AN93
Rev. 1.3 289
Power Line Related Noise

Figure 73. Odd Harmonics of 50 Hz Manifest as Horizontal Lines Spaced at 100 Hz

Causes:
Unbalanced phone line
High ac leakage supply
Poor CMR in modem
Solutions:
Fix the telephone line.
Ground the system to earth or float completely using a battery.
Use an analog supply with lower ac Leakage