Mixer-II Mixer can produce both sum and difference of the frequencies that are applied to it. This Radar requires two Antennas. We will get the overall output voltage $E_a$ of the array by adding the output voltages of each element present in that array, since all those radiation elements are connected in linear array. The advantage of double delay line canceller is that it rejects the clutter broadly. The rays that pass through the centre of the Lens are less refracted than the rays that pass through the edges of the Lens. We can classify the Radar Antennas into the following two types based on the physical structure. Radars can be used for various applications on ground, on sea and in space. << Previous Next >>. An adjustable pedestal signal has to be moved along the baseline till it coincides the signal deflections, which are coming from the horizontal position of the target. If the angular error signal is applied to a servo control system, then it will move the axis of the Radar Antenna towards the direction of target. In our subsequent chapters, we will discuss the Radars, which are useful for detecting non-stationary targets. In general, Radars use directional Antennas. The above tasks will be repeated for every newly transmitted signal. The frequency of the received signal will decrease, when the target moves away from the Radar. We can use the following standard form of Radar range equation in order to calculate the maximum range of Radar for given specifications. The block diagram of the FMCW Radar looks similar to the block diagram of CW Radar. $$r\left ( t \right )=p\left ( t \right )-p\left ( t-T_P \right )-\left [ p\left ( t-T_P \right )-p\left ( t-2T_P \right ) \right ]$$, $$\Rightarrow r\left ( t \right )=p\left ( t \right )-2p\left ( t-T_P \right )+p\left ( t-2T_P \right )\:\:\:\:\:Equation\:12$$. The frequency of the received signal will increase, when the target moves towards the direction of the Radar. It is shown that in a vari-ety of noisy environments, the noise radar always has a much lower LPI than the conventional LFM radar. This radiation should be effective with minimum losses. Squint angle is the angle between beam axis and rotation axis and it is shown in the above figure. It measures not only the speed of the target but also the distance of the target from the Radar. Radar is used for detecting the objects and finding their location. Therefore, the output of Full Wave Rectifier looks like as shown in the following figure. Mathematically, we can write the expression for Aperture efficiency $\epsilon_A$ as, According to the standard definition, Antenna Efficiency is the ratio of the radiated power of the Antenna to the input power accepted by the Antenna.. T. e. MIT Lincoln Laboratory. Mathematically, it can be represented as , $$\Rightarrow F=\frac{S_i/N_i}{S_o/N_o}$$. The output of Magnetron Oscillator and the output of Local Oscillator are applied to Mixer-I. The output of this amplifier is connected to Mixer. It uses single Antenna for both transmission and reception of signals with the help of Duplexer. It is placed in such a way that one of its foci coincides with the focus of the paraboloid. An electronic instrument, which is used for displaying the data visually is known as display. Following is the mathematical formula for angular frequency, $\omega$ , Following equation shows the mathematical relationship between the angular frequency $\omega$ and phase angle $\phi$ , $$\omega=\frac{d\phi }{dt}\:\:\:\:\:Equation\:2$$. Even though the value of the signal at point C is closer to threshold value, it is a missing detection. $$R_{Max}=\left [ \frac{\left ( 400\times 10^3 \right )\left ( 30 \right )\left ( 5^2 \right )}{4\pi\left ( 0.003 \right )^2\left ( 10 \right )^{-10}} \right ]^{1/4}$$. Now, let us discuss about these basic terms one by one. The IF amplifier shown in the figure amplifies the signal having frequency of $f_o\left (t-T \right )-f_o\left (t \right )+f_{IF}$. Hence, the Antenna is said to have its directivity in that particular direction. Pulse Radar uses single Antenna for both transmitting and receiving of signals with the help of Duplexer. The receiver is not directly connected to port3. So, consider a 4-port circulator and connect the transmitter, Antenna, receiver and matched load to port1, port2, port3 and port4 respectively. Substitute, $N_i=KT_oB_n$ in above equation. $$G=\frac{4\pi A_e}{\lambda^2}\:\:\:\:\:Equation\:8$$, $$R_{Max}=\left [ \frac{P_t\sigma A_e}{\left ( 4\pi \right )^2S_{min}}\left ( \frac{4\pi A_e}{\lambda^2} \right ) \right ]^{1/4}$$, $$\Rightarrow R_{Max}=\left [\frac{P_tG\sigma {A_e}^2}{4\pi \lambda^2 S_{min}}\right ]^{1/4}\:\:\:\:\:Equation\:9$$. The output of Local Oscillator is applied to both Mixer-I and Mixer-II. The Radar cannot transmit the signal during reception time. It is because of this reciprocity, the Lens can be used as an Antenna, as the same phenomenon helps in utilizing the same Antenna for both transmission and reception. An important characteristic of a radar receiver is the noise level within the receiver chain. For this purpose, Radar uses the principle of Doppler Effect for distinguishing the non-stationary targets from stationary objects. This effect is known as the Doppler effect. We can find the maximum values of field intensity pattern by using L-Hospital rule when both numerator and denominator of Equation 5 are equal to zero. This means, it allows the AC components of echo signals received from non-stationary targets, i.e., moving targets. An MTI Radar operates at a frequency of $6GHZ$ with a pulse repetition frequency of $1KHZ$. It is a two dimensional Radar display. Basic Principle of Radar Radar is used for detecting the objects and finding their location. The advantage of time domain delay line canceller is that it can be operated for all frequency ranges. This phenomenon can be reversed which means the light if sent from the left side, is converged at the right side of the Lens. It does not require any additional blocks. Now, let us discuss what angular tracking is. So, no signal has been reached to the receiver. So, the operating wavelength,$\lambda$ is equal to $0.03m$, when the operating frequency, $f$ is $10GHZ$. FM Transmitter It transmits the FM signal with the help of transmitting Antenna. It is the modified version of A-Scope. This topic is considered in the next lecture. $$R_{Max}=\left [\frac{P_tG\sigma A_e}{\left (4\pi\right )^2 S_{min}}\right ]^{1/4}\:\:\:\:\:Equation\:7$$. If a light source is assumed to be present at a focal point of a lens, which is at a focal distance from the Lens, then the rays get through the Lens as collimated or parallel rays on the plane wave front. So, we have to select the duration between the two clock pulses in such a way that the echo signal corresponding to present clock pulse will be received before the next clock pulse starts. The signal, which is produced by the transmitter has to reach the Antenna for the Antenna will transmit that signal during transmission time. Similarly, Antenna receives the signal having frequency of $f_l+f_c\pm f_d$ when the duplexer connects the Antenna to Mixer-II. The dipole Antenna or the horn Antenna, which acts as the receiver Antenna at its feed receives this signal, to convert it into electric signal and forwards it to the receiver circuitry. In E-Scope, intensity modulation takes place. The Antenna under study is termed as subject Antenna. We will get the following equation by substituting G a = 1 in Equation 6. h ( t) = s ( t 1 t) The above equation proves that the impulse response of Matched filter is the mirror image of the received signal about a time instant t 1. $$\left | E_a \right|=\left | \frac{\sin\left [\frac{n\Psi}{2}\right ]}{\sin\left [\frac{\Psi}{2}\right]} \right |\:\:\:\:\:Equation\:4$$. The reciprocal of pulse repetition time is called pulse repetition frequency, $f_P$. It is the modified version of B-Scope in order to provide the information about elevation angle of the target. Let us consider n isotropic radiation elements, which when combined form an array. The function of the transmitter is to transmit the Radar signal in the direction of the target present. Transmitter It transmits the pulse-modulated signal, which is a train of repetitive pulses. Branch-type Duplexer consists of two switches Transmit-Receive (TR) switch and Anti Transmit-Receive (ATR) switch. Though an Antenna radiates power, the direction in which it radiates matters is of much significance. Mathematically, it can be written as , $$\Psi=\frac{2\pi d\sin\theta }{\lambda }\:\:\:\:\:Equation\:1$$. Most of the Tracking Radars use the principle of tracking in angle. We can find the value of Doppler frequency $f_d$ by substituting the values of $V_r$ and $\lambda$ in Equation 4. The common parameter that specifies this is the System Noise Factor, which . Side Band Filter It allows only one side band frequencies, i.e., either upper side band frequencies or lower side band frequencies. The Radar, which operates with pulse signal for detecting stationary targets, is called the Basic Pulse Radar or simply, Pulse Radar. It is the modified version of A-Scope. The signal, which is produced by the transmitter has to reach the Antenna for the Antenna to transmit that signal during transmission time. Substitute, the above $S_{min}$ in the following standard form of Radar range equation. Simply, Gain of an Antenna takes the Directivity of Antenna into account along with its effective performance. The noise radar's exceptional performance in the above evaluations makes it a suitable radar system for a variety of military applications. Mathematically, we can write the expression for blind speed $v_n$ as , $$v_n=\frac{n\lambda}{2T_P}\:\:\:\:\:Equation\:7$$, $$\Rightarrow v_n=\frac{n\lambda f_P}{2}\:\:\:\:\:Equation\:8$$, $n$ is an integer and it is equal to 1, 2, 3 and so on. $$R_{Max}=\left [\frac{P_t \sigma {A_e}^2}{4\pi \lambda^2 S_{min}}\right ]^{1/4}$$. Display In general, it displays the amplified video signal on CRT screen. The Figure shows a block diagram of a typical superheterodyne receiver. Power of minimum detectable signal $S_{min}$ should be low. Therefore, we can say that the range of the target is said to be maximum range when the received echo signal is having the power equal to that of minimum detectable signal. The horizontal and vertical coordinates represent the range and echo amplitude of the target respectively. If the direction of the target and reference direction is not same, then there will be angular error, which is nothing but the difference between the two directions. There are two fundamental noise mechanisms in a photodetector: shot noise. Due to this, the range of the target seems to be smaller than the actual range. The axis of Radar Antenna is considered as the reference direction. The following figure shows the block diagram of FMCW Radar . In our subsequent chapters, we will discuss the operations of all these Radars in detail. If $\lambda$ is one wave length, then the number of wave lengths N that are present in a two-way communication path between the Radar and target will be equal to $2R/\lambda$. We know that one wave length $\lambda$ corresponds to an angular excursion of $2\pi$ radians. A knowledge of Lens is required to understand the working of Lens Antenna in depth. Gain of the transmitting Antenna $G$ should be high. It is nothing but the frequency response of the single delay line canceller. The signals having frequencies $f_l+f_c\pm f_d$ and $f_l$ are applied to Mixer-II. From Equation 4, we can conclude that the frequency response of the single delay line canceller becomes zero, when Doppler frequency $f_d$ is equal to integer multiples of reciprocal of pulse repetition time $T_P$. They use Doppler Effect for detecting non-stationary targets. The horizontal and vertical coordinates represent the range and height of the target respectively. Hence, the blocks corresponding to passive TR limiter are used in order to provide the protection to the receiver. The operation of MTI Radar with power oscillator transmitter is mentioned below. Among which, the difference of the frequencies will be of Intermediate Frequency (IF) type. Equation 9 represents the modified form of Radar range equation. It is an important parameter of parabolic reflector and its value varies from 0.25 to 0.50. We have to select proper threshold value based on the strength of the signal to be detected. So, the Mixer-I will produce the output having frequencies $f_o+f_l$ or $f_of_l$. From what we learnt so far, single Delay line canceller eliminates the DC components of echo signals received from stationary targets, when $n$ is equal to zero. In the military arena, low probability of intercept (LPI) and of exploitation (LPE) by the enemy are required, while in the civil context, the spectrum occupancy is a more and more important requirement, because of the growing request by . The output of FM Transmitter is also connected to Mixer-I. In this way, Duplexer isolates both transmitter and receiver sections. The applications of Radars are listed below. The ray diagram represents the focal point and the focal length from the source to the Lens. Peak power transmitted by the Radar $P_t$ should be high. This law when used along with a parabola helps the beam focus. Antenna transmits the pulse-modulated signal, when the duplexer connects the Antenna to the transmitter. Then the beam gets refracted and meets at a point called the focal point, at a focal distance from the Lens. It is the modified version of A-Scope. Mathematically, it can be represented as. Substitute the values of $\lambda$ and $f_P$ in the equation of first blind speed. The horizontal and vertical displacements of the blip represent the horizontal and vertical aiming errors respectively. The physical area of the aperture should also be taken into consideration, as the effectiveness of the radiation depends upon the area of the aperture, physically on the Antenna. Transmitter It transmits the pulse-modulated signal, which is a train of repetitive pulses. It is a two-dimensional Radar display. Radar signals should be transmitted at every clock pulse. The relation Ip = R Pin assumes that such a conversion is noise free. The output of phase detector can be connected to Delay line canceller. Substitute, the given parameters in the above equation. The amount of power, which is reflected back towards the Radar depends on its cross section. According to the standard definition, Aperture efficiency of an Antenna is the ratio of the effective radiating area (or effective area) to the physical area of the aperture.. These sources include radiometric noise, jammers, and interference. The Radar Displays can be classified into the following types. Because the intent of this chapter is to discuss optical detector and receiver properties, only noise associated with the photodetection process is discussed. $$\Rightarrow f_d=\frac{2\left ( 27.78 \right )\left ( 5\times 10^9 \right )}{3\times 10^8}$$. Therefore, the value of Doppler frequency, $f_d$ is $926HZ$ for the given specifications. i.e., $f_o\left (t \right )-f_{IF}$. Hence, $q\left ( t \right )$ will be the input of the second delay line canceller. Video Amplifier As the name suggests, it amplifies the video signal, which is obtained at the output of detector. However, in practical applications, Radar receives the echo signals due to stationary objects in addition to the echo signal due to that movable target. We will get the following mathematical relation from first delay line canceller. Optical Receiver Noise. We will get those modified forms of Radar range equation from the standard form of Radar range equation. Following figure shows the block diagram of CW Radar . This Radar requires two Antennas. The amount of such thermal noise is proportional to receiver bandwidth. So, the operating wavelength $\lambda$ is equal to $0.05m$, when the operating frequency f is $6GHZ$. If we consider $p$ as zero, then we will get the value of $\sin\theta$ as zero. $$v_1=\frac{1\times \lambda f_p}{2}=\frac{\lambda f_p}{2}$$, $$v_2=\frac{2\times \lambda f_p}{2}=2\left ( \frac{\lambda f_p}{2} \right )=2v_1$$, $$v_3=\frac{3\times \lambda f_p}{2}=3\left ( \frac{\lambda f_p}{2} \right )=3v_1$$. If there is no target, then the signal received will be just noise. $$f_d =\frac{1}{2\pi}\frac{d}{dt}\left ( \frac{4\pi R}{\lambda} \right )$$, $$\Rightarrow f_d =\frac{1}{2\pi}\frac{4\pi}{\lambda}\frac{dR}{dt}$$, $$\Rightarrow f_d =\frac{2V_r}{\lambda}\:\:\:\:\:Equation\:4$$. Figure 1: Block diagram of a Superheterodyne The superheterodyne receiver changes the rf frequency into an easier to process lower IF- frequency. 9.5K views 1 year ago Radar Systems This video lecture is about the Receiver Noise and Signal to Noise Ratio. We make use of First and third party cookies to improve our user experience. $$P_{de}=P_{dd}\left (\frac{\sigma}{4\pi R^2}\right )\:\:\:\:\:Equation\:3$$ It is also called Continuous Wave Frequency Modulated Radar or CWFM Radar. So, the total angle of excursion made by the electromagnetic wave during the two-way communication path between the Radar and target will be equal to $4\pi R/\lambda$ radians. Hence, all the waves reaching the aperture are in phase. This echo signal is the desired one. The output of subtractor is applied as input to Full Wave Rectifier. Of these two antennas, one Antenna is used for transmitting the signal and the other Antenna is used for receiving the signal. This signal is used as the reference signal. We will get O-Scope, by including an adjustable notch to A-Scope for measuring distance. Equate the right hand side terms of Equation 1 and Equation 2 since the left hand side terms of those two equations are same. We know that power density is nothing but the ratio of power and area. Switched Frequency Counter It is useful for getting the value of Doppler velocity. Doppler Amplifier As the name suggests, Doppler amplifier amplifies the signal, which is having Doppler frequency, $f_d$. Phase Detector It is used to produce the output signal having frequency $f_d$ from the applied two input signals, which are having the frequencies of $f_c+f_d$ and $f_c$. $$R_{min}=\frac{C\tau}{2}\:\:\:\:\:Equation\:6$$. If the Radar Antenna is aimed at the target incorrectly, then I-Scope displays the target as a segment instead of circle. So, the Mixer-II will produce the output having frequencies of 2$f_o+f_l\pm f_d$ or $f_l\pm f_d$. The line L represents the directrix on which the reflected points lie (to say that they are being collinear). The low-pass filter has a cutoff frequency with the same value as the bit rate. It measures only the speed of the target but not the distance of the target from the Radar. If CW Doppler Radar uses the Frequency Modulation, then that Radar is called the Frequency Modulated Continuous Wave (FMCW) Radar or FMCW Doppler Radar. There exists a feedback mechanism in the Tracking Radar, which works until the angular error becomes zero. The Balanced Duplexer consists of two TR tubes. In our subsequent sections, we will discuss more about these two Delay line cancellers. The type of feed where a pair of certain configurations are there and where the feed beam width is progressively increased while Antenna dimensions are held fixed is known as Gregorian feed. The operating frequency of MTI Radar, $f=6GHZ$. Parabolic Reflector Antennas are the Microwave Antennas. When the electromagnetic wave hits the shape of the parabola, the wave gets reflected onto the feed point. The shape of the parabola when used for the purpose of reflection of waves, exhibits some properties of the parabola, which are helpful for building an Antenna, using the waves reflected. The signals having frequencies of $f_o+f_l$ and $f_o\pm f_d$ are applied to Mixer-II. It is also known as secondary hyperboloid reflector or sub-reflector. The ratio of focal length to aperture size (i.e., $f/D$ ) is known as f over D ratio. It is the modified version of A-Scope. Pulse Radars can be classified into the following two types based on the type of the target it detects. By using this website, you agree with our Cookies Policy. The signal, which is received by the Antenna has to reach the receiver during reception time. Radars can be classified into the following two types based on the type of signal with which Radar can be operated. It is a two dimensional Radar display. We will get the values of second & third blind speeds as $50m/sec$& $75m/sec$ respectively by substituting the value of 1 in the equations of second & third blind speeds. It improves the Signal to Noise Ratio at output. The above equation proves that the impulse response of Matched filter is the mirror image of the received signal about a time instant $t_1$. According to the Doppler effect, we will get the following two possible cases . Following are the different properties of Parabola . Substitute, $R=R_{un}$ and $T=T_P$ in Equation 1. $E_1, E_2, E_3, , E_n$ are the output voltages of first, second, third, , nth radiation elements respectively. MTI Radar uses single Antenna for both transmission and reception of signals with the help of Duplexer. We will get the value of maximum unambiguous range of the target, $R_{un}$ by substituting the values of $C$ and $T_P$ in Equation 3. We know that Radar signals should be transmitted at every clock pulse. We will get the minimum range of the target, when we consider the time required for the echo signal to receive at Radar after the signal being transmitted from the Radar as pulse width. The line joining F and V is the axis of symmetry. We can find the value of Doppler frequency, $f_d$ by substituting the values of $V_r,f$ and $C$ in Equation 5. The angle between the direction of the target and the rotation axis determines the amplitude of the modulated signal. Receiver NoisePIN. They are given here . $$V_2=A\sin\left [ 2\pi f_d\left ( t-T_P\right )-\phi_0 \right ]\:\:\:\:\:Equation\:2$$. As shown in the figure, Radar mainly consists of a transmitter and a receiver. So, the Mixer-II will produce the output having frequency either $f_o\left (t-T \right )+f_o\left (t \right )-f_{IF}$ or $f_o\left (t-T \right )-f_o\left (t \right )+f_{IF}$. We know that the functionality of the circulator is that if we apply an input to a port, then it will be produced at the port, which is adjacent to it in the clockwise direction. $$\lambda =\frac{3\times 10^8}{10\times 10^9}$$. Here, the Mixer-II is used for producing the output, which is having the frequency $f_c\pm f_d$. $$H\left (f\right )=S^\ast\left (f\right )e^{-j2\pi ft_1}\:\:\:\:\:Equation\:2$$. Following figure shows an example of sequential lobing in polar coordinates. It shows the echo signal information visually on the screen. The function of each block of FMCW Radar is mentioned below. Noise is a random, usually unwanted, signal in a lot of applications: you can hear it in acoustic signals as an additional constant fizzle, you can see it as variation in brightness or color information in a picture or a video sequence, The signals having frequencies of $f_l$ and $f_c$ are applied to Mixer-I. It is also called Continuous Wave Frequency Modulated Radar or CWFM Radar. Let the spacing between the successive elements be d units. Both the axis of Radar Antenna and the direction of target will coincide when the angular error is zero. Detector It detects the signal, which is having Doppler frequency, $f_d$. Radar engineering: Correlation can help determine the presence of a target and its range from the radar unit. This Radar requires two Antennas. It is more suitable for manually tracking Radar. The Antennas radiate individually and while in an array, the radiation of all the elements sum up, to form the radiation beam, which has high gain, high directivity and better performance, with minimum losses. Obviously, the amount of radiation power will be increased when we use group of Antennas together. It is a Radar display, which uses intensity modulation. The value of the signal at point B is equal to threshold value. In this tutorial, we will discuss analog communication, modulation, types of modulation, demodulators, noise, transmitters, receivers, and other components of the communication system. So, Tracking Radar tracks the target by tracking one of the three parameters range, angle, Doppler frequency shift. We will get the subtractor output by subtracting Equation 2 from Equation 1. Mixer-II Mixer can produce either sum or difference of the frequencies that are applied to it. In this chapter, we discussed how the Pulse Radar works and how it is useful for detecting stationary targets. One of the major advantages . The Radar, which operates with continuous signal (wave) for detecting non-stationary targets, is called Continuous Wave Radar or simply CW Radar. Pulse Modulator It produces a pulse-modulated signal and it is applied to the Transmitter. If CW Doppler Radar uses the Frequency Modulation, then that Radar is called FMCW Doppler Radar or simply, FMCW Radar. If the Antenna beam continuously rotates for tracking a target, then it is called conical scanning. If the threshold value is used for detecting the presence of the target from the received signal, then that detection is called threshold detection. Similarly, a low threshold value should be chosen when the strength of the signal to be detected is low. What is communication? Actually, the circulator itself acts as Duplexer. Following these points, the parabolic reflectors help in producing high directivity with narrower beam width. Angular error is indicated in the above figure. Noise is the main factor that limits receiver sensitivity. The echo signals due to stationary objects (places) such as land and sea are called clutters because these are unwanted signals. Mathematically, we can write the expression for Directivity as , $$Directivity=\frac{U_{Max}\left (\theta,\phi\right )}{U_0}$$, $U_{Max}\left (\theta,\phi\right )$ is the maximum radiation intensity of subject Antenna. Among which, one Antenna is used for transmitting the signal and the other Antenna is used for receiving the signal. In the system-level design for both conventional radars and noise radars, a fundamental element is the use of waveforms suited to the particular application. We will get the output of Delay line canceller, by replacing $t$ by $t-T_P$ in Equation 1. We can observe that if the denominator of Equation 5 becomes zero, then the numerator of Equation 5 also becomes zero. Therefore, we have to choose the Radar in such a way that it considers only the echo signal due to movable target but not the clutters. Mathematically, it can be represented as, $$T_P=\frac{1}{f_P}\:\:\:\:\:Equation\:4$$, $$R_{un}=\frac{C\left ( \frac{1}{f_P} \right )}{2}$$, $$R_{un}=\frac{C}{2f_P}\:\:\:\:\:Equation\:5$$. If the Radar is used for detecting the movable target, then the Radar should receive only the echo signal due to that movable target. This phenomenon is called Convergence. This purpose will be achieved when the transmitter generates a signal at port1. Following is the block diagram of Pulse Radar . The output of Local Oscillator is connected to both Mixer-I and Balanced Detector. In Equation 3, there are two terms.