Top 10 RF Interview Questions and Answers

This page explains the answers to the top 10 RF interview questions. This list of RF interview questions and answers helps interviewees pass job interviews with ease in RF (Radio Frequency) domain positions. This page covers an RF questionnaire set prepared by experienced specialists in the RF domain.

This domain has always been in demand right from design, testing, and installation. Nowadays, RF firmware jobs are also available, which require knowledge of both baseband and RF technologies.

These top 10 RF interview questions and answers help engineers seeking RF jobs to crack the interview with ease. These questions are very useful as viva questions as well. 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.

Q1: What is the Return loss and VSWR? How are they related?

Both return loss and VSWR are used as a measure of reflection of E-M waves over coaxial cables, RF cables, or microstrip lines. It indicates how much power is reflected and how much power is absorbed at various points, especially at terminating and source points, and at places of impedance discontinuities.

Return Loss in a coaxial cable having $Z_0$ as characteristic impedance and $Z_L$ as terminating or load impedance can be expressed as follows:

Return Loss (dB) = $20 * Log_{10} (|(Z_L - Z_0) / (Z_L + Z_0)|)$

Where $Z_0 = \sqrt{(L/C)}$

Both Return loss and VSWR are related as mentioned in the following expression.

Return loss = $20 * log_{10}((VSWR + 1) / (VSWR - 1))$

VSWR ranges from 1 to infinity.

Q2: What is the 1dB compression point and 3rd order intercept point? What is the relation between both?

Both 1dB compression and 3rd order intercept points are used as performance measures of RF amplifiers, RF mixers, etc.

It provides the limit of the linear region and the point from where the device will move to saturation or the nonlinear region. The power output of an RF device should vary linearly according to the input power. The point from when the power output does not vary linearly with the input is referred to as the saturation or compression point. At this place, a 2 dB change in the input power results in only a 1dB change in the output power.

Let us understand the 3rd order intercept point with an example of two frequency signals $f_1$ and $f_2$ fed as input to the RF amplifier within the bandwidth limit of the amplifier. Normally, it should produce amplified $f_1$ and $f_2$ signals, but due to distortion in an amplifier, it produces harmonics at other frequencies. The second-order products include $f_1 - f_2$ and $f_1 + f_2$. The third-order products include $2f_1 +/- f_2$ and $2f_2 +/- f_1$.

The most troublesome components are $2f_1 - f_2$ and $2f_2 - f_1$, which fall within the amplifier bandwidth and the level of which is referred to as the 3rd order intercept point.

The 3rd order intercept point (TOI) is usually 10dB higher than the 1dB compression point.

Q3: Why is the isolator placed at the output of the amplifier?

An RF Isolator allows signals to flow only in one direction and hence prevents any reflection from going into the amplifier from the output port. Hence, it prevents damage to the amplifier device.

Q4: What is the carrier to interference ratio (C/I) and how is it related to SNR?

C/I refers to the ratio of carrier power to the interference power. SNR refers to the ratio of signal power to the noise power.

C/I applies to modulated waveforms, while SNR applies to unmodulated waveforms.

Q5: What is G/T of the antenna?

G/T is referred to as the figure of merit of the RF antenna. G stands for Antenna gain, and T stands for Antenna noise temperature.

Q6: Explain the difference between an RF circulator and an isolator

An RF circulator is a 3-port device, and an isolator is a 2-port device. Both allow signals to flow in only one direction and prevent signals from going in the other direction as per design.

RF circulators have two main types: clockwise and anti-clockwise. If ports are say P1, P2, and P3, then the isolator can pass the signal from P1 to P2, P2 to P3, and from P3 to P1, and not in the other direction if designed so; otherwise, it will pass the signal from P3 to P2, P2 to P1, and from P1 to P3.

Q7: Explain the transmission mode for EM waves in a microstrip line. Also, explain the types of microstrip lines

The Quasi TEM mode is used in a microstrip line. In a normal TEM mode, the E-field and H-field are perpendicular to each other and also perpendicular to the direction of propagation.

This interview question is very important to judge the microstrip line fundamentals of the interviewee.

Q8: What is the difference between harmonics and spurious signals? When are these signals generated in an RF circuit?

Harmonic and spurious frequencies are generated when RF mixer and amplifier devices are operating in a nonlinear region due to distortion.

Integer multiples of the input frequency are referred to as harmonics. Non-integer multiples of the input frequency are referred to as spurious signals.

Q9: What is image frequency rejection in an RF transceiver? What is the difference between homodyne and heterodyne architectures in an RF Receiver?

The pair of frequencies that produce the same output at the output of the RF receiver are referred to as images of each other.

For example, in a 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.

Homodyne and heterodyne are the two main architectures used in RF receivers.

Q10: What is the relation between dBm, dBW, and Watt?

All these are units of power measurement. dBm refers to decibels related to 1 milliwatt, dBW refers to decibels related to 1 watt.