Modulation format WWVB

1 modulation format

1.1 amplitude modulation
1.2 phase modulation

1.2.1 allowance carrier phase tracking receivers


1.3 station id

modulation format

wwvb transmits data @ 1 bit per second, taking 60 seconds send current time of day , date within century. there 2 independent time codes used purpose: amplitude-modulated time code, has been in use minor changes since 1962, , phase-modulated time code added in late 2012.

amplitude modulation

the wwvb 60 khz carrier, has normal erp of 70 kw, reduced in power @ start of each utc second 17 db (to 1.4 kw erp). restored full power time during second. duration of reduced power encodes 1 of 3 symbols:

if power reduced one-fifth of second (0.2 s), data bit value zero.
if power reduced one-half of second (0.5 s), data bit value one.
if power reduced four-fifths of second (0.8 s), special non-data marker, used framing.

each minute, 7 markers transmitted in regular pattern allows receiver identify beginning of minute , correct framing of data bits. other 53 seconds provide data bits encode current time, date, , related information.

before july 12, 2005, when wwvb s maximum erp 50 kw, power reduction 10 db, resulting in 5 kw signal. change greater modulation depth part of series of experiments increase coverage without increasing transmitter power.

phase modulation

an independent time code transmitted binary phase-shift keying of wwvb carrier. 1 bit encoded inverting phase (a 180° phase shift) of carrier 1 second. 0 bit transmitted normal carrier phase. phase shift begins 0.1 s after corresponding utc second, transition occurs while carrier amplitude low.

the use of phase-shift keying allows more sophisticated (but still simple modern electronics standards) receiver distinguish 0 , 1 bits far more clearly, allowing improved reception on east coast of united states wwvb signal level weak, radio frequency noise high, , msf time signal u.k. interferes @ times.

there no markers in amplitude modulated time code. minute framing instead provided fixed pattern of data bits, transmitted in last second of each minute , first 13 seconds of next one. because amplitude-modulated markers provide 0.2 s of full-strength carrier, more difficult decode phase modulation. phase-modulated time code therefore avoids using these bit positions within minute important information.

allowance carrier phase tracking receivers

added in late 2012, phase modulation has no effect on popular radio-controlled clocks consider carrier s amplitude, cripple (rare) receivers track carrier phase.

to allow users of phase tracking receivers time adjust, phase-modulated time code omitted twice daily 30 minutes, beginning @ noon , midnight mountain standard time (07:00 , 19:00 utc). provided enough opportunity receiver lock on wwvb carrier phase. allowance removed of march 21, 2013.

station id

prior addition of phase-modulated time code, wwvb identified advancing phase of carrier wave 45° @ ten minutes past hour, , returning normal (a −45° shift) 5 minutes later. phase step equivalent cutting , pasting  ⁄8 of 60 khz carrier cycle, or approximately 2.08 µs.

this station id method common narrowband high power transmitters in vlf , lf bands other intervening factors prevent normal methods of transmitting call letters.

when phase modulation time code added in late 2012, station identification eliminated; time code serves station identification.