*Question G8B05:
Which intermodulation products are closest to the original signal frequencies?*  

*Correct Answer is  C: Odd-order*  

### AI Response: (may contain errors)  

Intermodulation products are unwanted signals generated when two or more signals mix in a non-linear device, such as an amplifier. These products can appear at various frequencies, and their order determines how close they are to the original signal frequencies.

1. **Correct Answer: Odd-order**

   - **Explanation**: Odd-order intermodulation products (e.g., 3rd, 5th, 7th order) are closest to the original signal frequencies. This is because they are generated by mixing the original frequencies in such a way that the resulting products are near the original signals. For example, if you have two frequencies, $f_1$ and $f_2$, the 3rd-order products would be at $2f_1 - f_2$ and $2f_2 - f_1$, which are close to $f_1$ and $f_2$.

2. **Why the Other Choices are Incorrect**:

   - **Second harmonics (Choice A)**: These are multiples of a single frequency (e.g., $2f_1$, $2f_2$) and are not intermodulation products. They are further away from the original frequencies compared to odd-order products.

   - **Even-order (Choice B)**: Even-order intermodulation products (e.g., 2nd, 4th, 6th order) tend to fall further away from the original frequencies. For instance, a 2nd-order product like $f_1 + f_2$ or $f_1 - f_2$ is generally not as close to the original frequencies as odd-order products.

   - **Intercept point (Choice D)**: This is a measure of a device's linearity and is not related to the frequency of intermodulation products. It indicates the point at which the power of the intermodulation products equals the power of the fundamental signals.

3. **Useful Formulas**:

   - For odd-order products:  
     $$ \text{3rd-order: } 2f_1 - f_2, \, 2f_2 - f_1 $$
   - For even-order products:  
     $$ \text{2nd-order: } f_1 + f_2, \, f_1 - f_2 $$

Understanding these concepts helps in identifying and mitigating interference issues in radio systems.