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10 RF interview questions and answers

This page covers RF questionnaire set prepared by experienced specialists in RF domain. This domain has always demand right from design,testing and installation. Now-a-days RF firmware jobs are also available which requires knowledge of both baseband and RF technologies. These top 10 RF interview questions and answers help engineers seeking RF job to crack the interview with ease.These questions are very useful as viva questions also.

This questionnaire will help one pass the interview for various RF positions such as RF test engineer, RF Design Engineer, RF system Engineer, RF lead etc.

Question -1:What is the Return loss and VSWR? How are they related, pls. explain?
Answer -1: 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.
Read more

Question -2: What is 1dB compression point and 3rd order intercept point? What is the relation between both?
Answer -2: 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.
Refer P1dB versus TOI to know more about the difference between both.

Question -3: Why is the isolator placed at the output of the amplifier?
Answer -3: RF Isolator allows signal to flow only in one direction and hence prevent any reflection going into the amplifier from output port. Hence it prevents damage to the amplifier device, read more.

Question -4:What is carrier to interference ratio(C/I) and how it is related to SNR?
Answer -4: C/I refer to ratio of carrier power to the interference power. SNR refer to ratio of signal power to the noise power. C/I apply to modulated waveform while SNR applies to the unmodulated waveform, read more.



Question -5: What is G/T of the antenna?
Answer -5: G/T is referred as figure of merit of the RF antenna. G stands for Antenna gain and T stands for Antenna noise temperature. Refer satellite terminology page to know more about figure of merit of antenna. This is most often asked in the interview for satellite group position.

isolator vs circulator-RF interview questions and answers

Question -6: Explain difference between RF circulator and isolator.
Answer -6: RF circulator is a 3 port device and isolator is a 2 port device. Both allow signal to flow in any one direction and prevents signal going into the other direction as per design. RF circulator being having 3 ports, there are two main types clockwise and anticlockwise. If ports are say P1, P2 and P3 then isolator can pass signal from P1 to P2, P2 to P3 and from P3 to P1 and not in other direction if designed so otherwise it will pass signal from P3 to P2 and P2 to P1 and from P1 to P3. Refer RF isolator vs RF circulator page.

Question -7: Explain the transmission mode for EM waves in microstrip line. Also explain types of microstrip line.
Answer -7: 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.

Question -8: What is the difference between harmonics and spurious? When these signals are generated in RF circuit?
Answer -8: 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. Refer spurious versus harmonics page.

Question -9: What is image frequency rejection in RF transceiver? What is the difference between homodyne and heterodyne architecture in RF Receiver?
Answer -9: 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.

Question -10: What is the relation between dBm, dBW and Watt?
Answer -10: All these are units of power measurement , dBm refers to decibel related to 1 milliwatt, dBW refers to decibel related to 1 watt, Refer difference between dBm and dBW.

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