JJY(40kHz, 60kHz) HBG(75kHz) DCF77(77.5kHz), MSF(60kHz), WWVB(60kHz) all use AM to encode the time using different pulse widths to denote markers , 0 and 1 respectively. WWVB, JJY reduce the power by 10dB to encode 1, 0 etc so that a residual carrier is always present. WWVB also phase modulates the carrier at 1/3600Hz (period = 1hour) with a duty cycle of 5/60. MSF and HBG use ON/OFF keying. DCF77 encodes 1, 0 etc by reducing power to 25%, in addition DCF77 phase modulates the carrier with a PRBS. Since the minimum pulse width used is 0.1sec the receiver's effective bandwidth (when receiving WWVB, MSF or DCF77) need not be more than a few Hz.
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