimpose a fourth polar plot. OBSERVATIONS How does the directivity of the different configurations compare? ___________________________________________________________________________ How does the forward gain of the stacked yagis compare with the single yagi? ___________________________________________________________________________ How does the forward gain of the stacked yagis change when the driven element phasing is incorrect? ___________________________________________________________________________ RESULT
14. 14. PRACTICAL NO 5 OBJECT Implementation of Time Division Multiplexing system using matlab/simulink THEORY Time-division multiplexing (TDM) is a type of digital or (rarely) analog multiplexing in which two or more signals or bit streams are transferred apparently simultaneously as sub-channels in one communication channel, but are physically taking turns on the channel. The time domain is divided into several recurrent timeslots of fixed length, one for each sub-channel. A sample byte or data block of sub-channel 1 is transmitted during timeslot 1, sub-channel 2 during timeslot 2, etc. One TDM frame consists of one timeslot per sub-channel plus a synchronization channel and sometimes error correction channel before the synchronization. After the last sub-channel, error correction, and synchronization, the cycle starts all over again with a new frame, starting with the second sample, byte or data block from sub-channel 1, etc For multiple signals to share one medium, the medium must somehow be divided, giving each signal a portion of the total bandwidth. The current techniques that can accomplish this include • Frequency division multiplexing (FDM) • Time division multiplexing (TDM)-Synchronous vs statistical1 • Wavelength division multiplexing (WDM) • Code division multiplexing (CDM) Multiplexing: Two or more simultaneous transmissions on a single circuit Figure 5 Multiplexing Multiplexor (MUX) Demultiplexor (DEMUX) Time Division Multiplexing: Sharing of the signal is accomplished by dividing available transmission time on a medium among users. Digital signaling is used exclusively. Time division multiplexing comes in two basic forms: 1. Synchronous time division multiplexing, and 2. Statistical, or asynchronous time division multiplexing. Synchronous Time Division Multiplexing: The original time division multiplexing, the multiplexor accepts input from attached devices in a round-robin fashion and transmits the data in a never ending pattern. T-1 and ISDN telephone lines are common examples of synchronous time division multiplexing. If one device generates data at a faster rate than other devices, then the multiplexor must either sample the incoming data stream from that device more often than it samples the other devices, or buffer the faster incoming stream. If a device has nothing to transmit, the multiplexor must still insert a piece of data from that device into the multiplexed stream. So that the receiver may stay synchronized with the incoming data stream, the transmitting multiplexor can insert alternating 1s and 0s into the data stream. Three types popular today of Synchronous Time Division Multiplexing: • T-1 multiplexing (the classic) • ISDN multiplexing • SONET (Synchronous Optical NETwork)
15. 15. The T1 (1.54 Mbps) multiplexor stream is a continuous series of frames of both digitized data and voice channels. The ISDN multiplexor stream is also a continuous stream of frames. Each frame contains various control and sync info SONET – massive data rates Statistical Time Division Multiplexing: A statistical multiplexor transmits only the data from active workstations (or why works when you don’t have to). If a workstation is not active, no space is wasted on the multiplexed stream. A statistical multiplexor accepts the incoming data streams and creates a frame containing only the data to be transmitted. To identify each piece of data, an address is included. If the data is of variable size, a length is also included. More precisely, the transmitted frame contains a collection of data groups. A statistical multiplexor does not require a line over as high a speed line as synchronous time division multiplexing since STDM does not assume all sources will transmit all of the time! Good for low bandwidth lines (used for LANs) Much more efficient use of bandwidth! Matlab code for TDM: % *********** Matlab code for Time Division Multiplexing ************* clc; close all; clear all; % Signal generation x=0:.5:4*pi; % siganal taken upto 4pi sig1=8*sin(x); % generate 1st sinusoidal signal l=length(sig1); sig2=8*triang(l); % Generate 2nd traingular Sigal % Display of Both Signal subplot(2,2,1); plot(sig1); title('Sinusoidal Signal'); ylabel('Amplitude--->'); xlabel('Time--->'); subplot(2,2,2); plot(sig2); title('Triangular Signal'); ylabel('Amplitude--->'); xlabel('Time--->'); % Display of Both Sampled Signal subplot(2,2,3); stem(sig1); title('Sampled Sinusoidal Signal'); ylabel('Amplitude--->'); xlabel('Time--->'); subplot(2,2,4); stem(sig2); title('Sampled Triangular Signal'); ylabel('Amplitude--->'); xlabel('Time--->'); l1=length(sig1); l2=length(sig2); for i=1:l1 sig(1,i)=sig1(i); % Making Both row vector to a matrix sig(2,i)=sig2(i); end % TDM of both quantize signal tdmsig=reshape(sig,1,2*l1); % Display of TDM Signal figure
16. 16. stem(tdmsig); title('TDM Signal'); ylabel('Amplitude--->'); xlabel('Time--->'); % Demultiplexing of TDM Signal demux=reshape(tdmsig,2,l1); for i=1:l1 sig3(i)=demux(1,i); % Converting The matrix into row vectors sig4(i)=demux(2,i); end % display of demultiplexed signal figure subplot(2,1,1) plot(sig3); title('Recovered Sinusoidal Signal'); ylabel('Amplitude--->'); xlabel('Time--->'); subplot(2,1,2) plot(sig4); title('Recovered Triangular Signal'); ylabel('Amplitude--->'); xlabel('Time--->'); Results: Sinusoidal Signal Triangular Signal 10 8 5 6 Amplitude---> Amplitude---> 0 4 -5 2 -10 0 0 10 20 30 0 10 20 30 Time---> Time---> Sampled Sinusoidal Signal Sampled Triangular Signal 10 8 5 6 Amplitude---> Amplitude---> 0 4 -5 2 -10 0 0 10 20 30 0 10 20 30 Time---> Time--->
17. 17. TDM Signal 8 6 4 2 Amplitude---> 0 -2 -4 -6 -8 0 10 20 30 40 50 60 Time---> Recovered Sinusoidal Signal 10 5 Amplitude---> 0 -5 -10 0 5 10 15 20 25 30 Time---> Recovered Triangular Signal 8 6 Amplitude---> 4 2 0 0 5 10 15 20 25 30 Time--->
18. 18. PRACTICAL NO 6 OBJECT Implementation of pulse code modulation and demodulation using matlab/simulink THEORY ANALOG-TO-DIGITAL CONVERSION: A digital signal is superior to an analog signal because it is more robust to noise and can easily be recovered, corrected and amplified. For this reason, the tendency today is to change an analog signal to digital data. In this section we describe two techniques, pulse code modulation and delta modulation Pulse code Modulation (PCM): Pulse-code modulation (PCM) is a method used to digitally represent sampled analog signals, which was invented by Alec Reeves in 1937. It is the standard form for digital audio in computers and various Blu-ray, Compact Disc and DVD formats, as well as other uses such as digital telephone systems. A PCM stream is a digital representation of an analog signal, in which the magnitude of the analogue signal is sampled regularly at uniform intervals, with each sample being quantized to the nearest value within a range of digital steps. PCM consists of three steps to digitize an analog signal: 1. Sampling 2. Quantization 3. Binary encoding  Before we sample, we have to filter the signal to limit the maximum frequency of the signal as it affects the sampling rate.  Filtering should ensure that we do not distort the signal, ie remove high frequency components that affect the signal shape. Figure 6.1 Components of PCM encoder Sampling: • Analog signal is sampled every TS secs. • Ts is referred to as the sampling interval. • fs = 1/Ts is called the sampling rate or sampling frequency. • There are 3 sampling methods: – Ideal - an impulse at each sampling instant – Natural - a pulse of short width with varying amplitude – Flattop - sample and hold, like natural but with single amplitude value • The process is referred to as pulse amplitude modulation PAM and the outcome is a signal with analog (non integer) values •
19. 19. According to the Nyquist theorem, the sampling rate must be at least 2 times the highest frequency contained in the signal. Quantization: • Sampling results in a series of pulses of varying amplitude values ranging between two limits: a min and a max. • The amplitude values are infinite between the two limits. • We need to map the infinite amplitude values onto a finite set of known values. • This is achieved by dividing the distance between min and max into L zones, each of height ∆. ∆ = (max - min)/L Quantization Levels: • The midpoint of each zone is assigned a value from 0 to L-1 (resulting in L values) • Each sample falling in a zone is then approximated to the value of the midpoint. Quantization Error: • When a signal is quantized, we introduce an error - the coded signal is an approximation of the actual amplitude value. • The difference between actual and coded value (midpoint) is referred to as the quantization error. • The more zones, the smaller ∆ which results in smaller errors. • BUT, the more zones the more bits required to encode the samples -> higher bit rate Quantization Error and SNQR: • Signals with lower amplitude values will suffer more from quantization error as the error range: ∆/2, is fixed for all signal levels. • Non linear quantization is used to alleviate this problem. Goal is to keep SNQR fixed for all sample values. • Two approaches: • The quantization levels follow a logarithmic curve. Smaller ∆’s at lower amplitudes and larger ∆’s at higher amplitudes. • Companding: The sample values are compressed at the sender into logarithmic zones, and then expanded at the receiver. The zones are fixed in height. Bit rate and bandwidth requirements of PCM: • The bit rate of a PCM signal can be calculated form the number of bits per sample x the sampling rate Bit rate = nb x fs • The bandwidth required to transmit this signal depends on the type of line encoding used. Refer to previous section for discussion and formulas. • A digitized signal will always need more bandwidth than the original analog signal. Price we pay for robustness and other features of digital transmission. PCM Decoder: • To recover an analog signal from a digitized signal we follow the following steps: • We use a hold circuit that holds the amplitude value of a pulse till the next pulse arrives. • We pass this signal through a low pass filter with a cutoff frequency that is equal to the highest frequency in the pre-sampled signal. • The higher the value of L, the less distorted a signal is recovered.
20. 20. Matlab code of PCM: %********PCM************************************************** %the uniform quantization of an analog signal using L quantizaton levels% %****implemented by uniquan.m function of matlab %(uniquan.m) function [q_out,Delta,SQNR]=uniquan(sig_in,L) %usage % [q_out,Delta ,SQNR]=uniquan(sig_in,L) % L-number ofuniform quantization levels % sig_in-input signalvector % function output: % q_out-quantized output % Delta-quantization interval % SQNR- actual signal to quantization ratio sig_pmax=max(sig_in); % finding the +ve peak sig_nmax=min(sig_in); % finding the -ve peak Delta=(sig_pmax-sig_nmax)/L; % quantization interval q_level=sig_nmax+Delta/2:Delta:sig_pmax-Delta/2; %define Q-levels L_sig=length(sig_in); % find signal length sigp=(sig_in-sig_nmax)/Delta+1/2; % convert int to 1/2 to L+1/2 range qindex=round(sigp); % round to 1,2,.....L levels qindex=min(qindex,L); % eliminate L+1 as a rare possibility q_out=q_level(qindex); % use index vector to generate output SQNR=20*log10(norm(sig_in)/norm(sig_in-q_out)); % actual SQNR value end % sampandquant.m function executes both sampling and uniform quantization %sampandquant.m function [s_out,sq_out,sqh_out,Delta,SQNR]=sampandquant(sig_in,L,td,ts) % usage % [s_out,sq_out,sqh_out,Delta,SQNR]=sampandquant(sig_in,L,td,ts) % L-no. of uniform quantization levels % sig_in-input signal vector % td-original signal sampling period of sig_in % ts- new sampling period % NOTE: td*fs must be +ve integef % function outputs:
21. 21. % s_out-sampled output % sq_out-sample and quantized output % sqh_out-sample, quantized and hold output % Delta- quantization interval % SQNR-actual signal to quantization ratio if rem(ts/td,1)==0 nfac=round(ts/td); p_zoh=ones(1,nfac); s_out=downsample(sig_in,nfac); [sq_out,Delta,SQNR]=uniquan(s_out,L); s_out=upsample(s_out,nfac); sqh_out=upsample(sq_out,nfac); else warning('Error! ts/td is not an integer!'); s_out=[]; sq_out=[]; sqh_out=[]; Delta=[]; SQNR=[]; end end %********generation of PCM *****************************% clc; clear; clf; td=0.002; % original sampling rate rate 500 hz t=[0:td:1.]; %time interval of 1 sec xsig=sin(2*pi*t)-sin(6*pi*t); %n1hz +3 hz sinusoidals Lsig=length(xsig); Lfft=2^ceil(log2(Lsig)+1); Xsig=fftshift(fft(xsig,Lfft)); Fmax=1/(2*td); Faxis=linspace(-Fmax,Fmax,Lfft); ts=0.02; % new sampling rate =50 hz Nfact=ts/td; % send the signal through a 16-level uniform quantiser [s_out,sq_out,sqh_out1,Delta,SQRN]=sampandquant(xsig,16,td,ts); % obtaind the signal which is % - sampled,quantiser,and zero-order hold signal sqh_out % plot the original signal and PCM signal in time domain figrue(1); figure(1); subplot(211); sfig1=plot(t,xsig,'k',t,sqh_out1(1:Lsig),'b'); set(sfig1,'Linewidth',2); title('Signal {it g}({{it t}) and its 16 level PCM signal') xlabel('time(sec.)'); % send the signal through a 16-level unifrom quantiser [s_out,sq_out,sqh_out2,Delta,SQNR]=sampandquant(xsig,4,td,ts); % obtained the PCM signal which is % - sampled,quantiser,and zero_order hold signal sqh_out % plot the original signal and the PCM signal in time domain subplot(212); sfig2=plot(t,xsig,'k',t,sqh_out2(1:Lsig),'b'); set(sfig2,'Linewidth',2); title('Signal {it g}({it t}) and its 4 level PCM signal') xlabel('time(sec.)'); Lfft=2^ceil(log2(Lsig)+1); Fmax=1/(2*td); Faxis=linspace(-Fmax,Fmax,Lfft); SQH1=fftshift(fft(sqh_out1,Lfft));
22. 22. SQH2=fftshift(fft(sqh_out2,Lfft)); % Now use LPF to filter the two PCM signal BW=10; %Bandwidth is no larger than 10Hz. H_lpf=zeros(1,Lfft);H_lpf(Lfft/2-BW:Lfft/2+BW-1)=1; %ideal LPF S1_recv=SQH1.*H_lpf; s_recv1=real(ifft(fftshift(S1_recv))); s_recv1=s_recv1(1:Lsig); S2_recv=SQH2.*H_lpf; s_recv2=real(ifft(fftshift(S2_recv))); s_recv2=s_recv2(1:Lsig); % plot the filtered signal against the original signal figure(2); subplot(211); sfig3=plot(t,xsig,'b-',t,s_recv1,'b-.'); legend('original','recovered') set(sfig3,'Linewidth',2); title('signal{it g}({it t}) and filtered 16-level PCM signal') xlabel('time(sec.)'); subplot(212); sfig4=plot(t,xsig,'b-',t,s_recv2(1:Lsig),'b'); legend('original','recovered') set(sfig1,'Linewidth',2); title('signal{it g}({it t}) and filtered 4-level PCM signal') xlabel('time(sec.)'); Results: Signal {it g}({{it t}) and its 16 level PCM signal 2 1 0 -1 -2 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 time(sec.) Signal g( t) and its 4 level PCM signal 2 1 0 -1 -2 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 time(sec.)
23. 23. signal g(it t) and filtered 16-level PCM signal 2 original 1 recovered 0 -1 -2 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 time(sec.) signal g(it t) and filtered 4-level PCM signal 2 original 1 recovered 0 -1 -2 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 time(sec.)
24. 24. PRACTICAL NO 7 OBJECT Implementation of delta modulation and demodulation and observe effect of slope Overload using matlab/simulink THEORY Delta Modulation: Delta modulation (DM or Δ-modulation) is an analog-to-digital and digital-to-analog signal conversion technique used for transmission of voice information where quality is not of primary importance. DM is the simplest form of differential pulse-code modulation (DPCM) where the difference between successive samples is encoded into n-bit data streams. In delta modulation, the transmitted data is reduced to a 1-bit data stream. • This scheme sends only the difference between pulses, if the pulse at time tn+1 is higher in amplitude value than the pulse at time t n, then a single bit, say a “1”, is used to indicate the positive value. • If the pulse is lower in value, resulting in a negative value, a “0” is used. • This scheme works well for small changes in signal values between samples. • If changes in amplitude are large, this will result in large errors. Figure 1.1 the process of delta modulation Figure 1.2 Delta modulation components Figure 1.3 Delta demodulation components
25. 25. Mtalb Code: % *** Function for Delta Modulation*********** % (deltamod.m) function s_DMout=deltamod(sig_in,Delta,td,ts) % usage % s_DMout=deltamod(xsig,Delta,td,ts) % Delta-step size % sig_in-input signal vector % td-original signal sampling period of sig_in % NOTE: td*fs must be a positive integer; % S_DMout -DM sampled output % ts-new sampling period if (rem(ts/td,1)==0) nfac=round(ts/td); p_zoh=ones(1,nfac); s_down=downsample(sig_in,nfac); Num_it=length(s_down); s_DMout(1)=Delta/2; for k=2:Num_it xvar=s_DMout(k-1); s_DMout(k)=xvar+Delta*sign(s_down(k-1)-xvar); end s_DMout=kron(s_DMout,p_zoh); else warning('Error! ts/t is not an integer!'); s_DMout=[]; end end %********Delta Modulation **********************************% % togenerate DM signals with different step sizes, % Delta1=0.2,Delta2=Delta1,Delta3=Delta4 clc; clear; clf; td=0.002; % original sampling rate rate 500 hz t=[0:td:1.]; % time interval of 1 sec xsig=sin(2*pi*t)-sin(6*pi*t); % 1hz +3 hz sinusoidals Lsig=length(xsig); ts=0.02; % new sampling rate =50 hz Nfact=ts/td; % send the signal through a 16-level uniform quantiser Delta1=0.2; s_DMout1=deltamod(xsig,Delta1,td,ts); % obtaind the DM signal % plot the original signal and DM signal in time domain figrue(1); figure(1); subplot(311); sfig1=plot(t,xsig,'k',t,s_DMout1(1:Lsig),'b'); set(sfig1,'Linewidth',2); title('Signal {it g}({{it t}) and its DM signal') xlabel('time(sec.)'); axis([0 1 -2.2 2.2]); % Apply DM again by doubling the Delta Delta2=2*Delta1; s_DMout2=deltamod(xsig,Delta2,td,ts); subplot(312); sfig2=plot(t,xsig,'k',t,s_DMout2(1:Lsig),'b'); set(sfig2,'Linewidth',2);
26. 26. title('Signal {it g}({it t}) and DM signal with doubled stepsize') xlabel('time(sec.)'); axis([0 1 -2.2 2.2]); %*********** Delta3=2*Delta2; s_DMout3=deltamod(xsig,Delta3,td,ts); subplot(313); sfig3=plot(t,xsig,'k',t,s_DMout3(1:Lsig),'b'); set(sfig3,'Linewidth',2); title('Signal {it g}({it t}) and DM signal with quadrupled stepsize') xlabel('time(sec.)'); axis([0 1 -2.2 2.2]); Results: S ignal {it g}({{it t}) and its D M s ignal 2 1 0 -1 -2 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 tim e(s ec .) S ignal g( t) and D M s ignal w ith doubled s teps iz e 2 1 0 -1 -2 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 tim e(s ec .) S ignal g( t) and D M s ignal w ith quadrupled s teps iz e 2 1 0 -1 -2 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 tim e(s ec .)
27. 27. PRACTICAL NO 8 OBJECT Implementation of pulse data coding techniques for various formats using matlab/simulink. THEORY Data Encoding Methods: We can roughly divide line coding schemes into five broad categories, as shown in figure: 2.1. Figure 2.1 line coding scheme Non-Return to Zero (NRZ): Figure 2.2 NRZ • It is called NRZ because the signal does not return to zero at the middle of the bit • NRZ is the simplest representation of digital signals • One bit of data is transmitted per clock cycle • Bit values of 1and 0 are represented by high and low voltage signals, respectively NRZ-L (NRZ-Level), NRZ-I (NRZ-Invert): Figure 2.3 NRZ-L (NRZ-Level), NRZ-I (NRZ-Invert) • In NRZ-L the level of the voltage determines the value of the bit.
28. 28. • In NRZ-I the inversion or the lack of inversion determines the value of the bit Polar RZ: Return-to-Zero scheme: Polar biphase: Manchester and differential Manchester schemes: • In Manchester and differential Manchester encoding, the transition at the middle of the bit is used for synchronization • The minimum bandwidth of Manchester and differential Manchester is 2 times that of NRZ Bipolar Schemes: sometimes called multilevel binary • Three voltage levels: positive, negative, and zero • Two variations of bipolar encoding o AMI (alternate mark inversion)  0: neutral zero voltage  1: alternating positive and negative voltages o Pseudoternary  1: neutral zero voltage  0: alternating positive and negative voltages • AMI (alternate mark inversion) – The work mark comes from telegraphy and means 1. – AMI means alternate 1 inversion – The neutral zero voltage represents binary 0. – Binary 1s are represented by alternating positive and negative voltages. • Pesudotenary :
29. 29. – Same as AMI, but 1 bit is encoded as a zero voltage and the 0 bit is encoded as alternating positive and negative voltages. Multilevel Schemes: • The desire to increase the data speed or decrease the required bandwidth has resulted in the creation of many schemes. • The goal is to increase the number of bits per baud by encoding a pattern of m data elements into a pattern of n signal elements. • Different types of signal elements can be allowing different signal levels. • If we have L different levels, then we can produce Ln combinations of signal patterns. • The data element and signal element relation is • mBnL coding, where m is the length of the binary pattern, B means binary data, n is the length of the signal pattern, and L is the number of levels in the signaling. • B (binary, L=2), T (tenary, L=3), and Q (quaternary, L=4). • In mBnL schemes, a pattern of m data elements is encoded as a pattern of n signal elements in which 2m ≤ Ln 2B1Q (two binary, one quaternary) – m=2, n=1, and L=4 – The signal rate (baud rate) 2B1Q is used in DSL (digital subscriber line) technology to provide a high-speed connection to the Internet by using subscriber telephone lines 8B6T: • Eight binary, six ternary (8B6T)
30. 30. – This code is used with 100BASE-4T cable. – Encode a pattern of 8 bits as a pattern of 6 signal elements, where the signal has three levels (ternary). – 28=256 different data patterns and 36=478 different signal patterns. (The mapping is shown in Appendix D.) – There are 478-256=222 redundant signal elements that provide synchronization and error detection. – Part of the redundancy is also used to provide DC (direct-current) balance. • + (positive signal), - (negative signal), and 0 (lack of signal) notation. • To make whole stream DC-balanced, the sender keeps track of the weight 4D-PAM5 : • Four-dimensional five-level pulse amplitude modulation (4D-PAM5) – 4D means that data is sent over four wires at the same time. – It uses five voltage levels, such as -2, -1, 0, 1, and 2. – The level 0 is used only for forward error detection. – If we assume that the code is just one-dimensional, the four levels create something similar to 8B4Q. – The worst signal rate for this imaginary one-dimensional version is Nx4/8, or N/2. – 4D-PAM5 sends data over four channels (four wires). This means the signal rate can be reduced to N/8. – All 8 bits can be fed into a wire simultaneously and sent by using one signal element. – Gigabit Ethernet use this technique to send 1-Gbps data over four copper cables that can handle 1Gbps/8 = 125Mbaud – Multiline Transmission: MLT-3: • The multiline transmission, three level (MLT-3) • Three levels (+V, 0, and –V) and three transition rules to move the levels – If the next bit is 0, there is no transition – If the next bit is 1 and the current level is not 0, the next level is 0.
31. 31. – If the next bit is 1 and the current level is 0, the next level is the opposite of the last nonzero level. • Why do we need to use MLT-3? – The signal rate for MLT-3 is one-fourth the bit rate (N/4). – This makes MLT-3 a suitable choice when we need to send 100 Mbps on a copper wire that cannot support more than 32 MHz (frequencies above this level create electromagnetic emission). – Summary of line coding schemes: Matlab code: function [U P B M S]=nrz(a) % 'a' is input data sequence, % U = Unipolar, P=Polar, B=Bipolar, M=Mark and S=Space %Wave formatting, %Unipolar U=a; n= length(a); %POLAR
32. 32. P=a; for k=1:n; if a(k)==0 P(k)=-1; end end %Bipolar B=a; f = -1; for k=1:n; if B(k)==1; if f==-1; B(k)=1; f=1; else B(k)=-1; f=-1; end end end %Mark M(1)=1; for k=1:n; M(k+1)=xor(M(k), a(k)); end %Space S(1)=1; for k=1:n S(k+1)=not(xor(S(k), a(k))); end %Plotting Waves subplot(5, 1, 1); stairs(U) axis([1 n+2 -2 2]) title('Unipolar NRZ') grid on subplot(5, 1, 2); stairs(P) axis([1 n+2 -2 2]) title('Polar NRZ') grid on subplot(5, 1, 3); stairs(B) axis([1 n+2 -2 2]) title('Bipolar NRZ') grid on subplot(5, 1, 4); stairs(M) axis([1 n+2 -2 2]) title('NRZ-Mark') grid on subplot(5, 1, 5); stairs(S) axis([1 n+2 -2 2]) title('NRZ-Space') grid on Input a=[1 0 0 1 1] a= 1 0 0 1 1
33. 33. [U P B M S]=nrz(a) Output wavform Unipolar NRZ 2 0 -2 1 2 3 4 5 6 7 Polar NRZ 2 0 -2 1 2 3 4 5 6 7 Bipolar NRZ 2 0 -2 1 2 3 4 5 6 7 NRZ-Mark 2 0 -2 1 2 3 4 5 6 7 NRZ-Space 2 0 -2 1 2 3 4 5 6 7
34. 34. RACTICAL NO 9 OBJECT Implementation of Data decoding techniques for various formats using matlab/simulink Matlab Code: function [Ur Pr Br Mr Sr]=nrzRx(U,P,B,M,S) % 'a' is input data sequence % U = Unipolar, P=Polar, B=Bipolar, M=Mark and S=Space %Wave formatting %Unipolar Ur=U; n= length(P); %POLAR Pr=P; l=find(Pr<0); Pr(l)=0 %Bipolar n= length(B); Br=B; l=find(Br<0); Br(l)=1; %Mark n= length(M); for k=1:n-1; Mr(k)=xor(M(k), M(k+1)); end %Space n= length(S); S(1)=1; for k=1:n-1 Sr(k)=not(xor(S(k), S(k+1))); end %Plotting Waves n= length(Ur); subplot(5, 1, 1); stairs(Ur) axis([1 n+2 -2 2]) title('Unipolar NRZ Decoded') grid on n= length(P); subplot(5, 1, 2); stairs(P) axis([1 n+2 -2 2]) title('Polar NRZ Decoded') grid on n= length(Br); subplot(5, 1, 3); stairs(B) axis([1 n+2 -2 2]) title('Bipolar NRZ Decoded') grid on n= length(Mr); subplot(5, 1, 4); stairs(M) axis([1 n+2 -2 2]) title('NRZ-Mark Decoded') grid on n= length(Sr); subplot(5, 1, 5); stairs(S) axis([1 n+2 -2 2]) title('NRZ-Space Decoded')
35. 35. grid on *******************************end*********************************** Input U = 1 0 0 1 1 P = 1 -1 -1 1 1 B = 1 0 0 -1 1 M = 1 0 0 0 1 0 S = 1 1 0 1 1 1 Call [Ur Pr Br Mr Sr]=nrzRx(U,P,B,M,S) Output Pr = 1 0 0 1 1 Ur = 1 0 0 1 1 Pr = 1 0 0 1 1 Br = 1 0 0 1 1 Mr = 1 0 0 1 1 Sr = 1 0 0 1 1
36. 36. PRACTICAL NO 10 OBJECT Implementation of amplitude shift keying modulator and demodulator using matlab/simulink. THEORY ASK (amplitude shift keying) modulator: Amplitude-shift keying (ASK) is a form of modulation that represents digital data as variations in the amplitude of a carrier wave. The amplitude of an analog carrier signal varies in accordance with the bit stream (modulating signal), keeping frequency and phase constant. The level of amplitude can be used to represent binary logic 0s and 1s. We can think of a carrier signal as an ON or OFF switch. In the modulated signal, logic 0 is represented by the absence of a carrier, thus giving OFF/ON keying operation and hence the name given • ASK is implemented by changing the amplitude of a carrier signal to reflect amplitude levels in the digital signal. • For example: a digital “1” could not affect the signal, whereas a digital “0” would, by making it zero. • The line encoding will determine the values of the analog waveform to reflect the digital data being carried. Fig. 4.1 Ask modulator Fig. 4.1 Ask signal
37. 37. Matlab Code : % program for amplitude shift keying % clc; clear all; close all; s= [1 0 1 0]; f1=20; a=length (s); for i=1:a f=f1*s (1,i); for t=(i-1)*100+1:i*100 x(t)=sin(2*pi*f*t/1000); end end plot(x); xlabel('time in secs'); ylabel('amplitude in volts'); title('ASK') grid on; Results: A K S 1 0.8 m litu einv lts 0.6 o 0.4 0.2 d 0 a p - .2 0 - .4 0 - .6 0 - .8 0 -1 0 50 10 0 10 5 20 0 20 5 30 0 30 5 40 0 tim ins c e es
38. 38. PRACTICAL NO 11 OBJECT Implementation of frequency shift keying modulator and demodulator using matlab/simulink. THEORY FSK (frequency shift keying) modulator: Frequency-shift keying (FSK) is a frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a carrier wave. The simplest FSK is binary FSK (BFSK). BFSK literally implies using a pair of discrete frequencies to transmit binary (0s and 1s) information. Applications: Most early telephone-line modems used audio frequency-shift keying to send and receive data, up to rates of about 300 bits per second. Matlab Code: %*********FSK**************% clc; clear all; close all; s= [1 0 1 0]; f1=10; f2=50; a=length (s); for i=1:a if s(1,i)==1 freq=f1*s(1,i); for t= (i-1)*100+1:i*100 x(t)= sin(2*pi*freq*t/1000); end elseif s(1,i)==0 b=(2*s(1,i))+1; freq=f2*b; for t=(i-1)*100+1:i*100 x(t)= sin(2*pi*freq*t/1000); end end end plot(x); xlabel('title in secs'); ylabel('amplitude in volts') title ('FSK') grid on;
39. 39. Results FSK 1 0.8 0.6 0.4 amplitude in volts 0.2 0 -0.2 -0.4 -0.6 -0.8 -1 0 50 100 150 200 250 300 350 400 title in secs
40. 40. PRACTICAL NO 12 OBJECT Implementation of phase shift keying modulator and demodulator using matlab/simulink THEORY PSK (phase shift keying) modulator: Phase-shift keying (PSK) is a digital modulation scheme that conveys data by changing, or modulating, the phase of a reference signal (the carrier wave). Any digital modulation scheme uses a finite number of distinct signals to represent digital data. PSK uses a finite number of phases, each assigned a unique pattern of binary digits. Usually, each phase encodes an equal number of bits. Each pattern of bits forms the symbol that is represented by the particular phase. The demodulator, which is designed specifically for the symbol-set used by the modulator, determines the phase of the received signal and maps it back to the symbol it represents, thus recovering the original data. This requires the receiver to be able to compare the phase of the received signal to a reference signal — such a system is termed coherent (and referred to as CPSK). Alternatively, instead of using the bit patterns to set the phase of the wave, it can instead be used to change it by a specified amount. The demodulator then determines the changes in the phase of the received signal rather than the phase itself. Since this scheme depends on the difference between successive phases, it is termed differential phase-shift keying (DPSK). DPSK can be significantly simpler to implement than ordinary PSK since there is no need for the demodulator to have a copy of the reference signal to determine the exact phase of the received signal (it is a non-coherent scheme). In exchange, it produces more erroneous demodulations. The exact requirements of the particular scenario under consideration determine which scheme is used. Matlab Code: Initializing Variables: The first step is to initialize variables for number of samples per symbol, number of symbols to simulate, alphabet size (M) and the signal to noise ratio. The last line seeds the random number generators. nSamp = 8; numSymb = 100; M = 4; SNR = 14; seed = [12345 54321]; rand('state', seed(1)); randn('state', seed(2)); Generating Random Information Symbols Next, use RANDSRC to generate random information symbols from 0 to M-1. Since the % simulation is of QPSK, the symbols are 0 through 3. The first 10 data points are plotted. numPlot = 10;
41. 41. rand('state', seed(1)); msg_orig = randsrc(numSymb, 1, 0:M-1); stem(0:numPlot-1, msg_orig(1:numPlot), 'bx'); xlabel('Time'); ylabel('Amplitude'); Phase Modulating the Data Use MODEM.PSKMOD object to phase modulate the data and RECTPULSE to upsample to a sampling rate 8 times the carrier frequency. Use SCATTERPLOT to see the signal constellation. grayencod = bitxor(0:M-1, floor((0:M-1)/2)); msg_gr_orig = grayencod(msg_orig+1); msg_tx = modulate(modem.pskmod(M), msg_gr_orig); msg_tx = rectpulse(msg_tx,nSamp); h1 = scatterplot(msg_tx); Creating the Noisy Signal Then use AWGN to add noise to the transmitted signal to create the noisy signal at the receiver. Use the 'measured' option to add noise that is 14 dB below the average signal power (SNR = 14 dB). Plot the constellation of the received signal.
42. 42. randn('state', seed(2)); msg_rx = awgn(msg_tx, SNR, 'measured', [], 'dB'); h2 = scatterplot(msg_rx); Recovering Information from the Transmitted Signal Use INTDUMP to downsample to the original information rate. Then use MODEM.PSKDEMOD object to demodulate the signal, and detect the transmitted symbols. The detected symbols are plotted in red stems with circles and the transmitted symbols are plotted in blue stems with x's. The blue stems of the transmitted signal are shadowed by the red stems of the received signal. Therefore, comparing the blue x's with the red circles indicates that the received signal is identical to the transmitted signal. close(h1(ishandle(h1)), h2(ishandle(h2))); msg_rx_down = intdump(msg_rx,nSamp); msg_gr_demod = demodulate(modem.pskdemod(M), msg_rx_down); [dummy graydecod] = sort(grayencod); graydecod = graydecod - 1; msg_demod = graydecod(msg_gr_demod+1)'; stem(0:numPlot-1, msg_orig(1:numPlot), 'bx'); hold on; stem(0:numPlot-1, msg_demod(1:numPlot), 'ro'); hold off; axis([ 0 numPlot -0.2 3.2]); xlabel('Time'); ylabel('Amplitude');
43. 43. PRACTICAL NO 13 OBJECT Study of microwave components and instruments THEORY Microwave Components: • Connecting Devices: – Waveguide • Rectangular • Circular – Microstrip line – Strip line • Junctions: – E plane – H plane – EH plane or magic tee (hybride line) – Hybride ring • Microwave Source: – Multicavity klystron – Reflex klystron – Magnetron – Travelling Wave Tube (TWT) – Crossed Field Amplifier (CFA) – Backward oscillator • Semiconductor Source: – Gunn Diode – IMPATT, IMPATT, TRAPATT – Tunnel Diode • Microwave Amplifier: – Multicavity klystron – Travelling Wave Tube (TWT) – Gunn Diode – Parametric Amplifier • Switches: – PIN Diode Waveguides: A waveguide is a structure which guides waves, such as electromagnetic waves or sound waves. There are different types of waveguide for each type of wave. The original and most common meaning is a hollow conductive metal pipe used to carry high frequency radio waves, particularly microwaves. Waveguides differ in their geometry which can confine energy in one dimension such as in slab waveguides or two dimensions as in fiber or channel waveguides. In addition, different waveguides are needed to guide different frequencies: an optical fiber guiding light (high frequency) will not guide microwaves (which have a much lower frequency). As a rule of thumb, the width of a waveguide needs to be of the same order of magnitude as the wavelength of the guided wave. Principal of operation: Waves in open space propagate in all directions, as spherical waves. In this way they lose their power proportionally to the square of the distance; that is, at a distance R from the source, the power is the source power divided by R2. The waveguide confines the wave to propagation in one dimension, so that (under ideal conditions) the wave loses no power while propagating. Waves are confined inside the waveguide due to total reflection from the waveguide wall, so that the propagation inside the waveguide can be described approximately as a "zigzag" between the walls. This description is exact for electromagnetic waves in a rectangular or circular hollow metal tube. Rectangular Waveguide:
44. 44. It consists of a rectangular hollow metallic conductor. The electromagnetic waves in (metal-pipe) waveguide may be imagined as travelling down the guide in a zig-zag path, being repeatedly reflected between opposite walls of the guide. • Need to find the fields components of the em wave inside the waveguide – Ez Hz Ex Hx Ey Hy • We’ll find that waveguides don’t support TEM waves Modes of propagation: • TEM (Ez=Hz=0) can’t propagate. • TE (Ez=0) transverse electric – In TE mode, the electric lines of flux are perpendicular to the axis of the waveguide – TM (Hz=0) transverse magnetic, Ez exists – In TM mode, the magnetic lines of flux are perpendicular to the axis of the waveguide. – HE hybrid modes in which all components exists The cutoff frequency occurs when: 2 2  mπ   nπ  When ω c µε =  then γ = α + jβ = 0 2  +   a   b  2 2 1 1  mπ   nπ  or f c =   +  2π µε  a   b  Dominant mode- T10 Circular waveguide: It consists of a circular hollow metallic conductor. For same cutoff frequency the cylindrical waveguide longer then rectangular waveguide in cross- sectional area so it is more bulky Dominant mode- T11 Microstrip Line: Microstrip is a type of electrical transmission line which can be fabricated using printed circuit board [PCB] technology, and is used to convey microwave-frequency signals. It consists of a conducting strip separated from a ground plane by a dielectric layer known as the substrate. Microwave components such as antennas, couplers, filters, power dividers etc. can be formed from microstrip, the entire device existing as the pattern of metallization on the substrate. Microstrip is thus much less expensive than traditional waveguide technology, as well as being far lighter and more compact.
45. 45. Cross-section of microstrip geometry, Conductor (A) is separated from ground plane (D) by dielectric substrate (C). Upper dielectric (B) is typically air. The disadvantages of microstrip compared with waveguide are the generally lower power handling capacity, and higher losses. Also, unlike waveguide, microstrip is not enclosed, and is therefore susceptible to cross-talk and unintentional radiation It is behave as a parallel wire. Strip line: A stripline circuit uses a flat strip of metal which is sandwiched between two parallel ground planes, The insulating material of the substrate forms a dielectric. The width of the strip, the thickness of the substrate and the relative permittivity of the substrate determine the characteristic impedance of the strip which is a transmission line. As shown in the diagram, the central conductor need not be equally spaced between the ground planes. In the general case, the dielectric material may be different above and below the central conductor To prevent the propagation of unwanted modes, the two ground planes must be shorted together. This is commonly achieved by a row of vias running parallel to the strip on each side. Like coaxial cable, strip line is non-dispersive, and has no cut off frequency. Good isolation between adjacent traces can be achieved more easily than with microstrip. Cross-section diagram of strip line geometry. Central conductor (A) is sandwiched between ground planes (B and D). Structure is supported by dielectric (C). Waveguide Junction: E-type waveguide junction: It is called an E-type T junction because the junction arm, i.e. the top of the "T" extends from the main waveguide in the same direction as the E field. It is characterized by the fact that the outputs of this form of waveguide junction are 180° out of phase with each other. Waveguide E-type junction The basic construction of the waveguide junction shows the three port waveguide device. Although it may be assumed that the input is the single port and the two outputs are those on the top section of the "T", actually any port can be used as the input, the other two being outputs.
46. 46. To see how the waveguide junction operates, and how the 180° phase shift occurs, it is necessary to look at the electric field. The magnetic field is omitted from the diagram for simplicity. Waveguide E-type junction E fields It can be seen from the electric field that when it approaches the T junction itself, the electric field lines become distorted and bend. They split so that the "positive" end of the line remains with the top side of the right hand section in the diagram, but the "negative" end of the field lines remain with the top side of the left hand section. In this way the signals appearing at either section of the "T" are out of phase. These phase relationships are preserved if signals enter from either of the other ports. H-type waveguide junction: This type of waveguide junction is called an H-type T junction because the long axis of the main top of the "T" arm is parallel to the plane of the magnetic lines of force in the waveguide. It is characterized by the fact that the two outputs from the top of the "T" section in the waveguide are in phase with each other. Waveguide H-type junction To see how the waveguide junction operates, the diagram below shows the electric field lines. Like the previous diagram, only the electric field lines are shown. The electric field lines are shown using the traditional notation - a cross indicates a line coming out of the screen, whereas a dot indicates an electric field line going into the screen. Waveguide H-type junction electric fields It can be seen from the diagram that the signals at all ports are in phase. Although it is easiest to consider signals entering from the lower section of the "T", any port can actually be used - the phase relationships are preserved whatever entry port is ised.
47. 47. Magic T hybrid waveguide junction: The magic-T is a combination of the H-type and E-type T junctions. The most common application of this type of junction is as the mixer section for microwave radar receivers. Magic T waveguide junction The diagram above depicts a simplified version of the Magic T waveguide junction with its four ports. To look at the operation of the Magic T waveguide junction, take the example of whan a signal is applied into the "E plane" arm. It will divide into two out of phase components as it passes into the leg consisting of the "a" and "b" arms. However no signal will enter the "E plane" arm as a result of the fact that a zero potential exists there - this occurs because of the conditions needed to create the signals in the "a" and "b" arms. In this way, when a signal is applied to the H plane arm, no signal appears at the "E plane" arm and the two signals appearing at the "a" and "b" arms are 180° out of phase with each other. Magic T waveguide junction signal directions When a signal enters the "a" or "b" arm of the magic t waveguide junction, then a signal appears at the E and H plane ports but not at the other "b" or "a" arm as shown. One of the disadvantages of the Magic-T waveguide junction are that reflections arise from the impedance mismatches that naturally occur within it. These reflections not only give rise to power loss, but at the voltage peak points they can give rise to arcing when sued with high power transmitters. The reflections can be reduced by using matching techniques. Normally posts or screws are used within the E-plane and H-plane ports. While these solutions improve the impedance matches and hence the reflections, they still reduce the power handling capacity. Hybrid ring waveguide junction: This form of waveguide junction overcomes the power limitation of the magic-T waveguide junction. A hybrid ring waveguide junction is a further development of the magic T. It is constructed from a circular ring of rectangular waveguide - a bit like an annulus. The ports are then joined to the annulus at the required points. Again, if signal enters one port, it does not appear at allt he others.
48. 48. The hybrid ring is used primarily in high-power radar and communications systems where it acts as a duplexer - allowing the same antenna to be used for transmit and receive functions. During the transmit period, the hybrid ring waveguide junction couples microwave energy from the transmitter to the antenna while blocking energy from the receiver input. Then as the receive cycle starts, the hybrid ring waveguide junction couples energy from the antenna to the receiver. During this period it prevents energy from reaching the transmitter. Multicavity klystron: Gain of about 10-20 dB are typical with two cavity tubes. A higher overall gain can be achieved by connecting several two cavity tubes in cascade, feeding the output of each of the tubes to the input of the succeeding one. With four cavities, power gains of around 50 dB cab be easily achieved. The cavities are tuned the same frequency. Reflex klystron: In the reflex klystron (also known as a 'Sutton' klystron after its inventor), the electron beam passes through a single resonant cavity. The electrons are fired into one end of the tube by an electron gun. After passing through the resonant cavity they are reflected by a negatively charged reflector electrode for another pass through the cavity, where they are then collected. The electron beam is velocity modulated when it first passes through the cavity. The formation of electron bunches takes place in the drift space between the reflector and the cavity. Thevoltage on the reflector must be adjusted so that the bunching is at a maximum as the electron beam re-enters the resonant cavity, thus ensuring a maximum of energy is transferred from the electron beam to the RFoscillations in the cavity. The voltage should always be switched on before providing the input to the reflex klystron as the whole function of the reflex klystron would be destroyed if the supply is provided after the input. The reflector voltage may be varied slightly from the optimum value, which results in some loss of output power, but also in a variation in frequency. This effect is used to good advantage for automatic frequency control in receivers, and in frequency modulation for transmitters. The level of modulation applied for transmission is small enough that the power output essentially remains constant. At regions far from the optimum voltage, no oscillations are obtained at all. This tube is called a reflex klystron because it repels the input supply or performs the opposite function of a klystron. Magnetron:
49. 49. A cross-sectional diagram of a resonant cavity magnetron. Magnetic lines of force are parallel to the geometric axis of this structure All cavity magnetrons consist of a hot cathode with a high (continuous or pulsed) negative potential by a high-voltage, direct-current power supply. The cathode is built into the center of an evacuated, lobed, circular chamber. A magnetic field parallel to the filament is imposed by a permanent magnet. The magnetic field causes the electrons, attracted to the (relatively) positive outer part of the chamber, to spiral outward in a circular path rather than moving directly to this a node. Spaced around the rim of the chamber are cylindrical cavities. The cavities are open along their length and connect the common cavity space. As electrons sweep past these openings, they induce a resonant, high-frequency radio field in the cavity, which in turn causes the electrons to bunch into groups. A portion of this field is extracted with a short antenna that is connected to a waveguide (a metal tube usually of rectangular cross section). The waveguide directs the extracted RF energy to the load, which may be a cooking chamber in a microwave oven or a high-gain antenna in the case of radar. Travelling Wave Tube (TWT): A traveling-wave tube (TWT) is an electronic device used to amplify radio frequency signals to high power, usually in an electronic assembly known as a traveling-wave tube amplifier (TWTA). Cutaway view of a TWT (1) Electron gun; (2) RF input; (3) Magnets; (4) Attenuator; (5) Helix coil; (6) RF output; (7) Vacuum tube; (8) Collector. Crossed Field Amplifier (CFA): A crossed-field amplifier (CFA) is a specialized vacuum tube, first introduced in the mid-1950s and frequently used as a microwave amplifier in very-high-power transmitters. A CFA has lower gain and bandwidth than other microwave amplifier tubes (such as klystrons or traveling- wave tubes); but it is more efficient and capable of much higher output power. Peak output powers of many megawatts and average power levels of tens of kilowatts can be achieved, with efficiency ratings in excess of 70 percent. Backward oscillator: A backward wave oscillator (BWO), also called carcinotron (a trade name for tubes manufactured by CSF, now Thales) or backward wave tube, is a vacuum tubethat is used to generate microwaves up to the terahertz range. It belongs to the traveling-wave tube family. It is an oscillator with a wide electronic tuning range. An electron gun generates an electron beam that is interacting with a slow-wave structure. It sustains the oscillations by propagating a traveling wave backwards against the beam. The generated electromagnetic wave power has its group velocity directed oppositely to the direction of motion of the electrons. The output power is coupled out near the electron gun.