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RF general interview questions



What is the relation between dB and change of signal? +3 dB is 2 times bigger, +10 dB is 10 times bigger.

What is an octave? The bandwidth characterized by higher band being twice as much as lower band (100-200 MHz).

What is a decade? A bandwidth, where the higher band is 10x the lower band (100 MHz - 1 GHz).

Skin effect? The signal is reported on the surface of the object, but not inside, e.g. a solid brick wall.

VSWR? Voltage Standing Wave Ratio. The numeric representation of the device leak, written like X:1 where X is greater when the leak is greater and 1:1 is the absolute no-leakage VSWR.

Describe an impedance matching circuit. A circuit capable of changing the impedance with different values for incoming and outgoing impedance.



Describe circular polarization. The signal travels both vertically and horizontally.

What is the job of the amplifier? To increase the incoming signal.

What is noise figure? The level of noise that a low-noise amplifier (LNA) is capable of discriminating.

What is a transfer curve? It is a graph of the output power versus the input power of an amplifier.

What is the 911 point? The point where the input power results in flames on the other end.

What is saturation point? The point where the amplifier loses its linearity of the transfer curve.

What does balanced amplifier consist of? Two amplifiers stuck together.

What is the filters function? To filter out all the unwanted radio signals.

What filter types are there? Low-pass (all frequencies below are allowed to pass in), high-pass, bandpass (all frequencies of a given bandwidth are allowed) and band-reject.

What are saw filters used for? Very low frequency filtering.

What are mixers used for? To change the frequency of the signal.

What is a superheterodyne receiver? It is a receiver that uses two mixers in a row.

Describe VCO. Voltage-controlled oscillators change the frequency of the produced sine wave depending on the voltage they receive.

Describe more on the function of PLL ?

VLSI interview questions with answers



  • What are the various Design constraints used while performing Synthesis for a design?
    Ans: 1. Create the clocks (frequency, duty-cycle).
    2. Define the transition-time requirements for the input-ports
    3. Specify the load values for the output ports
    4. For the inputs and the output specify the delay values(input delay and ouput delay), which are already consumed by the neighbour chip.
    5. Specify the case-setting (in case of a mux) to report the timing to a specific paths.
    6. Specify the false-paths in the design
    7. Specify the multi-cycle paths in the design.
    8. Specify the clock-uncertainity values(w.r.t jitter and the margin values for setup/hold).
    19. Specify few verilog constructs which are not supported by the synthesis tool.


  • What are the various design changes you do to meet design power targets?
    Ans: Design with Multi-VDD designs, Areas which requires high performance, goes with high VDD and areas which needs low-performance are working with low Vdd's, by creating Voltage-islands and making sure that appropriate level-shifters are placed in the cross-voltage domains Designing with Multi-Vt's(threshold voltages), areas which require high performance, goes with low Vt, but takes lot of leakage current, and areas which require low performance with high Vt cells, which has low leakage numbers, by incorporating this design process, we can reduce the leakage power. As in the design , clocks consume more amount of power, placing optimal clock-gating cells, in the design and controlling them by the module enable's gives a lot of power-savings.
    As clock-tree's always switch making sure that most number of clock-buffers are after the clock-gating cells, this reduces the switching there by power-reduction.
    Incorporating Dynamic Voltage and Frequency scaling (DVFS) concepts based on the application , there by reducing the systems voltage and frequency numbers when the application does not require to meet the performance targets. Ensure the design with IR-Drop analysis and ground-bounce analysis, is with-in the design specification requirement. Place power-switches, so that the leakage power can be reduced. related information.


  • what is meant by Library Characterizing
    Ans: Characterization in terms of delay, power consumption,..

  • what is meant by wireload model
    Ans: In the synthesis tool, in order to model the wires we use a concept called as "Wireload models", Now the question is what is wireload models: Wireload models are statistical based on models with respect to fanout. say for a particular technology based on our previous chip experience we have a rough estimate we know if a wire goes for "n" number of fanin then we estimate its delay as say "x" delay units. So a model file is created with the fanout numbers and corresponding estimated delay values. This file is used while performing Synthesis to estimate the delay for Wires, and to estimate the delay for cells, technology specific library model files will be available

  • what are the measures to be taken to design for optimized area
    Ans: As silicon real-estate is very costly and saving is directly propotional to the company's revenue generation lot of emphasize is to design which has optimial utilization in the area-front. The steps to reduce area are
    If the path is not timing-critical, then optimize the cells to use the low-drive strength cells so that there will saving in the area. Abut the VDD rows Analyzing the utilization numbers with multiple floor-planning versions which brings up with optimized area targets.

  • what all will you be thinking while performing floorplan
    Ans: Study the data-flow graph of the design and place the blocks accordingly, to reducing the weighted sum of area, wire-length. Minimize the usuage of blocks other-than square shapes, having notches Place the blocks based on accessibility/connectivity, thereby reducing wire-length. Abut the memory, if the pins are one-sided, there-by area could be reduced. If the memory communicates to the outside world more frequently , then placing at the boundary makes much of a sense. Study the number of pins to be routed, with the minimum metal width allowed , estimate the routability issues. Study the architecture and application , so that the blocks which will be enabled should be scattered, to reduce the power-ground noise.


  • what are the measures in the Design taken for Meeting Signal-integrity targets
    Ans: As more and more devices are getting packed, results in more congested areas, and coupling capactiances dominating the wire-capacitance, creates SI violations. Let's see now by what are all the measures we can reduce/solve it.
    As clock-tree runs across the whole chip, optimizing the design for SI, is essential route the clock with double-pitch and triple spacing. In-case of SI violation, spacing the signal nets reduces cross-talk impacts.
    Shield the nets with power-nets for high frequency signal nets to prevent from SI.
    Enable SI aware routing , so that the tool takes care for SI
    Ensure SI enabled STA runs, and guarantee the design meeting the SI requirements
    Route signals on different layers orthogonal to each other
    Minimize the parallel run-length wires, by inserting buffers.




    COMMON INTERVIEW QUESTIONS AND ANSWERS


    Review these typical interview questions and think about how you would answer them. Read the questions listed; you will also find some strategy suggestions with it. You may use your brain to answer in better way.


    1. Tell me about yourself:
    The most often asked question in interviews. You need to have a short statement prepared in your mind. Be careful that it does not sound rehearsed. Limit it to work-related items unless instructed otherwise. Talk about things you have done and jobs you have held that relate to the position you are interviewing for. Start with the item farthest back and work up to the present.


    2. Why did you leave your last job?
    Stay positive regardless of the circumstances. Never refer to a major problem with management and never speak ill of supervisors, co-workers or the organization. If you do, you will be the one looking bad. Keep smiling and talk about leaving for a positive reason such as an opportunity, a chance to do something special or other forward-looking reasons.

    3. What experience do you have in this field?
    Speak about specifics that relate to the position you are applying for. If you do not have specific experience, get as close as you can.



    4. Do you consider yourself successful? You should always answer yes and briefly explain why. A good explanation is that you have set goals, and you have met some and are on track to achieve the others.

    5. What do co-workers say about you? Be prepared with a quote or two from co-workers. Either a specific statement or a paraphrase will work. Jill Clark, a co-worker at Smith Company, always said I was the hardest workers she had ever known. It is as powerful as Jill having said it at the interview herself.

    6. What do you know about this organization? This question is one reason to do some research on the organization before the interview. Find out where they have been and where they are going. What are the current issues and who are the major players?

    7. What have you done to improve your knowledge in the last year? Try to include improvement activities that relate to the job. A wide variety of activities can be mentioned as positive self-improvement. Have some good ones handy to mention.

    8. Are you applying for other jobs? Be honest but do not spend a lot of time in this area. Keep the focus on this job and what you can do for this organization. Anything else is a distraction.

    9. Why do you want to work for this organization? This may take some thought and certainly, should be based on the research you have done on the organization. Sincerity is extremely important here and will easily be sensed. Relate it to your long-term career goals.

    10. Do you know anyone who works for us? Be aware of the policy on relatives working for the organization. This can affect your answer even though they asked about friends not relatives. Be careful to mention a friend only if they are well thought of.

    11. What kind of salary do you need? A loaded question. A nasty little game that you will probably lose if you answer first. So, do not answer it. Instead, say something like, That's a tough question. Can you tell me the range for this position? In most cases, the interviewer, taken off guard, will tell you. If not, say that it can depend on the details of the job. Then give a wide range.

    12. Are you a team player? You are, of course, a team player. Be sure to have examples ready. Specifics that show you often perform for the good of the team rather than for yourself are good evidence of your team attitude. Do not brag, just say it in a matter-of-fact tone. This is a key point.

    13. How long would you expect to work for us if hired? Specifics here are not good. Something like this should work: I'd like it to be a long time. Or As long as we both feel I'm doing a good job.

    14. Have you ever had to fire anyone? How did you feel about that? This is serious. Do not make light of it or in any way seem like you like to fire people. At the same time, you will do it when it is the right thing to do. When it comes to the organization versus the individual who has created a harmful situation, you will protect the organization. Remember firing is not the same as layoff or reduction in force.

    15. What is your philosophy towards work? The interviewer is not looking for a long or flowery dissertation here. Do you have strong feelings that the job gets done? Yes. That's the type of answer that works best here. Short and positive, showing a benefit to the organization.

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