UNIT-4 Part A

1. What is kickback noise? [ N/D-16] It is basically the noise from the switching first stage on the input of the comparator. If the output of the first stage swings quickly in large range, it will make a glitch at the comparator inputs. The effects is worst if they are driven by high impedance (in switched cap circuit for instance). Then, you get an offset.

2. Define Navigation Navigation is the art of directing the movements of craft from one point to another along a desired

path.

3. What is the need of a Chronometer? With the help of Chronometer, the navigator was able to determine his longitude by noting the

transit time of heavenly bodies. 4. What is the need of Adcock direction finders?

The Adcock direction finders are designed to eliminate polarization errors by dispensing with the horizontal members.

5. What are the four methods of navigation?

Navigation by pilotage

Celestial or astronomical navigation

Navigation by dead –reckoning

Radio navigation

6. Define astronomical navigation Celestial navigation is accomplished by measuring the angular position of celestial bodies.

7. Define navigation by dead reckoning The position of the craft at any instant of time is calculated from the previously determined position,

the speed of its motion with respect to earth along with the direction of its motion and the time elapsed. 8. What is the important source of antenna effect? The important source is the asymmetry of the loop antenna with respect to the ground.

9. How the antenna effect is minimized? To minimize the antenna effect, the centre of the loop is earthed and its output is thereby balanced. 10. Give the disadvantage of loop direction finder.

The loop is small enough to be rotated easily. This results in a small signal pickups.

To facilitate manual operation, the loop is located near the receiver.

11. What are the errors arising in direction finders?

Errors due to abnormal polarization of the incoming wave

Errors due to abnormal propagation

Site errors

Instrumental errors

12. What are the two types of radio ranges in use?

Low frequency four course radio range

VHF Omni directional radio range

13. What are the sources of errors in VOR system?

Ground station and aircraft equipment

Site irregularities

Terrain features

Polarization

14. Define LORAN LORAN is Long Range Navigational Aid and is a pulse system. The ground station

transmit a train of pulses with fixed time relation between them and at the receiver, these pulses are identified and the delay between them is measured on a cathode ray oscilloscope

Part B

1. Explain in detail about Adcock Direction Finders and its advantages over loop antenna.

(8) [ N/D-16] Adcock antenna:

The Adcock antenna is an antenna array consisting of four equidistant vertical elements which can be used to transmit or receive directional radio waves.

The Adcock array was invented and patented by British engineer Frank Adcock in 1919, and has been used for a variety of applications, both civilian and military, ever since Although originally conceived for receiving low frequency (LF) waves, it has also been used for transmitting, and has since been adapted for use at much higher frequencies, up to ultra high .frequency (UHF).

In the early 1930s, the Adcock antenna (transmitting in the LF/MF bands) became a key feature of the newly created radio navigation system for aviation. The low frequency range (LFR) network, which consisted of hundreds of Adcock antenna arrays, defined the airways used by aircraft for instrument flying. The LFR remained as the main aerial navigation technology until it was replaced by the VOR system in the 1950s and 1960s.

The Adcock antenna array has been widely used commercially, and implemented in vertical antenna heights ranging from over 130 feet (40 meters) in the LFR network, to as small as 5 inches (13 cm) in tactical direction finding applications.

Radio direction finding:

Frank Adcock originally used the antenna as a receiving antenna, to find the azimuthal direction a radio signal was coming from in order to find the location of the radio transmitter; a process called radio direction finding.

Prior to Adcock's invention, engineers had been using loop antennas l to achieve directional sensitivity. They discovered that due to atmospheric disturbances and reflections, the detected signals included significant components of electromagnetic interference and distortions: horizontally polarized radiation contaminating the signal of interest and reducing the accuracy of the measurement.

Adcock—who was serving as an Army officer in the British Expeditionary force in wartime France at the time he filed his invention—solved this problem by replacing the loop antennas with symmetrically inter-connected pairs of vertical monopole or dipole antennas of equal length. This created the equivalent of square loops, but without their horizontal members, thus eliminating sensitivity to much of the horizontally polarized distortion. The same principles remain valid today, and the Adcock antenna array and its variants are still used for radio direction finding.

A radio direction finder (RDF) is a device for finding the direction, or bearing, to a radio source. The act of measuring the direction is known as radio direction finding or sometimes simply direction finding (DF). Using two or more measurements from different locations, the location of an unknown transmitter can be determined; alternately, using two or more measurements of known transmitters, the location of a vehicle can be determined. RDF is widely used as a radio navigation system, especially with boats and aircraft.

RDF systems can be used with any radio source, although the size of the receiver antennas are a function of the wavelength of the signal; very long wavelengths (low frequencies) require very large antennas, and are generally used only on ground-based systems. These wavelengths are nevertheless very useful for marine navigation as they can travel very long distances and "over the horizon", which is valuable for ships when the line-of-sight may be only a few tens of kilometres. For aerial use, where the horizon may extend to hundreds of kilometres, higher frequencies can be used, allowing the use of much smaller antennas. An automatic direction finder, often capable of being tuned to commercial AM radio transmitters, is a feature of almost all modern aircraft.

2. Explain in detail about VOR Receiving Equipment. (8) [ N/D-16]

VOR receiving equipment:

There are many different types of airborne equipment of different types of different degrees of complexity available. However all equipments have the following parts viz an antenna, a control box, a receiver, and navigation circuits. Antenna:

The antenna is a v-type dipole antenna. The control box contains an ON-OFF switch, frequency selector or tuner, and an aural volume control. The volume control regulates only the intensity of the signal going into the headset or the loudspeaker. Receiver:

The receiver is a conventional superhetrodyne receiver. The navigation circuits take the signals from the receiver, and measure the phase angle difference between the reference signal and the variable signal.

As the phase angle difference is defined fixed amount for each radial, it is therefore possible to determine the bearing of the aircraft from the VOR beacon, and this information can be presented visually.

Similarly, if the equipment can be adjusted to a desired bearing and indicate the relationship of the aircraft to the bearing and when the aircraft has to reach the bearing, it is possible to preset tracks then fly to and continue along them.

The visual indicators comprise a manually operated Omni bearing selector, a deviation indicator, and a TO / FROM indicator, and these are normally combined in one instrument known as an Omni bearing indicator.

The information available from the navigation circuits is presented on the deviation indicator, and a TO / FROM indicator with relation to the setting of the Omni bearing selector. Information derived from the VOR may also be presented on a radio magnetic indicator.

A block diagram of the airborne equipment receiver is shown in the figure. The VOR enables a pilot to select, identify and locate a line of position from a particular VOR beacon. The information can be obtained.

3. Explain in detail radar direction determination. The angular determination of the target is determined by the directivity of the antenna.

Directivity, sometimes known as the directive gain, is the ability of the antenna to concentrate the transmitted energy in a particular direction. An antenna with high directivity is also called a directive antenna. By measuring the direction in which the antenna is pointing when the echo is received, both the azimuth and elevation angles from the radar to the object or target can be determined. The accuracy of angular measurement is determined by the directivity, which is a function of the

size of the antenna. Radar units usually work with very high frequencies.

Reasons for this are: quasi – optically propagation of these waves. High resolution (the smaller the

wavelength, the smaller the objects the radar is able to detect). Higher the frequency, smaller the

antenna size at the same gain. The Electromagnetic waves almost behave like light beams and

well can be calculated in accordance with optical rules. This isn't surprising either since light also

only is regarded as an electromagnetic wave. The difference only consists in the frequency which

is much higher at the light than at electromagnetic waves in the radar frequency range.

• The quasi-optically sight is a little further than the visual view because of this one at deeper

frequencies more effective diffraction. Well, the radar horizon lies far away than the visual horizon.

Obstacles on the way there affect also differently. A couple of trees affect the visual sight fatally

while electromagnetic waves can possibly penetrate this obstacle. An observer location in larger

height is just as effectively as a higher one antenna location for the electromagnetic waves in the

optics since by the bend of the earth's surface flat objects disappear very fast behind the horizon.

Radar line of sight

• The electromagnetic waves follow the rules of the optics in higher frequencies (>100 MHz). All

radar unit systems almost without exception work in this frequency domain. Well, therefore the wave fronts also propagate to quasi-optical rules.

• The earth's curvature may prevent the radar seeing a target within the maximum range given by the radar range equation. Therefore results a „dead zone” for every radar system in which one targets can't be detected.

True bearing The True Bearing (referenced to true north) of a radar target is the angle between true north and a line pointed directly at the target. This angle is measured in the horizontal plane and in a clockwise direction from true north. (The bearing angle to the radar target may also be measured in a clockwise direction from the centerline of your own ship or aircraft and is referred to as the relative bearing.)

The

antennas of most radar systems are designed to radiate energy in a one-directional lobe or beam

that can be moved in bearing simply by moving the antenna. As you can see in the Figure 2, the

shape of the beam is such that the echo signal strength varies in amplitude as the antenna beam

moves across the target. In actual practice, search radar antennas move continuously; the point of

maximum echo, determined by the detection circuitry or visually by the operator, is when the beam

points direct at the target. Weapons-control and guidance radar systems are positioned to the point

of maximum signal return and maintained at that position either manually or by automatic tracking

circuits.In order to have an exact determination of the bearing angle, a survey of the north direction

is necessary. Therefore, older radar sets must expensively be surveyed either with a compass or

with help of known trigonometrically points. More modern radar sets take on this task and with help

of the GPS satellites determine the north direction independently.

Transfer of bearing information

• The rapid and accurate transmission of the bearing information between the turntable with the mounted antenna and the scopes can be carried out for servo systems and counting of azimuth change pulses. Servo systems are used in older radar antennas and missile launchers and works with help of devices like synchro torque transmitters and synchro torque receivers. In newer radar units we find a system of azimuth- change pulses (ACP). In every rotation of the antenna a coder sends many pulses, these are then counted in the scopes. Newer radar units work completely without or with a partial mechanical motion. These radars employ electronic phase scanning in bearing and/or in elevation (phased- array-antenna).

4. Explain in detail about sparkles in thermometer code and metastability in flash ADC. (16) [ N/D-16]

Flash ADCs are made by cascading high-speed comparators. Figure 1 shows a typical flash ADC block

diagram. For an N-bit converter, the circuit employs 2N-1 comparators. A resistive-divider with 2N resistors provides

the reference voltage. The reference voltage for each comparator is one least significant bit (LSB) greater than the

reference voltage for the comparator immediately below it. Each comparator produces a 1 when its analog input

voltage is higher than the reference voltage applied to it. Otherwise, the comparator output is 0. Thus, if the analog

input is between VX4 and VX5, comparators X1 through X4 produce 1s and the remaining comparators produce 0s. The

point where the code changes from ones to zeros is the point at which the input signal becomes smaller than the

respective comparator reference-voltage levels.

Figure 1. Flash ADC architecture. If the analog input is between VX4 and VX5, comparators

X1 through X4 produce 1s and the remaining comparators produce 0s.

This architecture is known as thermometer code encoding. This name is used because the design is similar

to a mercury thermometer, in which the mercury column always rises to the appropriate temperature and

no mercury is present above that temperature. The thermometer code is then decoded to the appropriate

digital code.

The comparators are typically a cascade of wideband low-gain stages. They are low gain because at high

frequencies it is difficult to obtain both wide bandwidth and high gain. The comparators are designed for

low-voltage offset, so that the input offset of each comparator is smaller than an LSB of the ADC.

Otherwise, the comparator's offset could falsely trip the comparator, resulting in a digital output code that is

not representative of a thermometer code. A regenerative latch at each comparator output stores the result.

The latch has positive feedback, so that the end state is forced to either a 1 or a 0. Given these basics,

some adjustments are needed to optimize the flash converter architecture.

Sparkle Codes

Normally, the comparator outputs will be a thermometer code, such as 00011111. Errors can cause an

output like 00010111, meaning that there is a spurious zero in the result. This out-of-sequence 0 is called a

sparkle, which is caused by imperfect input settling or comparator timing mismatch. The magnitude of the

error can be quite large. Modern converters like the MAX109/MAX104 employ an input track-and-hold in

front of the ADC along with an encoding technique that suppresses sparkle codes.

Metastability

When the digital output from a comparator is ambiguous (neither a 1 nor a 0), the output is defined as

metastable. Metastability can be reduced by allowing more time for regeneration. Gray-code encoding,

which allows only 1 bit in the output to change at a time, can greatly improve metastability. . Thus, the

comparator outputs are first converted to gray-code encoding and then later decoded to binary, if desired.

Another problem occurs when a metastable output drives two distinct circuits. It is possible for one circuit to

declare the input a 1, while the other circuit thinks that it is a 0. This can create major errors. To avoid this

conflict, only one circuit should sense a potentially mestatable output.

Input Signal-Frequency Dependence

When the input signal changes before all the comparators have completed their tasks, the ADC's

performance is adversely impacted. The most serious impact is a drop-off in signal-to-noise ratio (SNR)

plus distortion (SINAD) as the frequency of the analog input frequency increases.

Measuring spurious-free dynamic range (SFDR) is another good way to observe converter performance.

The "effective bits" achieved by the ADC is a function of input frequency; it can be improved by adding a

track-and-hold (T/H) circuit in front of the ADC. The T/H circuit allows dramatic improvement, especially

when input frequencies approach the Nyquist frequency, as shown in Figure 2 (taken from the MAX104

data sheet). Parts without T/H show a significant drop-off in SFDR.

Clock Jitter

SNR is degraded when there is jitter in the sampling clock. This becomes noticeable for high

analog-input frequencies. To achieve accurate results, it is critical to provide the ADC with a

low-jitter, sampling clock source.

5. Explain in detail DECCA Navigation system.

DECCA is a hyperbolic electronic navigation system.

Salient features of DECCA Navigation system.

Operates in LF band.

Between 70 to 120 kHz.

Uses unmodulated continuous waves.

In DECCA navigation system the fix is obtained by measuring the phase difference

between the signals of the two stations which is phase locked.

DECCA chain consists of 4 stations, 1 master & 3 slaves.

The master station is at the centre and three slaves at the corners of a triangle.

This arrangement gives the three sets of hyperbolic position lines, one set corresponding

to the master and each slave.

Fix is obtained over a considerable area by the intersection of two hyperbolic lines.

In DECCA system each transmitter has different frequency so the direction from each

station will differentiate by the receiver.

Generally harmonically related frequencies radiated by each transmitters and phase

measurements done at common harmonic frequency which is obtained at the receiver by

using multiplying circuits.

DECCA Reception:

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Range and Accuracy