*Question E4E01:
What problem can occur when using an automatic notch filter (ANF) to remove interfering carriers while receiving CW signals?*  

*Correct Answer is  A: Removal of the CW signal as well as the interfering carrier*  

### AI Response: (may contain errors)  

When using an automatic notch filter (ANF) to remove interfering carriers while receiving CW (Continuous Wave) signals, the main issue is that the ANF might inadvertently remove the CW signal itself. Here's why:

1. **Correct Answer: Removal of the CW signal as well as the interfering carrier**

   - **Explanation**: An automatic notch filter is designed to detect and suppress narrowband interference, such as a carrier signal. However, CW signals are also narrowband by nature. If the ANF is not carefully adjusted, it may interpret the CW signal as interference and notch it out, leading to the removal of the desired CW signal along with the interfering carrier.

2. **Why the other choices are incorrect**:

   - **Choice B: Any nearby signal passing through the DSP system will overwhelm the desired signal**
     - This is not directly related to the function of an ANF. The ANF specifically targets narrowband interference, not the overall signal processing capability of the DSP system.

   - **Choice C: Excessive ringing**
     - Ringing is a potential issue with filters, especially sharp ones, but it is not the primary concern with ANFs in the context of CW signals. Ringing typically affects the transient response of a filter, not the removal of the desired signal.

   - **Choice D: All these choices are correct**
     - Since choices B and C are not correct in this context, choice D is also incorrect.

In summary, the main concern with using an ANF while receiving CW signals is the risk of the filter mistakenly removing the CW signal itself, as it may be perceived as interference due to its narrowband nature.