Communication Systems

Step 1: Recall the block diagram of a standard superheterodyne FM receiver. The received signal from the antenna is processed in a specific order to extract the original message.

RF Amplifier (B): The first stage is a tuned Radio Frequency amplifier. Its purpose is to amplify the very weak signal from the antenna and provide some initial selectivity to reject signals far from the desired frequency (like the image frequency).
Mixer (A): The amplified RF signal is then fed to a mixer, along with the signal from a local oscillator (LO). The mixer performs frequency conversion, down-converting the RF signal to a fixed, lower Intermediate Frequency (IF).
IF Amplifier (D): The signal at the IF is then passed through several stages of IF amplification. Most of the receiver's gain and selectivity are provided in this section.
Limiter (C): Before demodulation, the IF signal in an FM receiver is passed through a limiter. The limiter is a circuit that removes any amplitude variations from the signal. Since the information in FM is purely in the frequency variations, these amplitude variations are considered noise and must be removed for a high-quality output.
(After the limiter comes the FM demodulator/discriminator, followed by an audio amplifier.)

Step 2: Form the correct sequence. Based on the standard architecture, the correct sequence of the given blocks is RF amplifier Mixer IF amplifier Limiter. This corresponds to the sequence B, A, D, C.

Step 1: Understand PSK (Phase Shift Keying). In coherent PSK, the receiver must generate a local carrier wave that is perfectly synchronized in phase with the incoming carrier wave. This is used as a reference to determine the phase of the received signal. Generating this coherent carrier is complex and adds to the cost and complexity of the receiver.
Step 2: Understand DPSK (Differential Phase Shift Keying). In DPSK, the information is encoded in the difference in phase between consecutive symbols, not in the absolute phase. For example, a '1' might be represented by no phase change, and a '0' by a 180° phase change. The receiver demodulates the signal by comparing the phase of the current symbol with the phase of the previous symbol.
Step 3: Compare the two. Because DPSK uses the previous symbol as its own phase reference, it does not need a separate, locally generated coherent carrier. This simplifies the receiver design significantly. While DPSK has a slightly higher bit error rate than ideal coherent PSK for the same SNR, its major advantage is the avoidance of complex carrier recovery circuits.