Silicon Laboratories SI2493/57/34/15/04 - Manual (Page 289)

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