Coherent detection is a signal processing technique used in communication systems and signal demodulation to extract information from a modulated signal accurately, even in the presence of noise and other impairments. The term "coherent" refers to maintaining a constant phase reference between the transmitted and received signals, allowing for better demodulation accuracy.
In the context of AC (alternating current) signal demodulation, coherent detection is often used to recover the original message signal from a modulated carrier signal. AC signal demodulation involves the extraction of the original information (message) signal that was embedded onto a carrier signal. This is commonly seen in various modulation techniques such as amplitude modulation (AM) and frequency modulation (FM).
Here's how coherent detection works and its applications in AC signal demodulation:
Coherent Detection Principle:
Coherent detection involves multiplying the received signal with a locally generated reference signal that has the same frequency and phase as the carrier signal used for modulation. The local oscillator generates this reference signal. By multiplying the received signal with this reference signal, the result is a product that contains the desired message signal in addition to other components.
Carrier Recovery:
In order to perform coherent detection accurately, the receiver needs to recover the carrier frequency and phase of the received signal. This is typically achieved through techniques like phase-locked loops (PLLs) or frequency-locked loops (FLLs). These circuits help synchronize the local oscillator's frequency and phase with that of the received carrier signal.
Demodulation:
Once the carrier frequency and phase are recovered and the reference signal is generated, the received signal is multiplied with this reference signal. This multiplication process effectively shifts the frequency content of the received signal from the carrier frequency to near-zero frequency (baseband). The result of this multiplication is then passed through a low-pass filter to remove high-frequency components and noise, leaving behind the original message signal.
Applications:
Coherent detection finds applications in various AC signal demodulation scenarios, including:
AM Demodulation: In amplitude modulation (AM), coherent detection helps recover the original message signal by multiplying the received AM signal with a locally generated carrier signal. This process extracts the variations in the amplitude of the carrier signal, which represent the message signal.
FM Demodulation: In frequency modulation (FM), coherent detection assists in recovering the original message signal by tracking the changes in the frequency of the received signal. The phase and frequency information contained in the reference signal allow for accurate demodulation.
Coherent detection offers better performance in demodulating signals compared to non-coherent methods because it takes advantage of the phase and frequency information of the received signal. However, it requires more complex hardware and additional processing compared to non-coherent methods, which can make it more challenging to implement in certain applications.