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

Choices:  
A: Removal of the CW signal as well as the interfering carrier  
B: Any nearby signal passing through the DSP system will overwhelm the desired signal  
C: Excessive ringing  
D: All these choices are correct    

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

To remove steady carriers, an automatic notch filter (ANF) continuously searches for and cancels narrow, constant-frequency tones. A CW signal during key-down is exactly a single audio tone, so the ANF can mistake your desired CW tone for an interfering carrier and notch it out.

Why the correct answer is correct (A)
- During each key-down, CW is a steady sine wave at the audio beat note. The ANF identifies it as a tone and places a notch there, reducing or removing the desired CW signal.

Why the other choices are incorrect
- B: “Any nearby signal…will overwhelm the desired signal” describes AGC pumping or front-end overload, not an ANF-specific behavior.
- C: “Excessive ringing” is more associated with extremely narrow high-Q filters or some noise blankers. While DSP artifacts can occur, the hallmark ANF problem on CW is notching the wanted tone, not ringing.
- D: Not all listed problems apply; the key issue is removal of the CW signal.

Useful formulas and rules of thumb
- Morse dit duration at a given speed:
  $$T_{\text{dit}} = \frac{60}{50\,\text{WPM}} = \frac{1.2}{\text{WPM}} \text{ s}$$
  A dah is $3T_{\text{dit}}$.
- If the ANF adaptation time constant $\,\tau_{\text{ANF}}$ is shorter than the key-down time ($T_{\text{on}}$), the ANF will lock onto and notch the CW tone:
  $$\tau_{\text{ANF}} \lesssim T_{\text{on}} \;\Rightarrow\; \text{likely notching of CW}$$
- Rule of thumb: Turn ANF off for CW. Use a narrow CW IF filter or an audio peaking filter (APF) to enhance the desired tone without risking removal.