1. Tell me what is G/T of the antenna?
G/T is referred as figure of merit of the RF antenna. G stands for Antenna gain and T stands for Antenna noise temperature. This is most often asked in the interview for satellite group position.
2. Tell us how Does Ultra Sonic Testing Work?
High frequency sound waves are very directional, and they will travel through a medium (like a piece of steel or plastic) until they encounter a boundary with another medium (like air), at which point they reflect back to their source. By analyzing these reflections it is possible to measure the thickness of a test piece, or find evidence of cracks or other hidden internal flaws.
3. Explain me what Is The Unit Of Vibration?
Important parameters relating to vibration pickups/vibrometers are described below: (1) Vibration frequency Unit: Hz (Hertz) Symbol: f Refers to the number of times a vibrating object vibrates per second. The inverse of a vibration frequency is referred to as the period (T), T=1/f.
4. Tell me what Issues Can Be Found With Vibration Analysis?
Vibration analysis can help diagnose many issues ranging from improper lubrication to electrical defects. Some of the most common issues found are: Roller bearing defects, Equipment imbalance, Coupling misalignment, Looseness, Resonance, Gear defects, etc.
5. Tell me can I see non-metallic subsurface features with GPR?
GPR is extremely accurate when it comes to locating metallic and non-metallic objects. GPR systems work by sending a tiny pulse of energy into the ground from an antenna. An integrated computer records the strength and time required for the return of reflected signals. Any subsurface variations, metallic or non-metallic, will cause signals to bounce back. When this occurs, all detected items are revealed on the computer screen in real-time as the GPR equipment moves along. Users can even tell from the signal returned whether the feature in question is metallic or non-metallic.
6. Do you know can GPR be used with GPS?
Yes. GSSI's systems can integrate with most GPS systems. The GPS position data files and GPR scans are automatically matched within our systems so that the resulting data shows proper GPS position.
7. Tell me what did the elephant say?
Some interviewers like to see your reactions to whacky questions like, “If an elephant walked into this room, what would it say?” There is no right/ wrong answer to this one. The interviewer is looking for quick thinking, an ability to handle unprepared situations, a calm demeanour and even a sense of humour. Don't go overboard on the humour or sarcasm though. Avoid statements that convey negativity or desperation.
The term Wavelength is basically distance from wavecrest to wavecrest along direction of travel of EM wave. The unit is centimeter.
Bistatic radar has two antennas; one for transmission and the other for reception. These antennas either located side by side or they are located far away.
10. Explain me a Basic RADAR System?
Given below are 6 major parts of a RADAR System:
☛ A Transmitter: It can be a power amplifier like a Klystron, Travelling Wave Tube or a power Oscillator like a Magnetron. The signal is first generated using a waveform generator and then amplified in the power amplifier.
☛ Waveguides: The waveguides are transmission lines for transmission of the RADAR signals.
☛ Antenna: The antenna used can be a parabolic reflector, planar arrays or electronically steered phased arrays.
☛ Duplexer: A duplexer allows the antenna to be used as a transmitter or a receiver. It can be a gaseous device that would produce a short circuit at the input to the receiver when transmitter is working.
☛ Receiver: It can be super heterodyne receiver or any other receiver which consists of a processor to process the signal and detect it.
☛ Threshold Decision: The output of the receiver is compared with a threshold to detect the presence of any object. If the output is below any threshold, the presence of noise is assumed.
11. Tell me can GPR be used to find gold?
While GPR is in use with many professional mining companies, the technology is not well suited to finding coins, gold nuggets or buried treasure. Metal detectors are more suitable. GSSI recommends checking with local, state and federal laws before beginning any treasure-hunting activities.
12. Tell me what are the other terms that are synonymous with ground penetrating radar?
Ground Penetrating Radar is also known as GPR, Ground Probing Radar, Ground Radar, and Georadar.
13. Explain how Often Should Vibration Analysis Be Performed?
Several factors such as equipment runtime, criticality, running environment conditions, etc. help determine how often vibration needs to be performed. On equipment that runs year round, quarterly vibration analysis is recommended as a minimum frequency. However, many manufacturing environments require monthly vibration analysis on critical machinery.
14. Explain me what Instrument Is Used To Measure Vibration?
Accelerometers are instruments for measuring, displaying, and analyzing acceleration and vibration. Vibration sensors cover sensors and other instruments used for measuring vibration and acceleration.
15. Explain me what Is An Ultrasonic Flaw Detector?
Sound waves traveling through a material will reflect in predictable ways off of flaws such as cracks and voids. An ultrasonic flaw detector is an instrument that generates and processes ultrasonic signals to create a waveform display that can be used by a trained operator to identify hidden flaws in a test piece. The operator identifies the characteristic reflection pattern from a good part, and then looks for changes in that reflection pattern that may indicate flaws.
16. Explain me what Are The Potential Limitations Of Ultrasonic Testing?
Ultrasonic flaw detection requires a trained operator who can set up a test with the aid of appropriate reference standards and properly interpret the results. Inspection of some complex geometries may be challenging. Ultrasonic thickness gages must be calibrated with respect to the material being measured, and applications requiring a wide range of thickness measurement or measurement of acoustically diverse materials may require multiple setups. Ultrasonic thickness gages are more expensive than mechanical measurement devices.
The pair of frequencies which produce the same output at the output of the RF receiver are referred as images of each other. For example in C-band satellite receiver, 3700MHz and 5785 MHz produce the same 70MHz as output. Hence here 5785MHz is the image frequency for 3700MHz and vice versa, Refer RF measurements tutorial to know more about this and other RF measurements. Homodyne and heterodyne are the two main architectures used in RF receiver. Refer heterodyne receiver vs homodyne receiver to find the difference between them.
Both 1dB compression and 3rd order intercept points are used as performance measure of RF amplifier, RF mixer etc. It provides the limit of linear region and point from where device will move to saturation or nonlinear region.
Power output of RF device should vary according to the input power linearly. The point from when power output does not vary linearly with the input device that point is referred as saturation or compression point. At this place 2 dB changes in the input power results in only 1dB change in the output power.
Let us understand 3rd order intercept point with example of two frequency signals f1 and f2 fed as input to the RF amplifier within the bandwidth limit of amplifier. Normally it should produce amplified f1 and f2 signals but due to distortion in an amplifier it produces harmonics at other frequencies. The second order products include f1-f2 and f1+f2. The third order products include 2f1 +/- f2 and 2f2 +/- f1. The most troublesome components are 2f1-f2 and 2f2-f1, which falls within the amplifier bandwidth and level of which is referred as 3rd order intercept point.
3rd order intercept point (TOI) is usually 10dB higher than the 1dB compression point.
For equipment that is difficult to access while it is in operation, MTI can either temporarily or permanently mount sensors to easily take readings on the equipment.
20. Tell me what is the Return loss and VSWR? How are they related?
Both return loss and VSWR is used as a measure of reflection of E-M waves over coaxial cable or RF cable or microstrip line. It gives how much power is reflected and how much power is absorbed at various points specially at terminating and source points and at places of impedance discontinuities. Return Loss in a coaxial cable having Z0 as characteristic impedance and ZL as terminating or load impedance can be expressed as follows:
Return Loss (dB) = 20*Log10((ZL-Z0)/(ZL+Z0))
Where Z0= (L/C)0.5
Both Return loss and VSWR are related as mentioned in the following expression.
Return loss = 20 log ((VSWR+1) / (VSWR-1))
VSWR ranges from 1 to infinity.
Harmonic and spurious frequencies are generated when RF mixer and amplifier devices are operating in nonlinear region due to distortion. Integer multiple of input frequency is referred as spurious. Non integer multiple of input frequency is referred as spurious.
The term PRF refers to Pulse Repetition Frequency which is no. of peak power pulses transmitted per second.
3D radar produces three dimensional position data of the target. It covers range, azimuth and also height.
Quasi TEM mode is used in a microstrip line. In a normal TEM mode, E-field and H-field are perpendicular to each other and also perpendicular to the direction of propagation. Refer difference between TEM and Quasi TEM wave. This interview question is very important to judge microstrip line fundamental of interviewee.
25. Tell us what Other Types Of Instruments Are Available?
Ultrasonic imaging systems are used to generate highly detailed pictures similar to x-rays, mapping the internal structure of a part with sound waves. Phased array technology originally developed for medical diagnostic imaging is used in industrial situations to create cross-sectional pictures. Large scanning systems are used by the aerospace industry and metalworking suppliers to check for hidden flaws in both raw materials and finished parts. Ultrasonic pulser/receivers and signal analyzers are used in a variety of materials research applications.
