1 Answer
In a **superheterodyne receiver**, the goal is to convert the received radio frequency (RF) signal into a lower intermediate frequency (IF) for easier processing. This is achieved by mixing the RF signal with a local oscillator (LO) signal. However, this process can create unwanted signals, particularly a signal at a frequency that’s the same distance from the LO as the desired RF signal. This unwanted signal is called the **image signal**, and it can interfere with the proper reception of the intended signal.
### What is Image Rejection?
**Image rejection** refers to the process or technique used to suppress or eliminate this unwanted image signal, leaving only the desired signal at the intermediate frequency.
### How does the image signal arise?
When you mix the RF signal with the LO signal, you create two new frequencies — one at the sum of the two frequencies (LO + RF) and one at the difference (LO - RF). The desired signal is typically the difference (LO - RF), which becomes the IF.
However, there's another signal at a frequency that’s symmetric to the desired frequency relative to the LO. This is called the **image frequency**, and it’s located at:
\[
f_{\text{image}} = f_{\text{LO}} + (f_{\text{LO}} - f_{\text{RF}}) = 2f_{\text{LO}} - f_{\text{RF}}
\]
So, while the desired signal is at **LO - RF**, the image signal is at **LO + RF**. Both signals can produce the same IF after mixing, but only one is the actual signal of interest.
### Why is Image Rejection Important?
If the image signal is not rejected, it could also get converted to the intermediate frequency and interfere with the desired signal. This would degrade the quality of reception, cause distortion, and make it difficult to separate the signals at the IF stage.
### How is Image Rejection Achieved?
Image rejection is typically handled by **filtering** the signal before mixing or using a more complex design in the receiver, such as:
1. **Pre-mixer filtering**: A bandpass filter placed before the mixer to ensure that only the desired RF signal is allowed to pass through and reach the mixer, blocking the image signal.
2. **Image-reject mixers**: Special mixers that are designed to reject the image signal by exploiting the symmetry of the mixing process. These mixers use multiple stages or clever circuitry to cancel out the image frequency.
3. **Dual conversion receivers**: Sometimes, the signal is mixed twice with two different local oscillators, which helps to avoid or reduce the impact of the image frequency.
In simple terms, **image rejection** is crucial for clear, accurate reception of the desired radio signal without interference from unwanted frequencies.
### What is Image Rejection?
**Image rejection** refers to the process or technique used to suppress or eliminate this unwanted image signal, leaving only the desired signal at the intermediate frequency.
### How does the image signal arise?
When you mix the RF signal with the LO signal, you create two new frequencies — one at the sum of the two frequencies (LO + RF) and one at the difference (LO - RF). The desired signal is typically the difference (LO - RF), which becomes the IF.
However, there's another signal at a frequency that’s symmetric to the desired frequency relative to the LO. This is called the **image frequency**, and it’s located at:
\[
f_{\text{image}} = f_{\text{LO}} + (f_{\text{LO}} - f_{\text{RF}}) = 2f_{\text{LO}} - f_{\text{RF}}
\]
So, while the desired signal is at **LO - RF**, the image signal is at **LO + RF**. Both signals can produce the same IF after mixing, but only one is the actual signal of interest.
### Why is Image Rejection Important?
If the image signal is not rejected, it could also get converted to the intermediate frequency and interfere with the desired signal. This would degrade the quality of reception, cause distortion, and make it difficult to separate the signals at the IF stage.
### How is Image Rejection Achieved?
Image rejection is typically handled by **filtering** the signal before mixing or using a more complex design in the receiver, such as:
1. **Pre-mixer filtering**: A bandpass filter placed before the mixer to ensure that only the desired RF signal is allowed to pass through and reach the mixer, blocking the image signal.
2. **Image-reject mixers**: Special mixers that are designed to reject the image signal by exploiting the symmetry of the mixing process. These mixers use multiple stages or clever circuitry to cancel out the image frequency.
3. **Dual conversion receivers**: Sometimes, the signal is mixed twice with two different local oscillators, which helps to avoid or reduce the impact of the image frequency.
In simple terms, **image rejection** is crucial for clear, accurate reception of the desired radio signal without interference from unwanted frequencies.