Inside the La Crosse 1235UA UltrAtomic Radio Controlled WWVB (Atomic) Wall Clock


Tear down photos

Initial Impressions

Detailed performance plots

The following is a 10-day plot of the error in a 1235UA UltrAtomic clock. For this test probes were attached to each of the stepper motor wires. Timing was obtained with a set of picPET timer chips which compared UTC ticks and UltrAtomic ticks. The plot shows that over 10 days the UltrAtomic clock stayed within 1/5th to 1/10th of a second. The plot also shows frequent corrections made to the clock's timing in order to keep it sync'ed.

50 days (mean -70 ms, stdev 50 ms):

ADEV+MDEV (Allan deviation, Modified Allan deviation):

TDEV (Time deviation):

Leap second video (31-Dec-2016)

How a LF radio clock or PC (NTP) or GPS receiver handles leap seconds is interesting to those who are picky about precise time. Most PC's as well as home or commercial radio "atomic" clocks ignore leap seconds. The good news is that this La Crosse 404-1235UA WWVB "atomic" clock correctly handled the positive leap second at the end of December 2016. In the video below you can see the second hand pause for an extra second before the top of the hour. Strictly speaking, a positive leap second is an extra second called 23:59:60 but for a clock with gears and analog hands pausing at the 59th second for 2 seconds before jumping to the 00th second is as good as it gets.

Pick your format:   IMG_3126.MOV   IMG_3126.m4v   IMG_3126.mpg

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