RF Interview Questions and Answers: Academic FAQ

This page provides a collection of frequently asked questions (FAQs) related to Radio Frequency (RF) engineering, particularly useful for interview preparation and viva sessions.

The following is a list of RF FAQs categorized around RF design, RF development, and RF testing:

RF FAQs

1. If the frequency were 12GHz, what would be the wavelength? a) 25mm b) 2.5mm c) 0.25 mm d) 250mm

2. Explain S parameters.

3. What is the 1dB compression point of an amplifier?

4. What is the 3rd order intercept point of an amplifier?

5. What is the typical thumb rule between P1dB and TOI point?

6. Write the Noise figure equation.

   The noise figure (NF) is often expressed in decibels (dB) and can be calculated using the following equation:

   $NF = 10 \log_{10}(F)$

   Where F is the noise factor, which is defined as:

   $F = \frac{SNR_{in}}{SNR_{out}}$

   $SNR_{in}$ is the signal-to-noise ratio at the input, and $SNR_{out}$ is the signal-to-noise ratio at the output.

7. What is the input and output back-off of an amplifier?

8. What is the noise floor?

9. For a given permittivity of 4 and permeability of 2, what is the wave impedance? a) 266.5 b) 533.0 c) 377.0 d) 133.2

10. What is the VSWR and Return loss equation?

    • VSWR (Voltage Standing Wave Ratio):

      $VSWR = \frac{1 + |\Gamma|}{1 - |\Gamma|}$

      Where $\Gamma$ is the reflection coefficient.

    • Return Loss:

      $Return\ Loss = -20 \log_{10} |\Gamma|$

      Return loss is typically expressed in dB.

11. Explain the difference between a Duplexer and a Diplexer?

12. Which TEM mode is supported in a Micro-strip line?

13. What is the noise temperature of the LNA?

14. Where is the LNA typically placed in a wireless receiver and why?

15. A 1km long microwave link uses two antennas each having 30dB gain. If the power transmitted by one antenna is 1w at 3GHz the power received by the other antenna is a) 98.6 m watts b) 63.4 m watts c) 76.8 m watts d) 55.2 m watts

16. A rectangular air-filled wave-guide has a cross-section of 4cm x 10 cm. The minimum frequency which can propagate in the wave-guide is a) 1.5 GHz b) 2 GHz c) 2.5 GHz d) 3.5 GHz

17. What is the frequency range if the antenna is operating at Ku-band? a) 4-8GHz b) 8-12GHz c) 12-18GHz d) 18-27GHz

18. What is the typical range of the near-field region?

19. The far-field region is commonly taken to exist at (D is the maximum overall dimension of the source) a) Distances greater than $2D^2/\lambda$ from the source b) Distances lesser than $\lambda/4$ from the source c) Distances greater than $\lambda/4$ from the source d) Distances lesser than $2D^2/\lambda$ from the source

20. If source impedance is 50 Ohms and the load impedance is 100 Ohms, what will be the line impedance of the Quarter Wave Transformer? a) 14.14 Ohms b) 50 Ohms c) 1.414 Ohms d) 70.7 Ohms

21. Zigbee network is mainly used for a) Signaling and Monitoring b) High-Speed Data Transfer c) Voice Transfer d) Video Transmission

22. Which type of transmission line will have the maximum value of characteristic impedance a) Open Wire line b) Coaxial Cable c) Twin lead line d) None

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

24. What is the function of filters?

    To filter out all the unwanted radio signals.

25. 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.

26. What are saw filters used for?

    Very low frequency filtering.

27. What are mixers used for?

    To change the frequency of the signal.

28. What’s a super-heterodyne receiver?

    It’s a receiver that uses two mixers in a row.

29. Describe VCO.

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

30. What do you mean by EIRP?

    Effective Isotropic Radiated Power