Topic: Communication Systems
A coherent QPSK demodulator is required on ground for receiving data from a LEO satellite. What should be optimum order of PLL in the carrier phase tracking loop?
- A. $1^{st}$ order
- B. $2^{nd}$ order
- C. $3^{rd}$ order
- D. None of the above
Answer: C. $3^{rd}$ order
Explanation: Low Earth Orbit (LEO) satellites experience high and rapidly changing Doppler shifts (Doppler rate or acceleration of phase). A 3rd order Phase-Locked Loop (PLL) is required to track phase acceleration with zero steady-state error.